Efficient Autonomous Dew Water Harvesting by Laser Micropatterning: Superhydrophilic and High Emissivity Robust Grooved Metallic Surfaces Enabling Filmwise Condensation and Radiative Cooling

Abstract

The present work explores a unique yet unexplored synergy between the properties of laser micropatterned metallic surfaces and the requirements for an autonomous dew water harvesting candidate material. Laser-patterned aluminum surfaces achieved simultaneously high infrared emissivity (up to 0.95 in the atmospheric window) and superhydrophilic wettability (water contact angle of 0°), key properties enabling passive radiative cooling and filmwise condensation dynamics respectively. The generation of micrometric-sized grooves during laser processing plays a fundamental role in both properties, as they provide a broadband enhancement of the emissivity based on multiscale topographies and oxide layers, while limiting the growth of the water film during condensation through strong capillary wicking forces. As a result, the patterned aluminum surfaces display self-cooling capacities under radiative deficit conditions as well as low water retention levels (three times lower than the untreated dropwise condensation counterparts). The promising results obtained lead to the construction and evaluation of a real size outdoors autonomous dew water harvesting system based on those surfaces, demonstrating the scalability of the technology. A 70% improvement in the collected dew water in comparison to a state-of-the-art reference material is consistently measured during 1-year outdoor study, proving the robustness of the surfaces and their performance.

1 Introduction

It is a fact that the world faces a growing water scarcity problem: according to the UN's 2023 Sustainable Development Goals Report, about 28% of the world's population, i.e., 2.2 billion people, do not currently have access to safely managed drinking water.^[1] Together with an increase in the efficiency of water usage and a reduction in water pollution levels, exploring alternative clean water sources is one of the required actions to face this crucial global challenge.

In this line, atmospheric water harvesting in the form of dew or fog is one of the currently studied strategies for collecting residue-free clean water.^[2-4] Several examples in nature as spider silk structures, cacti spines or the skin of different species of beetles and frogs bring inspiration for the development of materials and systems to harvest water from the atmosphere.^[3-8]

Dew and fog are two distinct atmospheric water sources involving different physical mechanisms for its collection. In dew, water vapor condenses when it interacts with a surface at a temperature below the dew point temperature of the air. Extraction of water condensation latent heat is then required in this process. In contrast, fog water is already in liquid state as tiny drops are suspended in the air. Harvesting fog is therefore an easier process in technical terms and it also provides usually a higher water yield: dew water yield is typically below 0.3 mm (1 mm = 1 l m⁻²) per night^[9] while several tens of mm per night can be harvested with fog collectors.^[10] However, the ambient conditions required for fog harvesting are more demanding, restricting it to mountainous areas, while dew is available in a wider range of places across the planet.^{[11, 12]}

The most direct implication of the dependence of dew formation on the condensation phase change is the important role that surface wettability plays in the process. For condensation to occur, water molecules must overcome an energy barrier arising from the newly formed interphases when a liquid nucleus appears. Different from nucleation within the bulk gaseous phase (i.e., homogeneous nucleation), liquid nucleation is eased on top of a surface (i.e., heterogeneous nucleation). Here, the higher the wettability of the surface, the lower is the nucleation energy barrier, which vanishes as the water contact angle of the surface goes to zero.^[13-15] Apart from the nucleation rate, surface wettability defines also how the water is distributed across the condensation surface. In simple terms (refer to literature for more exhaustive discussion^{[15, 16]}), patterns of individual drops (i.e., dropwise condensation) are typically formed on hydrophobic surfaces. In contrast, water tends to spread and form a continuous film (i.e., filmwise condensation) in hydrophilic surfaces.

In this regard and in spite of a higher nucleation rate in hydrophilic surfaces, the filmwise condensation regime typically observed in those is often reported as detrimental for water harvesting applications. First, easier water removal from the surface once it is condensed (a crucial step for actual water harvesting) is commonly attributed to dropwise condensation surfaces,^[17-23] via either drop shedding, sliding, jumping, or capillarity-driven movements in comparison to water accumulation in big puddles in filmwise condensation surfaces.^{[16-18, 22, 24]} And second, this fast removal of condensed droplets reduces the average water thickness on the surface in comparison to the relatively thicker water films appearing in filmwise condensation. Hence, the conduction thermal resistance for heat transfer towards the cold source is lower in the dropwise regime, which then increases the efficiency of the condensation process.^{[20, 21, 24-28]} For those reasons, surfaces combining both hydrophobic and hydrophilic areas have been extensively studied to combine the advantages they offer.^{[6, 18, 19, 23, 29-34]}

Still, a careful analysis of the already existing literature and of the physics underlying the dew phenomenon still favors, to the authors’ opinion, the selection of homogeneously hydrophilic surfaces exhibiting filmwise condensation dynamics for dew water harvesting. First, it has been demonstrated that, with an adequate design, the water film of hydrophilic surfaces can be effectively evacuated, exploiting the advantage they offer in terms of nucleation in comparison to hydrophobic surfaces.^{[35, 36]} And secondly, in natural overnight dew condensation, the energy is extracted via infrared thermal radiation towards the atmosphere. In that situation, the condensation latent heat is evacuated directly from the liquid/gas interphase and not by conduction through the liquid itself,^{[9, 15]} which out rules the argument of a better conduction heat transfer in dropwise condensation.

Radiative cooling is an interesting approach to avoid the inclusion of external energy inputs in dew harvesting systems, avoiding the extra costs associated and allowing the autonomous functioning of the devices.^{[37, 38]} This strategy exploits the atmospheric transparency in the wavelength range from 8 to 13 µm. As it happens overnight with leaves in nature, a surface with high emissivity in that wavelength range can experiment a radiative deficit using the outer space as a radiation sink, lowering in consequence its temperature below that of the surrounding air without external action.^[39] Dew condensation takes place then when the surface temperature goes below the dew point temperature. Once the surface is wet, the surface emissivity tends towards the water emissivity irrespective of the initial value, but condensation starts in any case earlier in surfaces with higher emissivity, which makes them more efficient for autonomous dew water harvesting.^{[40, 41]}

In spite of this, the vast majority of recent works studying materials for dew harvesting do not consider a radiative cooling scenario, leaving a lack of research effort on the development of high emissivity materials as required for an autonomous functioning. And also, as discussed before, many studies in recent literature overlook the dew harvesting potential of hydrophilic surfaces exhibiting filmwise condensation. This work aims precisely at the development of a dew water harvesting system based on high emissivity filmwise condensation surfaces.

Besides, one of the crucial performance parameters for dew harvesting systems is the stability and robustness of the condensation surfaces under outdoor environments, where they are exposed to sun UV irradiation, rain, strong winds, abrasion by dust particles, etc. In that regard, the development of materials with robust properties is still a challenge faced by recent studies in the literature.^[42-44] Metallic materials are among the best candidates for providing robust outdoor performance. However, metal-based surfaces have been seldom employed for autonomous dew harvesting nor other radiative cooling applications^{[39, 45, 46]} due to their inherently low emissivity, which can nevertheless be enhanced by proper surface engineering.^{[47, 48]}

In this scenario, the technique of laser texturing is investigated here as the tool to achieve the desired surface properties. Born with the main aim of modifying surface topography, this technique has broadly shown the capability of tuning the wettability of a huge variety of materials^[49-52] including metals,^{[53, 54]} allowing the generation of surfaces ranging from superhydrophilic to superhydrophobic behavior.^[55-57] Besides, the generation of high infrared emissivity metal and semiconductor surfaces by laser texturing has also been widely demonstrated.^[58-65] However, despite perfectly fitting the above-explained requirements for autonomous dew harvesting, the exploration of water condensation by radiative cooling on hydrophilic laser-textured surfaces has not been yet reported to the best of the authors’ knowledge.

2 Results and Discussion

2.1 Surface Properties

2.1.1 Surface Topography

Samples from a 1.6 mm thickness plate of 6061 aluminum alloy were employed in this study (details are included in Experimental Section (Material)). Laser texturing was performed with nanosecond-pulsed laser irradiation, which on metals leads typically to heating, melting, and evaporation of material in the irradiated area. In this scenario, the main texturing mechanism is the sideways displacement of the molten pool under the action of the recoil pressure generated by the expanding metal vapor at the center on the laser-target interaction region (see Figure 1a. Detailed analyses of these mechanisms can be found in literature^{[66, 67]}). With the help of a galvo-scan and a F-Theta lens, the focused laser beam was guided through the surface as desired (Refer to Experimental Section (Laser Processing) for further details on the laser processing setup and treatment procedure).

Figure 1

a) Schematic of the laser texturing process. b) Surface topography characterization of the aluminum surfaces, including SEM and optical profilometry results. c) Surface chemistry characterization, including XPS and EDS analyses with O/Al atomic ratio. d) Wettability characterization results via water contact angle. e) Wicking behavior of laser textured surfaces, showing the fast upward motion of water from a liquid drop brought into contact with the surface at the bottom (refer to Figure S3, Supplementary Material for extended results). Uncertainty ranges correspond to standard deviation of 3 independent measurements.

Several works in the literature highlight the benefits of grooved surfaces on the efficient evacuation of condensed water.^{[36, 44, 68-70]} Following this idea, a scanning strategy of parallel straight lines was adopted. Along each line, the sideways displacement of the molten pool is repeated, allowing the generation of parallel grooves, as shown in Figure 1a,b.

The geometry of the grooves can be controlled by changing the irradiation conditions employed during the laser treatment. Different groove depths were tested in this work by varying the accumulated energy per unit area deposited along the scanning lines, controlled here by tuning the scanning speed. As illustrated by scanning electron microscopy (SEM) and optical profilometry results included in Figure 1b, the depth of the grooves is highly dependent on the scanning speed employed, with the total depth increasing from (17 ± 3) µm up to (73 ± 4) µm when the speed is decreased from 35 to 10 mm s⁻¹. Reducing the scanning speed increases the number of laser pulses per unit area, increasing the penetration to deeper levels as the molten material is evacuated by the vapor recoil pressure,^[71] explaining the observed results.

Apart from the main groove geometry, smaller surface features can be found in the laser-textured surfaces. As shown in the higher magnification SEM images of Figure 1b, recast material features in the order of tens of µm can be found along each groove. These are common features of nanosecond pulsed laser texturing of metals, arising from melt displacement and resolidification dynamics under the laser pulse chain.^[72-75] A profilometry analysis of these topography features (see Figures S1 and S2 and corresponding discussion in Supplementary Material, Section A) also revealed an increase of their height and period with the reduction of the scanning speed, linked to a higher volume of molten material involved in each displacement cycle with the higher spatial accumulation of laser pulses. Besides, asperities in the micro-submicron range can also be observed covering the whole treated surface in the higher magnification SEM images of Figure 1b, probably linked to a combination of protrusions grown by local melting phenomena, redeposition of molten particles and oxidation processes.^{[74, 76-78]}

Obviously, the most direct consequence of this rough groove texture is the increase of the surface roughness, with increased S_a parameter from (0.36 ± 0.04) µm for the untreated aluminum surface up to (24 ± 2) µm for the lowest scanning speed employed. This increase in roughness implies an increment of the actual surface area of the treated materials, growing up to a factor of more than 5 times for the lowest scanning speed as calculated from the S_dr parameter (values of S_dr and area ratio to untreated material are included in Figure 1. See definition of S_dr parameter in the Experimental Section (Surface Topography Characterization)).

2.1.2 Surface Chemistry

The second most direct effect of the laser treatments performed is the oxidation of the outer layers of the material during the air exposure of the hot metal. X-ray photoelectron spectroscopy (XPS) analyses (details included in Experimental Section (Surface Chemistry Characterization)) reveal an increase of the oxidation of the material with the laser treatments (see Figure 1c, left panel), with increasing O/Al atomic ratio from 1.08 ± 0.05 for the untreated alloy to an average of 1.78 ± 0.04 for the laser-treated surfaces. Furthermore, energy-dispersive X-ray spectroscopy (EDS) analyses (details included in Experimental Section (Surface Chemistry Characterization)) reveal an increasing oxidation trend for lower scanning speeds (Figure 1c, right panel), with measured O/Al ratios increasing from 0.524 ± 0.009 to 0.64 ± 0.01 and 0.88 ± 0.03 respectively for treatments under 35, 20 and 10 mm s⁻¹ scanning speed (for reference measured EDS O/Al ratio for the untreated material was 0.073 ± 0.005). The reason why this trend with the scanning speed is not visible in XPS is probably due to the low surface penetration of the technique, in the order of 10 nm, whereas EDS analyses gather information for depths up to 1 µm. This points to the formation of a thicker oxide layer in the surfaces treated with lower scanning speeds, consistent with increased heat penetration depth enabling deeper oxygen diffusion within the hot metal.

2.1.3 Surface Wettability

The wetting properties of the surfaces change drastically after the laser treatment. As shown in Figure 1d, untreated surfaces are just moderately hydrophilic with an average contact angle of (71 ± 3)°. However, surfaces become superhydrophilic after the laser treatment (contact angle of 0°).

Furthermore, a fast extension of the water drops was evident during contact angle measurements, revealing the high wickability of the laser-textured surfaces, as previously reported.^{[79, 80]} This behavior was confirmed by the characterized dynamics of the fast capillary rise of water along the surfaces (refer to Experimental Section (Surface Wettability Characterization)). The results included in Figure 1e and extended in Figure S3a (Supplementary Material) show how water travels upwards more than 20 mm along the grooved surfaces in just 1 second driven by capillary forces.

These extreme wetting properties are a consequence of both the oxidation and the developed grooved topography.

First, the formation of aluminum oxides increases the polar component of the surface energy and therefore its attraction to water molecules, which are also polar.^[81] This results in the reduction of the nucleation barrier for water condensation, and hence of the required temperature drop below the dew point for the condensation to actually start (supersaturation), which is of course interesting for dew harvesting.

Secondly, from a hydrophilic chemistry provided by the oxides, increasing the surface roughness enhances furthermore the wettability, as predicted by the classic wetting model of Wenzel.^[82] In this scenario, the presence of grooves enables the superhydrophilic behavior forcing the water to extend along the surface filling the topography channels by capillary wicking.^[83] Washburn-like dynamics^[84] with the wicked height rising proportionally to time^0.5 were detected for all tested samples (Figure S3b, Supplementary Material), also observed in previous literature studying wicking on grooved surfaces.^[85] Faster wicking dynamics were observed for surfaces processed with lower scanning speeds (Figure S3b, Supplementary Material), corresponding to deeper grooves and hence to thicker water films with lower influence from the friction with the bottom surface.^[86] The strong capillary observed in the grooved surfaces is beneficial for reducing the retention of the condensed water, as will be discussed in Section 2.2.3.

However, as known in the field of laser surface processing, the initial high wettability of laser textured metals is easily lost over time with ambient air exposure: the high surface energy linked to the presence of metal oxides and unsaturated metal atoms acts as the driving force for the adsorption of non-polar organic molecules present in the surrounding air,^{[87, 88]} which typically results then in a progressive transition to hydrophobic wetting over time as the surface polarity is decreased.^[89-93] As hydrophilicity is an interesting property for the dew harvesting application, and to prevent this wetting transition, a boiling water treatment was applied following previous research^{[18, 94]} (details included in Experimental Section (Boiling Post-Processing)). Immersion of laser-textured metals in boiling water leads to stable high wettability surfaces due to the formation of metal hydroxides,^{[89, 94]} with interesting nucleation properties due to their excellent affinity with water molecules.^{[95, 96]}

In this work, SEM images revealed the growth of a micro/submicrometric structure after the water boiling treatments for both untreated and laser-treated surfaces (see Figure S4a, Supplementary Material). At the same time, both XPS and EDS analyses reveal the increase of the O/Al atomic ratio for both unprocessed and processed surfaces after boiling (see Figure S4b,c, Supplementary Material). In high resolution XPS spectra, an increase of Al2p peak height and a reduction of its FWHM can be detected (see Figure S4d, quantification in caption, Supplementary Material), which is consistent with the presence of crystalline Al compounds. In agreement with results in literature,^{[94, 97]} all these experimental evidences point to the formation of Al hydroxides structures during the boiling treatment. And more importantly for the application, the superhydrophilic behavior of boiled laser-textured surfaces was maintained for the whole 1 year duration of the outdoor study later described in Section 2.3.

2.1.4 Surface Emissivity

As discussed in the introduction, a high surface emissivity in the atmospheric window is key for a radiative cooling-driven autonomous dew harvesting material. The infrared reflectivity spectra of untreated and laser treated surfaces as characterized by Fourier transform infrared (FTIR) microscopy are shown in Figure 2a (refer to Experimental Section (Surface Emissivity Characterization) for details). As shown in the figure, the reflectivity decreases drastically with the laser treatment over the whole wavelength range, which therefore implies an increase in the absorption and thus in the emissivity of the surfaces by virtue of Kirchhoff's law. The broadband high absorption properties of the laser-textured surfaces are also obvious, with low reflectivity values all over the whole studied wavelength range. Normal emissivity values averaged in the wavelength range of the atmospheric window (8 to 13 µm) were calculated from the reflectivity spectra and are shown in Figure 2b. As shown, the emissivity increases from 0.03 ± 0.02 for the untreated material up to 0.840 ± 0.007, 0.87 ± 0.01, and 0.92 ± 0.02 respectively for laser-textured materials under 35, 20, and 10 mm s⁻¹ scanning speed. This increase in emissivity can be explained on the basis of the above mentioned changes in roughness and oxidation, as explained in the following paragraphs.

Figure 2

Infrared properties of the studied surfaces. a) Infrared reflectivity spectra of untreated and laser-treated surfaces with and without water boiling treatment. Specific absorption bands can be detected for the boiled surfaces as indicated. b) Average emissivity in the range (8–13) µm calculated from reflectivity spectra measured at quasi-normal incidence for untreated and laser-treated materials with different scanning speed with and without water boiling treatment. Uncertainty ranges correspond to standard deviation of 3 independent measurements.

As demonstrated theoretically and experimentally by many works, adding roughness to a surface increases its emissivity.^[98-101] When the roughness features are larger than the considered wavelength, the increase of emissivity can be understood as a direct result of the increased surface area: the intrinsic emissivity of the material does not change, but there is more surface available for emission, which increases the overall amount of radiation leaving the same equivalent flat surface even taking into account partial reabsorption at internal reflections within the roughness features. Note that this is an equivalent interpretation of the increase of absorptivity due to ray trapping in surface cavities predicted by geometric optics, which would indirectly imply an increase of the emissivity by virtue of Kirchhoff's law.^{[99, 100, 102]} When roughness features are much smaller than the wavelength, these can act as a refractive index gradient between that of the air and the material, facilitating the coupling of electromagnetic waves between the two media, which therefore increases the absorptivity/emissivity of the surface.^{[99, 103]} Roughness features on the order of the wavelength imply wave effects as diffraction and can also increase the absorption and hence the emissivity by allowing resonance phenomena like excitation of surface waves.^{[99, 104]}

Therefore, for the range of wavelengths in the atmospheric window (8 to 13 µm), the increase of emissivity due to the added roughness with the laser treatment has probably contributions from the 3 regimes above mentioned. A rigorous analysis would then require numerical simulations to solve the EM field along the interface, which is out of the scope of this paper. A qualitative explanation of the observed trends is given here. First, the main periodicity due to presence of the grooves every 100 µm leads presumably to an enhanced emissivity linked to an increase of surface area, especially for the deeper grooves obtained with lower scanning speeds (see Figure 1b), which explains, at least partially, the increase of emissivity with the reduction of the scanning speed seen in Figure 2a. Second, the recast material features in the order of tens of µm found along the grooves (see Figures 1b and S1, Supplementary Material) can be linked to an increase of emissivity both through geometrical and wave effects. The increase of size of these features for lower scanning speeds (Figure S2, Supplementary Material) could presumably explain part of the observed increase of emissivity with the reduction of the scanning speed (Figure 2a). Lastly, the micron/submicron asperities covering the laser-textured surfaces (see Figure 1b) could also contribute to the increase of emissivity facilitating the EM energy coupling through the gradual refractive index effect above mentioned. Therefore, the broadband high absorption properties of the laser-textured surfaces displayed in Figure 2a are presumably the result of the irregular and multiscale nature of the topography features (shown in Figure 1b and Figure S1, Supplementary Material) which enable high absorption levels through all the measured wavelength spectrum (i.e., 2.5–4.3 µm). This is actually a typical feature observed in laser-textured surfaces,^{[63, 105-107]} which contrasts with narrow high absorption peaks arising from more regular and clean topography patterns.^[108-111]

Besides the contribution of the increased roughness, the observed increase in emissivity after the laser treatments probably also has a contribution arising from surface oxidation. The role of surface oxidation on the emissivity of metals has also been widely demonstrated both theoretically and experimentally.^[112-114] An oxide layer is itself a layer of dielectric material with optical properties different from metal ones. The interference of light rays between the air/oxide and the oxide/metal boundaries reduces the reflectivity. Considering no transmission through the metal, this implies an increase in absorptivity, and therefore, by virtue of Kirchhoff's law, of the emissivity. Results of laser-texturing experiments performed in argon atmosphere show how a reduced oxidation is correlated with a decrease in the emissivity (see results in Figure S5, Supplementary Material), demonstrating the role of oxidation in the emissivity of the laser-textured surfaces.

Furthermore, a visible effect on emissivity was also detected after the boiling treatments were performed to stabilize the hydrophilic behavior of the laser textured surfaces. As shown in Figure 2a,b, the emissivity increases consistently after the water boiling treatment for either untreated or laser-treated surfaces. The reason for this increase would be linked to the formation of Al hydroxides (as discussed in Section 2.1.3), probably through a combination of a change in the index of refraction, the growth of the net oxide/hydroxide layer thickness on top of the metal substrate and also changes in roughness at micro/submicrometric level as shown in Figure S4a (Supplementary Material).

The reflectivity spectra shown in Figure 2a can actually confirm the formation of aluminum hydroxide structures during water boiling of the surfaces. As shown, specific absorption bands appear in the spectra for the boiled samples either treated or not treated, matching the bands appearing on IR analyses of boehmite (AlOOH) structures in literature:^{[94, 97, 115-117]} bands at 3336 and 3120 cm⁻¹ correspond respectively to asymmetric and symmetric hydroxyl stretching modes; band at 1064 cm⁻¹ correspond to bending modes of H bonds in octahedral structure of boehmite OH-Al═O. The higher relative absorption at 3336 cm⁻¹ versus 3120 cm⁻¹ together with the lack of a high frequency shoulder for the peak at 1064 cm⁻¹ point specifically to the formation of pseudo-boehmite,^[115] which is in agreement with experimental findings of IR analysis of aluminum surfaces after water boiling treatments present in literature.^{[94, 97]}

The emissivity of the surfaces was also characterized during water condensation. A small but noticeable increase from the emissivity of the dry surface to that of water was measured (results included in Figure S6, Supplementary Material), in agreement with specific studies on the matter.^{[40, 41]} Besides, a slightly larger change in the emissivity with condensation was measured in the valleys of the main groove pattern in comparison to the ridges. This can be explained by considering the tendency of the water film to grow by filling of the valleys during condensation (Video S2, Supplementary Material).

2.2 Surface Performance

2.2.1 Radiative Cooling

To investigate whether the observed increases in emissivity can be employed to enable the overnight radiative cooling of the laser-textured surfaces, as required for autonomous dew harvesting, experiments were performed in a radiative cooling chamber developed by Troseille et. al.^[118] In this chamber the material faces a radiation sink, providing an available radiative deficit similar to the one found in outdoors dew condensation conditions. Under this configuration and in the absence of condensation the resulting radiative cooling of the material will depend on its emissivity (refer to Experimental Section (Radiative Cooling Properties Characterization) for further details on the setup). Figure 3 shows the temperature evolution in the radiative cooling setup for untreated and laser-treated materials. It can be seen how the laser-treated material is able to be cooled by radiation down to about 5 °C below the ambient temperature, while the cooling is barely 2 °C for the untreated material. As all environmental conditions (temperature, humidity, airflow, and available radiative deficit) were the same during the tests for both materials, it seems clear that the reason for a higher radiative cooling of the laser-treated surfaces is their much higher emissivity as shown in the previous section. Data in Figure 3 evidences how a proper surface engineering allows the transformation of a reflective metallic surface into an efficient radiation emitter.

Figure 3

Radiative cooling experiment results for both untreated and laser-treated material. Results for laser-treated material correspond to the treatment conditions selected for the outdoors experiment (after boiling, i.e. as in outdoor testing conditions) based on scalability criteria (see Section 2.3.2), the behavior being representative of all laser-textured surfaces.

2.2.2 Water Condensation

Once the radiative cooling properties of the laser-textured material were proved, its water collection properties were studied by condensation experiments. In all the experiments described in this and the next section, the materials were cooled down by conduction and not by radiation. Although the condensation dynamics do depend partly on the cooling mechanism, the following described experiments did reveal the main water collection behaviors which were also observed when performing outdoors experiments with radiative cooling (see Section 2.3.3).

The most direct result of the laser texturing treatments performed is the change of the condensation regime of the surface from dropwise to filmwise. As shown in Figure 4a, individual drops are formed during condensation on the untreated material. In contrast, for the laser treated material in Figure 4b individual drops cannot be detected as water condenses forming a continuous film along the surface due to the high hydrophilicity and capillary effects along the grooves. Video S1 (Supplementary Material) shows the qualitative macroscopic dynamics of the condensation on both surfaces (i.e., drop formation for untreated material and growth and water film extension along the treated surface). Video S2 (Supplementary Material) includes a microscopic detail of the condensation process. The common dropwise coalescence-driven growth of drops on the untreated surface is clearly visible. For the laser-treated material individual drops/wetting patches were hardly detected due to the high roughness and hydrophilicity of the surface. Instead, it can be seen how the valleys of the grooves are being rapidly filled by capillary wicking, forming a water film that covers the whole surface. This behavior was observed for all laser-treated surfaces.

Figure 4

Water condensation and collection properties of the tested surfaces. a-b) Water condensation regime for untreated and laser-treated material. c-f) Water collection dynamics for untreated and laser-treated material. g) Evolution of collected and accumulated water volumes per unit area for untreated and laser-treated material. h) Corner-end configuration, showing a small drop concentrating the water flowing down through the water film. i) Tile design.

2.2.3 Water Collection

To evaluate how this change from dropwise to filmwise condensation behavior impacts the water collection properties of the material, experiments were performed inside a climatic chamber. There, surfaces were placed in vertical position and cooled down by conduction in the back face, and exposed to a humid atmosphere. Water condensing on the surfaces and being detached was then measured with an analytical balance (details included in Experimental Section (Water Collection Properties Characterization)). Qualitatively, for the untreated material it can be observed in Figure 4c,e how the water collection events are triggered by coalescence events, by which drops become too large to be held by surface adhesion and slide through the surface by gravity, gathering further water volume while intercepting drops on its way downwards. In contrast, for laser-textured surfaces the filmwise condensation leads to the formation of a big water puddle at the bottom of the sample (see Figure 4d,f), which is released periodically once it reaches a certain volume that cannot be held by surface adhesion forces. Video S3 (Supplementary Material) shows the water collection dynamics for both surfaces.

Quantitatively, Figure 4g includes the plots of water volume per unit area collected over time representative for untreated (plotted in black) and laser-treated surfaces (plotted in green). Two differences between the water collection behaviors of both materials are evident from the graph: first, the laser-treated material starts collecting water much earlier than the untreated material and second, the collection rate for the treated material is constant whereas the collection slope varies highly for the untreated surface. Both differences were found to be direct consequences of the change in the condensed water configuration from dropwise to filmwise with the laser treatment as discussed in the following.

With respect to the collection dynamics, the stepped shape of the curve for the laser-treated material confirms the behavior observed in Video S3 (Supplementary Material), with each collection event occurring periodically at puddle instability. In contrast and as discussed, for the untreated material, the collection occurs randomly by drop sliding events. Thus, before the first collection event from the untreated surface, there is a large water volume accumulated on the surface forming drops. Once the drops become too large and start sliding, the collection starts then with a high rate (as shown in Figure 4g), as sliding drops trap others on its way down the surface. However, once the surface is depleted of big drops, the sliding events stop and the collection is slowed down (at ≈7000 s in Figure 4g) while the smaller drops remain on the surface while growing until they can slide again, repeating the cycle.

Under this scenario, what is also visible from the plot in Figure 4g is that, despite its oscillations, the untreated material does reach approximately the same amount of collected water as the treated surface when all big drops have swept down the surface (around 7000s in the graph). This points to an equal condensation rate for both surfaces, with an estimated value of (8.3 ± 0.5) 10⁻⁶ ml cm⁻² s⁻¹ as averaged from the collected mass data for all the laser-treated materials trials performed in this work. This value was confirmed by measurements using a conductive flux sensor (refer to Experimental Section (Water Collection Properties Characterization)). Being both surfaces studied in the same environment and at the same temperature under conducive cooling, differences in condensation rate could only arise from either nucleation barrier differences depending on surface wettability or differences in the heat conduction resistance of the liquid phase which might vary between the drop pattern and the film. The nucleation barrier difference is only truly present when the surfaces are dry and still the relatively high supercooling degree employed (9.2 °C below dew point) overrides this effect in this case.^[22] Regarding the liquid thermal resistance, it has been proven to be negligible with respect to the diffusion resistance of water vapor through the concentration boundary layer, which dominates heat transfer in the presence of non-condensable gases (i.e, when humid air is used as in this experiment in comparison to a pure water vapor environment).^{[15, 18, 119]} Then the equal condensation rate for both surfaces seems to be a reasonable hypothesis which can also explain the observed collection dynamics.

The evolution of the accumulated water over time on both surfaces is also plotted in Figure 4g. Accumulated water was calculated by the difference between condensed and collected water for each surface, considering equal water condensation dynamics in both surfaces (as discussed in the previous paragraph and as depicted with a dashed straight line in Figure 4g). As shown, the time-averaged amount of water accumulated is clearly smaller for the laser-treated material (plotted in yellow), averaging (4.3 ± 0.5) 10⁻³ mL cm⁻² with an oscillation of amplitude constrained within the puddle volumes before and after each release event. In contrast, the cycle of drop growth/sliding for the untreated material implies larger time-averaged amounts of accumulated water (plotted in grey), roughly about (13 ± 3) 10⁻³ mL cm⁻² (i.e., about three times more than the treated material), despite the collected volumes can be momentarily equal to those of the treated surface. This means that, although a relatively large water volume is accumulated at the bottom puddle of the treated samples, the fact that water forms only a thin film across the rest of the surface reduces the average amount of water stored, in comparison to the scattered growing drops pattern for the untreated material. This basically indicates that the filmwise condensation regime of the laser textured surfaces enables the concentration of the condensed water in a smaller region of the surfaces, favoring a more efficient gravity driven collection for the same amount of water condensed in the sample.

The efficient accumulation of water at the bottom part of the surface with the laser-treatment explains also the faster initial water collection in comparison to the untreated material. The average time for the first collection event, detected on average at (3010 ± 490) s for the untreated material, is reduced up to 3 times with the laser treatments performed in this work. From the perspective of the dew harvesting application, an early and efficient collection of the condensed water is crucial, as the time window for water collection is limited to the duration of the night, with water evaporating and sunrise.

These improvements would not only be a consequence of the superhydrophilic nature of the textured surfaces, but also due to the presence of grooves themselves and the capillarity effects they induce. Several works in the literature highlight the efficiency of grooves to evacuate the condensed water by forcing the liquid to flow downwards by capillary wicking.^{[36, 44, 68-70, 83]} Our observations point to a reduction of the film thickness due to the capillary suction along the grooves in comparison to a filmwise condensation situation on a flat surface: first, hydrophilic (after water boiling) but no textured surfaces were also tested, displaying filmwise-like condensation dynamics which improve the dropwise performance of the bare aluminum surface but do not reach the best performance levels of the laser-textured surfaces (see results in Figure S7, Supplementary Material); and secondly, estimations of the average water film thickness across the laser-treated surfaces from the water collection data return values of (20 ± 2) µm, (27 ± 3) µm and (42 ± 5) µm respectively for laser-textured materials under 35, 20 and 10 mm s⁻¹ scanning speed (see details in Experimental Section (Water Collection Properties Characterization)). In comparison, the average film thickness for a 5 cm long flat vertical plate under filmwise condensation estimated from the Nusselt model^[120] is ≈66 µm, with the difference being more significant when considering longer surfaces (see calculations in Experimental Section (Water Collection Properties Characterization)).

In fact, computing the average groove depth from profilometry data (i.e., the mean value of the oscillating periodic profile, see Figure 1b) gives (11 ± 1) µm, (22 ± 2) µm and (44 ± 4) µm for 35, 20, and 10 mm s⁻¹ scanning speeds respectively, values which are close to the experimental estimations of the film thickness. This points to a water film thickness upper bound by the groove size. Film thickness limitation by groove patterns in condensation surfaces has been extensively studied as an strategy to improve the heat transfer performance.^[121-123] Along a curved groove profile, the liquid film is evacuated laterally from the ridges towards valleys due to Laplace pressure differences.^[124] Besides, in general terms, the presence of water above micrometric-sized grooves or posts invaded by water in wicking surfaces is not an energetically favorable configuration.^{[83, 86, 125]} Once in the valley, the liquid is forced to flow along the grooves by capillary suction, aided by gravity in our experiments.

The water collection behavior of laser-textured surfaces processed with the different scanning speeds was also studied. All laser-treated surfaces displayed the film + bottom puddle configuration above described, with similar stepped water collection plots (results are shown in Figure S7, Supplementary Material). No significant differences were found in the collection slopes, reinforcing the idea of equal water condensation rate for all tested surfaces. However, differences in the time elapsed for first collection event were detected (see inset in Figure S7, Supplementary Material), being longer for the lowest scanning speed (first collection event detected on average at (1080 ±280) s, (1010 ±340) s and (1810 ±470) s respectively for laser-textured materials under 35, 20 and 10 mm s⁻¹ scanning speed). The increase of groove depth with lower scanning speeds (Figure 1b) can explain the increased delay for the first collection event due to 2 reasons. First, for the puddle to be formed at the bottom, grooves need to be filled by water, which takes logically a longer time for deeper grooves.^[44] And second, the time required for the puddle to be released is higher for deeper grooves as the capillary forces holding up the puddle are also larger, as demonstrated by wicking characterization experiments (see Figure S3b, Supplementary Material). Furthermore, given the equal and constant condensation and collection rates for all laser-treated surfaces, a reduction in the initial delay for collection implies directly a lower amount of water permanently accumulated in the surface, an of course interesting property to pursue for an efficient dew harvesting system.

An extra improvement in the collection performance of the superhydrophilic laser-treated surfaces was achieved by exploring further the idea of concentrating the condensed water in a smaller region of the surface. Examples in nature^[126] and in relevant experimental studies^[35] demonstrate how an adequate geometry facilitates the gravity-driven water drainage at the bottom part of wetted surfaces. Following these ideas, the water retention levels of surfaces with different combinations of simple surface geometries (flat end and corner end) and groove orientations (vertical, horizontal, and grid) were studied (see details in Experimental Section (Water Collection Properties Characterization)). Results (included in Figure S8a, Supplementary Material) reveal the lowest water retention achieved with the combination of vertical grooves with a corner-end configuration, shown in Figure 4h. The corner end facilitates the concentration of the puddle volume in a smaller surface, allowing the gravity to overcome the capillary forces more easily. Besides and as previously discussed, in comparison to other orientations, the water evacuation efficiency of vertical grooves^{[36, 44, 68, 69]} or channels^{[34, 119, 127]} has been previously documented, in line with the results observed in this work. Water collection results (Figure S8b) reveal again a similar collection rate for flat and corner end surfaces with vertical grooves. However, the collection starts earlier for the corner end surface (initial delay of (680 ±70) s versus (1010 ±340) s for the corresponding flat end surface), as a result of the puddle concentration capability of the corner geometry. This implies an extra reduction in the amount of water stored in the surface by reduction of the puddle size. Besides, a very consistent and repeatable collection behavior was observed for the corned ended samples, which encouraged the use of this geometry shape for the development of the outdoors dew harvesting system, as discussed in the next section.

2.3 Dew Harvesting System

2.3.1 Condensation Tiles

Observing the efficient concentration and collection of the condensed water achievable with the laser-textured surfaces and in order to exploit it further, the geometry shown in Figure 4i is proposed: a rectangular tile with a corner end, in which all the water condensed across the whole surface would flow downwards within the film and could be efficiently collected from the small puddle formed at the bottom corner.

The design was tested with a small scale model (w = 50 mm, L = 152.5 mm, α = 90° following nomenclature in Figure 4i) under conductive cooling conditions. Results shown in Video S4 (Supplementary Material) show how water condensed on the whole tile surface is successfully recovered from the small bottom puddle that detaches easily from the tile corner, validating the proposed design.

2.3.2 Scalability Considerations

The results presented in Section 2.1 revealed several trends in the fundamental surface properties (i.e., topography and chemistry) when varying the scanning speed of the laser treatments. As explained and as a result of these changes, higher emissivity but also higher amount of retained water were observed for lower scanning speed. Initially, this points to a trade-off between the optimization of both performance factors for a dew harvesting system with respect to the scanning speed of the treatments.

However, when considering a real scale system (in the order of tens to hundreds of squared meters^{[9, 128]}) it is also important to evaluate the time required to manufacture the condensation surfaces. With the current laser processing parameters employed, processing 1 m² would require between 79 and 278 h depending on the laser scanning speed employed.

Therefore, in order to reduce the processing time, a second laser setup was employed (as described in Experimental Section (Laser Processing)), with similar characteristics (wavelength, pulse length, pulse frequency) but with a higher available power (80.7 W vs 6.1 W). This allowed the increase of the scanning speed (100 mm s⁻¹ vs 10–35 mm s⁻¹) while obtaining a similar grooved surface. A larger laser spot was also employed with the second setup, which eases the generation of wider grooves and therefore the increase of the hatching distance required to cover completely the surface (240 µm vs 100 µm). Despite these changes, the laser processing conditions with the second setup are still comparable to those of the first setup in terms of the accumulated energy per unit area (refer to calculations in Experimental Section (Laser Processing), giving 288.8 J cm⁻² for the second setup versus 115.6–404.5 J cm⁻² for the first setup depending on the scanning speed employed). With the new set of laser processing parameters the processing time for a 1 m² surface is significantly reduced to less than 12 hours.

The properties and performance of the surfaces textured with the new set of laser processing parameters were evaluated. Results are displayed in Figure S9 (Supplementary Material). In general, the surfaces exhibit very similar properties to the ones described in Section 2.1: grooved surfaces with superhydrophilic behavior and high broadband infrared emissivity of 0.95 ±0.01 after water boiling, the highest value obtained in this work. Similarly to all laser-treated surfaces studied in this work, the new surfaces show filmwise condensation, with constant and stepped water collection dynamics linked to periodic puddle release events. A condensation rate of (8.7 ± 0.5) 10⁻⁶ mL cm⁻² s⁻¹ was measured, essentially the same as with the other surfaces. The delay for the first water collection with these surfaces was measured to be (1690 ± 20) s. In general terms, the surfaces textured with the new set of laser processing parameters perform similar to the surfaces processed with 10 mm s⁻¹ described in the previous sections, but with the advantage of a huge 24-fold reduction in the processing time.

As discussed in the introduction, the robustness of the condensation surfaces under outdoor environments is of crucial importance for any dew harvesting system.^[42-44] In this work, the aging of the surfaces was evaluated by exposing them to the same outdoor environment as the one surrounding the actual dew harvesting setup described in the next section. As shown in Figure S9 (Supplementary Material), after 4 months of outdoor aging no significant changes were observed in the surface properties. Particularly, wettability, emissivity, and water collection properties were maintained after ambient exposure.

Therefore, given the similar performance to the other treatments, its demonstrated stability and robustness plus the great reduction in the processing time, the surface treatment achieved with the new set of laser parameters was selected for the real scale dew harvesting system described in the next section.

2.3.3 Outdoors Performance of Real Scale Tiles

Once the tile design was validated and the treatment conditions adapted for scaling, large scale tiles were manufactured in order to test the dew harvesting performance of laser-textured surfaces in an outdoors setup. Tiles of dimensions w = 20 cm, L = 54.7 cm, α = 90° following nomenclature in Figure 4i were laser-treated with the conditions selected in the previous section and mounted in 2 rows covering a 1 m² condenser facing the sky with 30° inclination with respect to the horizontal as depicted in Figure 5a. In this design, water condensing in a filmwise regime and released from the top row is collected in the bottom row, as illustrated by Figure 5b. Atmospheric variables, tiles temperature, and the collected water volume were registered for 1 year from 22/07/2023 to 22/07/2024. A total of 66 dew events were detected during this period. Further details on the setup and measurements are included in Experimental Section (Outdoors Dew Harvesting Experiments).

Figure 5

a) Developed dew harvesting system based on laser-textured tiles. b) Schematic representing water collection dynamics in the system. c) Evolution of air temperature, dew point temperature, surface temperature and water collection values for a representative dewy night (22-23/10/2023). d) Daily dew yield data though the year studied for tiles and painted condensers.

Figure 5c shows data for a dewy night representative of the observations made throughout the testing year. In particular, the temperature evolution of the air (grey), dew point (black), and tiles (purple) are plotted. It can be seen how after sunset the temperature of the tiles goes below the dew point temperature thanks to the radiative cooling properties of the surface, starting dew condensation. After some time, collection of dew water begins, plotted in green. As shown, the water collection displays a constant rate, linked to a steady state condensation from an overall constant temperature difference between the surface and the dew point as shown in the graph. By the end of the night, the temperature of tiles goes up with the increase in the air temperature and collection stops definitely at sunrise. The delay observed between the time at which the temperature of the tiles goes below dew point, marking the start of the condensation, and the start of water collection corresponds to the time required to fill the grooves with water and form the puddles at the tip of each tile so that further water condensed can be collected. To serve as a comparison, water collection data from a second condenser located right next to the one with tiles was analyzed. This second condenser (shown in Figure S10a, Supplementary Material) consists of a flat surface coated with high infrared emissivity paint (0.977 ± 0.005 in the wavelength range from 8 to 13 µm) and hydrophilic wetting properties (average contact angle of θ = (53 ± 5)°). Water collected with this painted condenser is plotted in orange in Figure 5c. As shown, the painted condenser also displays a more or less constant collection behavior with a similar rate to the tiles condenser. However, the collection starts later for the painted condenser, ending also with sunrise, which leads to a lower total collected water volume at the end of the night. Both the similar collection rate for both condensers and the earlier collection start for the tiles condenser was confirmed as general trends after analysing the year data (see graphs corresponding to other representative dewy nights in Figure S11, Supplementary Material). As explained in the following paragraphs, the reason for the better performance of the tiles condenser was found to be related to the efficient drainage of water from the filmwise condensation laser-textured grooved surfaces.

The surface temperature evolution of both condensers was found to be consistently similar (Figure S11d, Supplementary Material). This is not surprising considering that, apart from starting with similar emissivity values when the surface is dry, once water is condensed, the emissivity of both surfaces will tend towards the water emissivity^{[40, 41]} and therefore the radiative cooling degree of both surfaces will be similar in steady state. As both condensers are also exposed to the same environment, it is reasonable then to consider equal water condensation rates in both surfaces. Therefore, the consistently later start of water collection for the paint condenser implies a larger collection delay since both condenser surfaces reach the dew point temperature at the same time. Then, under the assumption of similar condensation rates this necessarily implies a larger steady state volume of water accumulated in the painted surface which is not collected.

As mentioned before, the reason for this difference was found in the water condensation dynamics. In the tiles, water condenses in a filmwise manner thanks to the laser-textured superhydrophilic grooves as shown previously. However, the painted surface exhibits dropwise condensation, with drops scattered all over the surface (see Figure S10b, Supplementary Material). Water from a dropwise condensation surface can only be collected when the drops are large enough to start sliding above a critical size.^[129] Precisely, this was observed for the untreated metallic surface in the laboratory experiments (Figure 4e,g), which also led then to a longer collection delay in comparison to the laser-treated surface.

However, in contrast to the trends observed in the laboratory experiments where the dropwise condensation surface did reach collection levels similar to those of the filmwise surface after drops start sliding (Figure 4g), in the outdoors experiment the dropwise condensation surface exhibits consistent lower water collection values which never reach those of the filmwise condensation in the tiles (Figure 5c, Figure S11, Supplementary Material). The reason for this difference is likely a much larger surface size for the outdoors condenser: as shown in Figure 4e, due to the relative size between the drops and the untreated surface in the lab experiments, just a few sliding events can completely wipe all the water from it. In contrast, this is no longer true for the 1 m² surface of the condensers. There, drop sliding events can occur at some points of the surface while drops are growing unaltered in other locations simultaneously, leading overall to a constant collection rate as observed in the data (Figure 5c, Figure S11, Supplementary Material).

Therefore, it is clear that the condensate dynamics on the grooved filmwise condensation laser-textured surfaces reduce the amount of water stored before the start of collection in comparison to the dropwise counterparts. As shown in Figure 5d, which includes the total amount of dew water collected per day for both condensers through the analyzed year, the reduction in the accumulated water leads to a consistently better performance of the tiles condenser. For the registered dew events above 0.05 mm per night, an estimated average of (0.036 ± 0.009) mm extra water per night was collected from the tiles condenser thanks to the reduction of the amount of water stored in the sample, which implies about 70% more water collected on average per night, a ratio which is maintained when considering the total dew yield for the analyzed year: 3.55 mm for the tiles condenser versus 2.07 mm for the painted condenser. Besides, the results show the durability of laser-textured surfaces under outdoor exposure, maintaining a robust performance over the 1 year-long study. No residue build-up on the grooved surfaces was detected during the duration of the study, with water flow remaining unaltered. As well, no signs of metal contamination were detected when analyzing water condensed on laser-textured surfaces (see Experimental Section (Analysis of Collected Water)).

Regarding the role of environmental variables on the performance of the dew harvesting system, most of the detected dew events occurred with high relative humidity (82% of events with humidity above 85%), with a positive correlation between dew yield and relative humidity (see Figure S12a, Supplementary Material), both logical results considering easier nucleation for higher supersaturation degree. Regarding wind effects, the majority of nights with a significant degree of radiative cooling of the surfaces (at least 3 °C below the air temperature, a threshold surpassed during most of the significant dew events observed) happened for wind speeds below 1.5 m s⁻¹, with higher speeds clearly diminishing the temperature difference with the air due to convective heating (see Figure S12b, Supplementary Material). This trend was found to hold for all wind directions, with no significant differences detected among them. For further discussions on the effects of the environmental variables on dew yield, the reader is referred to published literature.^[130-134]

Considering all registered dew events (Figure 5d), an average of 0.055 mm of water per night was collected with the tiles condenser, with a maximum of 0.263 mm and typically between 0.1 and 0.2 mm during nights with favorable environment conditions. These values lie within those typically reported for planar condensers in the state-of-the-art literature^{[9, 128, 135]} (refer to Table 4 in ref. [128] for comparable data). This is indeed a promising result taking into account that, on the one hand and in contrast to the majority of works in literature, the condensers in this work were placed in an urban environment, where radiative cooling is limited by a suboptimal sky view degree due to the presence of near buildings^{[136, 137]} and with overall higher air temperature and lower humidity in comparison to rural areas,^[138-140] both factors highly hindering dew condensation^{[9, 128]} (about 2 to 8 times less daily yield than in countryside locations according to comparative studies^[138-140]). And on the other hand, the performance of the developed condenser is mainly solely based on the engineering of the properties of the condensation surfaces, while it still matches the dew yield of systems on which the surfaces are boosted with architectures specially designed to enhance the radiative cooling rate,^[135] not implemented here.

Furthermore, the observed improvements in dew yield linked to lower levels of accumulated water for the filmwise grooved surface in comparison to the dropwise condenser (≈0.036 mm) become interesting when considering that an important fraction of the condensed water in dew harvesting systems is typically accumulated as pinned drops, which are lost by evaporation at sunrise. Pinned water amounts lie roughly within 0.01 to 0.1 mm and typically above 0.04 mm.^{[130, 133, 141, 142]} As demonstrated here, the approach of filmwise condensation in grooved surfaces limiting the film thickness is an interesting strategy for an efficient collection of the condensed water.

3 Summary and Outlook

The results presented in this paper demonstrate the feasibility of exploiting the properties of the laser-micropatterned metallic surfaces for dew harvesting purposes. With a single processing step, the surfaces achieve simultaneously high broadband infrared emissivity (up to 0.95 in the atmospheric window) and superhydrophilicity (contact angle of 0°), key properties enabling radiative cooling and filmwise dynamics respectively, and both related to the groove pattern formation during laser texturing. The filmwise grooved laser-textured surfaces display 3 times lower water retention levels than the dropwise counterparts thanks to the limitation of the water film thickness provided by the grooves. As well, radiative cooling with metallic surfaces was proven to be feasible thanks to a drastic broadband emissivity enhancement with micropatterning. As demonstrated here, results from laboratory experiments can be scaled up to build a real scale autonomous dew condenser with robust and stable surface properties under long-term outdoor exposure. A 70% improvement in the collected dew water was measured by comparing the developed condenser with a state-of-the-art paint-coated dropwise condenser.

From this first prototype, additional improvements can be already envisaged. First, a further optimization of the laser treatment parameters seeking minimum water retention levels on the surface by controlling the topography patterns' shape, size, and distribution without compromising emissivity or the manufacturing time would be interesting. Also, the exploration of more complex condenser geometries would also be beneficial to isolate the surfaces from wind heating effects and improve the sky view factor while reducing the exposure to atmosphere reirradiations.^{[9, 128, 135, 143]} Besides those improvements, a scalation of the system to a commercial level will require also an economic analysis considering the costs of the material and its fabrication.

In conclusion, the present work demonstrates the uniqueness of the synergy between the properties of the laser micropatterned metallic surfaces and the requirements of a radiative cooling-driven dew condensation application in terms of emissivity and wettability, deserving further research effort. Also, the dew water collection efficiency of filmwise condensation in grooved surfaces seems clear, a strategy with a promising extrapolation to further materials and manufacturing techniques.

4 Experimental Section

Material

The 6061 aluminum alloy (AMAG rolling GmbH, compositional analysis shown in Table 1) was employed in this study as a 1.6 mm thick plate in T6 temper. Samples were cut by mechanical methods to the size required by each experimental procedure and cleaned with acetone and ethanol.

Table 1. Composition of the employed 6061 aluminum alloy.

Element	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Others each	Others total	Al
Wt. %	0.74	0.45	0.21	0.13	0.87	0.16	0.05	0.06	0.02	0.04	Balance

Laser Processing

Two different laser processing equipments were used to prepare the laser-treated surfaces. The first laser setup based on a Nd:YVO₄ diode end-pumped source (Rofin) was employed for assessing the main differences in surface properties and condensation behavior between untreated and treated materials. A second laser setup with a higher power Nd-doped fiber source (Raycus), was employed to scale up the processing to the dimensions required for the real outdoors dew harvesting setup developed in this work (shown in Figure 5a and further described in Experimental Section (Outdoors Dew Harvesting Experiments)). Both sources emit 1064 nm nanosecond pulsed laser radiation and were used with a similar optical system consisting of galvo mirrors and F-Theta scan lens. Surface treatments were based on the scanning of the focused laser bean through the surface following parallel scanning lines at constant speed. To study effects of topography dimensions in the surface condensation performance, scanning speed was varied for the Nd:YVO₄ laser source setup. Also, the effects of oxidation were evaluated by performing experiments in air and in argon, the latter being supplied with two gas nozzles of 2 × 40 mm² cross section located on each side of the processing area for a total gas rate of 90 L min⁻¹. Specific details on laser beam, optical systems, and employed processing conditions are included in Table 2. Overall, processing parameters were selected targeting superhydrophilic surfaces with regular grooves of different depths, which was done on the basis of previous research.^{[56, 88, 91, 94, 144]}

Table 2. Laser beam and optical system properties as well as processing parameters employed during the texturing experiments.

Laser beam and optics properties
Laser source	Wavelength [nm]	Pulse length FWHM [s]	M^2	F-theta lens effective focal Length [mm]
Nd:YVO4	1064	20 × 10^−9	<1.3	160
Fiber	1064	110 × 10^−9	< 1.5	254

Processing conditions
Laser source	Power [W]	Spot size 1/e^2 [µm]	Frequency [Hz]	Pulse energy density [J cm^−2]	Scanning speed [mm s^−1]	Accumulated En. Dens. [J cm^−2]	Hatching distance [µm]
Nd:YVO4	6.1	163	2 × 10^4	1.5	10, 20, 35	404.5, 202.2, 115.6	100
Fiber	80.7	302	2 × 10^4	5.6	100	288.8	240

The calculation of the accumulated energy per unit area delivered to the material was performed considering the variation in the overlap between a surface area equal in size to the laser spot and each one of the laser pulses arriving as the spot moves along the scanning line, as explained on the following paragraphs. Two simplifications were made for this calculation: first, the power density distribution in the beam cross-section was considered to be uniform, while it is actually Gaussian; secondly, overlapping effects between parallel scanning lines were not considered, as they affect relatively a small area fraction at the sides of the track width where the power density is small.

Then, is easy to show that the area fraction of the laser beam with diameter ϕ_SPOT that overlaps with a circular material area of the same diameter with a distance d between centers can be expressed as (see calculations in Supplementary Material, section H.1):

$$$$\begin{eqnarray} OF\;\left( d \right) &=& \frac{{Overlapp\;Area\;\left( d \right)}}{{Spot\;Area}}\nonumber\\ &&= \frac{2}{{\pi \;{\phi _{SPOT}}}}\left( {arcos\left( {\frac{d}{{{\phi _{SPOT}}}}} \right){\phi _{SPOT}} - d\sqrt {1 - {{\left( {\frac{d}{{{\phi _{SPOT}}}}} \right)}^2}} } \right)\end{eqnarray}$$$$(1)

Adding up the contributions from all the pulses overlapping with the considered circular material area as d varies leads to a net number of pulses per spot N_SPOT,NET (see calculations in supplementary material, section H.2):

$$$$\begin{equation} {N_{SPOT,NET}} = \frac{8}{{3\pi }}\cdot\frac{{f{\phi _{SPOT}}}}{v}\end{equation}$$$$(2)

Then, the accumulated energy per unit area E_SUP,CUM follows directly from the individual pulse energy density E_SUP as: E_SUP,CUM = E_SUP · N_SPOT,NET, which allows the comparison of irradiation conditions under different combinations of laser power, spot size, pulse frequency, and scanning speed values and was used to calculate the values shown in Table 2.

Boiling Post-Processing

As discussed in Section 2.1.3, in order to avoid the commonly observed hydrophilic to hydrophobic wettability transition of laser-textured metals, a boiling post-processing treatment was applied following previous research.^{[18, 94]} Then, after laser processing, samples were immersed in boiling Milli-Q distilled water for 2 h and air-dried afterwards.

Surface Topography Characterization

Qualitative topography analyses were performed by scanning electron microscopy (SEM, JSM 6010 LA) in secondary electrons (SE) mode.

Quantification of the area roughness parameters was performed by white light interferometry optical profilometry (Profilm 3D, Filmetrics) following definitions in ISO 25 178. Particularly, the arithmetic mean height of the surface S_a and the ratio of increment in the interfacial area to the area of a perfectly flat surface S_dr were analyzed (note that S_dr is 0% for a perfectly flat surface, so the area increase ratio for a rough surface can be computed as 1 + S_dr/100).

Profiles in the direction parallel to the grooves were extracted from the profilometry scans. For each condition, joint data from 6 independent profiles with individual length of 390 and 0.707 µm resolution was analyzed. The spatial periodicity of the distributions was evaluated by applying the Fast Fourier Transform (FFT) algorithm to the profile data. Being $$\widehat {x( f )}$$ the computed complex amplitude for the spatial frequency f, the power spectral density distribution was calculated as $$PSD\;( f ) = \frac{{2{{| {\widehat {x( f )}}| }^2}}}{{{f_s}N}}$$, where f_s is the spatial sampling frequency (1.417 points µm⁻¹ for the employed resolution) and N the total number of points. The cumulative power distribution was calculated by trapezoidal integration of PSD (f) for the frequency range corresponding to the spatial period range of [1.41, 60] µm. Obtained values were relativized to the total signal power in the range to allow the comparison between profiles obtained with different scanning speeds, which lead to different amplitude values and therefore different total signal power.

Surface Chemistry Characterization

Detailed surface chemistry characterization was obtained by X-ray photoelectron spectroscopy (XPS, Thermofisher NEXSA spectrometer). Aluminum Kα x-ray radiation (1486.6 eV) focused in a spot of 400 µm was employed to stimulate the photoelectron emission. A flood gun was used to minimize surface charging. General assessment of the elemental composition was based on survey spectra obtained with a 100 eV pass energy. Before those, a sputter cleaning step of the surfaces with Ar ions (1000 eV, 2 × 1 mm² spot, 60 s) was applied to reduce the contributions from adventitious carbon contamination and improve the signal to noise ratio. For the elements of interest, the chemical state was assessed by peak deconvolution from high resolution spectra at 30 eV pass energy. These were obtained prior to the sputter cleaning step to avoid reduction of species by Ar ions. Adventitious carbon C1s peak at 284.8 eV was employed as binding energy reference.

Energy-dispersive X-ray spectroscopy (EDS) was performed with the SEM equipment to provide a complementary surface composition analysis and study compositional changes in deeper layers of the material. Mean and standard deviation of 3 independent measurements are reported for each sample.

Surface Wettability Characterization

Surface wettability was evaluated by sessile drop water contact angle measurements using a goniometer (Kruss DSA25) equipped with an automatic liquid dose system. Distilled water drops of 2 µL volume were gently deposited on the surface of the samples, following procedures described in the European standard EN 828:2013. Mean and standard deviation of 3 independent measurements are reported for each sample.

The wicking properties of the laser-textured surfaces were evaluated by analyzing the dynamics of the capillary rise of water along the surfaces following procedures in literature.^{[79, 80]} 5 µL water drops were put in contact with the bottom side of vertically placed textured surfaces with vertical grooves. The capillary-driven rise of the water was recorded with a high speed camera (Photron) at 250 fps.

Surface Emissivity Characterization

Surface emissivity was quantified indirectly by means of reflectivity measurements, assuming absorptivity and emissivity are equal (Kirchoff's law). Thus, medium wave infrared reflectivity was characterized by an FTIR microscopy imaging system (based on a Perkin Elmer Spectrum 3 FTIR spectrometer). A reference gold surface was used to perform the background prior measurements. Spectra were acquired at quasi-normal incidence angle between 4000 and 700 cm⁻¹ (i.e., 2.5–14.3 µm), with a resolution of 8 cm⁻¹. For each analyzed sample, a total area of 350 × 450 µm² was analyzed, divided into 63 sub-areas of 50 µm. 15 scans at a speed of 1 cm s⁻¹ were performed in each sub-area to generate its individual spectrum. Reported spectra are the average of the 63 spectra for each sample. Transmittance was checked to be zero over the studied spectral range.

Emissivity was also measured during water condensation on the surfaces. A temperature controlled-stage (Pike Technologies, S-100R) was placed on the FTIR microscopy imaging system, cooling the studied surfaces to 4 °C. To initiate condensation, humid air at a temperature of 20 °C and 90% relative humidity (dew point at 16 °C) was injected into the stage chamber through two inlet gas nozzles, and the chamber was sealed by a Plexiglas cover. Humid air was obtained by bubbling air in a bath of ultrapure water.

Complementary surface emissivity characterization was performed on the basis of infrared thermography. An IR Camera (FLIR A655sc) was employed to register the infrared emission (7.5 to 14 µm) of surfaces previously back heated with a heat gun to temperatures between 40 and 60 °C. Emissivity was estimated then with an infrared image processing software (FLIR ResearchIR) on the basis of the temperature of the surface, which was measured either via a K-type thermocouple sticked to the surface with thermal conductive paste and aluminum tape or directly with the thermal camera over an area of the measured sample covered by insulating black tape of known 0.96 infrared emissivity (Scotch Super 88). Temperature measurements by both methods led to matching results. Correction of environment reflections was performed by first measuring the apparent reflected temperature from a diffuse reflector (rough aluminum film). A total area of 10 × 10 mm² was analyzed for each surface condition. The accuracy of this method was considered lower than the one obtained via the FTIR equipment, so it was only employed for preliminary assessment of surface emissivity and as a first check for emissivity changes during aging studies of the surfaces.

Radiative Cooling Properties Characterization

The radiative cooling properties of the surfaces were evaluated with the experimental setup developed by Trosseille et. Al.^[118] Briefly, the samples are placed inside a climatic chamber maintaining a constant 30 °C temperature and 20% relative humidity (water condensation was prevented in these dry conditions, to focus only on the cooling capacities of the studied surfaces). Samples are placed on top of an insulating foam block. Radiative cooling is achieved by placing a 15 × 15 × 15 cm³ box filled with 1 kg of solid carbon dioxide (Ø1 cm × 3–5 cm bars), which exchanges radiation with the studied sample through aluminum coated surfaces acting as infrared mirrors with very low emissivity. In this configuration, the radiation emitted from the sample is absorbed by the solid carbon dioxide, which barely emits radiation due to its low temperature. Thus, the radiative cooling of the sample is enabled. A combined heat flux sensor placed next to the sample is used to monitor radiative and total heat flux, including convection (heat conduction was neglected as both the sample and the sensor are placed on top of the insulating foam block). An available radiative deficit of 67 W m⁻² was employed during the experiments as measured by these sensors. This value matches well the observed radiative deficit values in outdoors dew condensation conditions, within 25–100 W m.^{−2[145, 146]} The convective heating flux of the surface from the surrounding air was 25.5 W m⁻² as measured by the heat flux sensors. A thermocouple placed in the back side of the surface with conductive paste was used to monitor the sample temperature over time, providing the data displayed in Figure 3. Figure 6 includes a schematic of the experimental setup.

Figure 6

Schematic of the radiative cooling setup. Besides the elements mentioned in the text and as shown, the radiation exchange was performed through an air flow channel limited by IR-transparent windows. The channel provides thermal insulation between the surface and the radiation sink compartments. Also. the air flow avoids excessive cooling of the windows, preventing water condensation on their surfaces and the subsequent loss of their required IR transparency.

Qualitative Condensation Behavior Characterization

The condensation dynamics on untreated and laser-treated surfaces were first qualitatively analyzed by observations under an optical microscope (Leica). A Peltier element was employed to cool down the surfaces down to 6.6 °C under 20 °C ambient temperature and 50% relative humidity (dew point at 9.3 °C). The process was monitored by optical cameras (The Imaging Source) periodically registering images from the surface under study at different magnifications.

Water Collection Properties Characterization

Water collection experiments were performed inside a climatic chamber (Weiss WKL 100), maintaining a constant 30 °C temperature and a relative humidity of 70%. A vertical 173 × 173 × 5 mm³ steel plate back-cooled by a Peltier element (Laird Technologies) acted as a cold surface, kept at 14.7 °C, as checked by a K-type thermocouple mounted on the surface, well below the dew point at the working conditions (23.9 °C). About 50 × 50 × 1.6 mm³ aluminum samples were stuck to the cold steel plate with conductive paste to ensure thermal contact. Water condensing on the surfaces over time was collected by an absorbent tissue located below, which was itself placed on top of an analytical balance (Denver Instruments), which therefore measures the evolution of collected water mass over time. Care was put to prevent any water condensing in cold surfaces distinct from the working one reaching the balance's tissue by placing more absorbent tissues surrounding the sample under study. Evaporation of water from the balance's tissue between collection events was evident from raw mass versus time plots, which were corrected after the experiments to evaluate the true amount of collected water. The process was monitored by optical cameras (The Imaging Source) periodically registering images from the surface under study.

The condensation rates estimated with the mass data from the balance were verified by heat flux measurements. A (30 × 30 mm²) conduction sensor was placed between a testing surface of the same size and the cold back plate, all of them in tight contact through thermal paste. In this configuration, the sensor measures the conduction heat flux evacuated at the back surface of the sample which equates, under steady state conditions, to the convective and condensation latent heat flux coming to the surface from the air. By subtracting the measured flux in experiments with humidity (convection + condensation) and without humidity (convection only) the condensation latent heat flux and hence the condensation rate could be estimated, matching the values obtained from the mass data.

Film thickness on laser-textured surfaces was estimated on the basis of the condensation rate and the time elapsed from the beginning of the experiment to the first formation of a puddle at the bottom part of the surfaces, the moment at which all the surface is already covered by the water film (film spreading across the surface was also evident from the captured images due to reflectivity variations. Also, the instant of full film coverage matched that of puddle appearance). Alternatively, film thickness was estimated from the plots of accumulated water volume over time for the small tiles (depicted in Figure 4i), which were selected for this estimation for 2 main reasons. First, the large surface size (70 cm² in comparison to the 25 cm² square samples) and the smaller puddle volume with the bottom corner geometry reduce the error introduced when calculating the film thickness from the accumulated water values: there is always some water stored as a remanent puddle even after each release event, an amount which is then averaged across the surface and hence leads to an overestimation of the actual average film thickness (this overestimation was considered to be too high for the 25 cm² square samples due to a larger puddle and smaller sample size). And second the longer length of the tiles can prove further how the grooves limit the growth of the film thickness in comparison to the Nusselt estimations for a flat filmwise condensation surface. For a 50 mm wide, 170 mm long tile ending in a 45° corner and textured with an average groove depth of (22 ± 2) µm (corresponding to the laser treatment at 20 mm s⁻¹), the estimated film thickness is (30 ± 2) µm, which is fairly close to the (27 ± 3) µm estimation from the camera images for the square sample with the same groove depth. In comparison, the average film thickness from the Nusselt model for a surface of 170 mm length is ≈90 µm.

The puddle release behavior from different combinations of surface geometry and groove orientations for laser-treated samples was first tested in ambient conditions (i.e., outside the chamber): 20 × 20 × 1.6 mm³ samples were mounted at a slope of 45°. 10 µL drops were then sequentially added at the top part of the samples with a pipette (water was gently added at different locations along the width of the sample to ensure proper water distribution). During this process and similarly to what was observed inside the climatic chamber, water flows down by gravity as a thin film and accumulates at the bottom part forming a puddle. The number of drops added before a puddle release event was registered, as well as the amount of water released in each event (measured with an analytical balance (Nahita)), which allowed the estimation of the water retention levels for each sample configuration and preselection of the best surfaces to further testing inside the climatic chamber.

Aging Tests

To study the robustness of the laser-textured surfaces for a dew harvesting application, measurements of wettability, emissivity, and water collection were performed after 4 months of outdoors exposure following the procedures explained in the paragraphs above.

Outdoors Dew Harvesting Experiments

The dew harvesting performance of the laser-textured surfaces was evaluated in real conditions with an outdoors experiment located at the terrace of the Institute de Physique du Globe de Paris in Paris, France. The setup, shown in Figure 5a, consists of a 1 m² condenser with 30° inclination with respect to the horizontal. This angle allows the best compromise between radiative cooling and water collection by gravity.^[38] The condenser was placed facing SSW direction (205.9°). The setup includes a collection gutter that conducts the condensed water to a pluviometer, registering the collected volume over time. Following the design described in Section 2.3, the laser-textured tiles were placed on top of the condenser structure as shown in Figure 5a. Fixation was made by means of 5 industrial plastic hooks and loops 25 × 25 mm² squares (3M) to reduce the thermal contact of the tiles with the condenser structure. Foam was also placed on the borders of the tiles to reduce the impact of the wind convection under the tiles. A thermocouple (Campbell) was installed on the back of the tiles to register its temperature. The setup includes also a separate weather station registering ambient temperature, pressure, relative humidity, wind speed, and wind direction. All sensors are connected to a central datalogger station (Campbell), with a sampling rate of 5s, storing average values every 15 minutes. Wind speed was taken as the arithmetic average of the values in the range, while the wind direction was taken from the averaged vector. For the collected water volume the value stored is the accumulated value for the 15 min period. Dew condensation events were easily distinguished from precipitation in form of rain or snow during data postprocessing on the basis of both the comparison of surface temperature of the tiles with the air and dew point temperatures and the collected volumes (generally much progressive and lower amounts collected by dew in comparison to precipitation water). As explained in Section 2.3.3, data from a second condenser placed in the same location and orientation was analyzed to serve as a comparison reference. The second condenser consists of a flat 1m2 surface coated with a paint designed for enabling dew harvesting. Wetting and emissivity properties of the paint coatings were analyzed following the procedures described in “Surface Wettability Characterization” and “Surface Emissivity Characterization”, respectively. The whole system was checked periodically to ensure proper functioning during the duration of the study.

Analysis of Collected Water

Water collected from condensation on laser-textured surfaces was analyzed for potential trace metal contamination (Al, Cr, Cu, Fe) by inductively coupled plasma optical emission spectroscopy (ICP-OES). No significant metallic concentration was detected, with all values below the detection limit of 4 10⁻³ mg L⁻¹ and hence in compliance with the World Health Organization guidelines for drinking water quality.^[147]

Acknowledgements

The authors wish to acknowledge: C. Serra from the Nanotechnology and Surface Analysis group at CACTI, Universidade de Vigo, for assisting with the XPS analyses; the technical staff from Electron Microscopy group at CACTI, Universidade de Vigo; R. Lomba from the Food Security group at CACTI, Universidade de Vigo, for assisting the ICP analyses; J.F. Álvarez from CINTECX, Universidade de Vigo, for assisting with high speed camera recordings; M. Thorey from Laboratoire des Energies de Demain at Université Paris Cité, for giving advice on the radiative cooling setup; A. Abalde from LaserON Research Group at Universidade de Vigo for assisting in the manufacturing of the large tiles for the outdoors experiment; L. Quartier and X. Benoit from the PMMH Laboratoire at ESPCI Paris for assisting in the setup of the outdoors dew harvesting system.

This work was partially supported by the Government of Spain [PID2020-117900RB-I00 (MCI/AEI/FEDER, UE), EQC2018-004315-P (AEI/FEDER UE) and Margarita Salas Fellowships], and by Xunta de Galicia (ED431C 2023/25 and ED481B-2023-030).

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

P.P.Á., A.M., A.R., J.P., and D.B. conceived the original idea and designed the study. Laser texturing experiments were performed by P.P.Á. under the advice of A.R. and J.P. Characterization and analysis of surface topography, chemistry (EDS), wettability, and emissivity (IR Camera) were performed by P.P.Á. N.L. performed the FTIR measurements for the reflectivity spectra under the advice of T.B. Experiments in the radiative cooling chamber were performed by P.P.Á. under the advice of L.R. Water condensation and collection experiments within the lab setups and aging studies were performed by P.P.Á. under the advice of A.M. and D.B. Design, setup and data analysis of the outdoors dew harvesting system was performed by P.P.Á. under the advice of A.M. and D.B. The original manuscript draft was written by P.P.Á. under the advice of A.M., A.R., J.P., and D.B. Figures were prepared by P.P.Á. under the advice of J.P., D.B., A.R., and A.M. All authors contributed by reviewing the final draft of the manuscript.

Open Research

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.