Surfaces with modified morphology and wettability arrangement: a potential medium for water harvesting in desertification areas

The above achievements imply that the utilization of trace quantity irrigation rather than conventional infiltrating or drop irrigation will also ensure the survival of flora while significantly reducing the consumption and cost produced by water-related works. Thirdly, drought-endurable plants such as Agropyron mongolicum, which is characterized by high cell osmotic pressure and slow metabolism, can be irrigated even by utilizing dew water. During our research in Yulin city (ShaanXi, China), we have also found that large number of dew droplets formed on the surface of local plants early in the morning. As a result, trace quantity irrigating by dew water generated from the condensation of vapor shows unique potential in realizing an in situ trace irrigation (water supply does not depend on direct water transportation, but on condensate formed by local gas–liquid transition) and reducing cost produced by water-related works to a minimum.

The surface of such material will be modified to possess an alternating hydrophilic and hydrophobic pattern. A non-uniformly wettability distribution is a critical factor that results in the promotion of dewing efficiency. When a dew droplet is lying on a spot with a wettability gradient, the contact angle (CA) at the front and the rear of the droplet will be different (Beysens 2018). According to the Young-equation, the contact angle difference causes an imbalance of the inner pressure inside the dew droplet, which leads to the motion of the dew drop in the direction of the wettability gradient. Based on Young-equation, the driven force formed by contact angle difference is F = γ_lg (cosθ_a–cosθ_b)Δl, where θ_a, θ_b represent the front and rear contact angles (Figure 2) of the dew droplet that is lying on the hydrophilic–hydrophobic junction; γ_lg represents the surface tension of liquid–gas interfacial boundary. Driven by the force F, droplets migrate and collide with dew drops located on the hydrophilic section of hybrid surfaces. Therefore, the growth rate of the volume of droplets which lie on the hydrophilic section of a hybrid surface will be accelerated by not only the diffusion of water molecules but also the inter-droplet collision and fusion. The literature stated that after a droplet reaches its stable volume corresponding to the current temperature, the influence of inter-droplet collision on the rate of droplet growth is much greater than that of diffusion of water molecules which is caused by moisture gradient between the atmosphere and the droplet surface (Sikarwar et al. 2012). Eventually, large droplets fall from the surface and are collected by soil or containers when the gravity is greater than the adhesion force. In a word, currently, the hybrid surface, i.e., hybrid material for water harvesting is material whose surface consists of alternating hydrophilic and hydrophobic patterns which motivates the droplet migration in the direction of the hydrophilic section by an imbalanced internal force caused by a contact angle difference. Consequently, dew droplets grow due to inter-droplet collision and detach the surface due to gravity.

Water harvesting materials which have only been modified on surface wettability arrangement have been studied since 1992 (Chaudhury & Whitesides 1992). Plenty of comprehensive reviews have been offered in which the preparation processes and dewing efficiencies in condensation chamber of such materials have been reported (Brown & Bhushan 2016; Zhang et al. 2016b; Sun & Guo 2019; Chen & Zhang 2020; Yang et al. 2020; Lv et al. 2022). What these researchers had in common is that they focused on how to increase the value of the wettability gradient on the surface of water harvesting materials, i.e., to develop new preparation processes that will upgrade hydrophilic–hydrophobic patterns to superhydrophilic-superhydrophobic patterns, however, other factors or manufacturing processes that may enhance nucleation seem to be neglected by most of the previous works. For the last few years, while the development of hybrid surfaces with alternating hydrophilic and hydrophobic patterns attracted scientists' attention, other research on the influence of micromorphology on vapor nucleation has gotten underway. It has been calculated theoretically that the Gibbs' free energy barrier of gas–liquid transition on a cold concave surface is smaller than those on a convex or a flat surface. The smaller the radius of the concave surface is, the lower the value of the Gibbs' free energy barrier of condensation is and the easier the dew droplet begins to form, i.e., the smaller the subcooling temperature required to form a stable water embryo in comparison with the subcooling required in standard condition (Qian & Ma 2012; Yan et al. 2016; Cheng et al. 2020; Hu et al. 2021a). Therefore, the microstructure and wettability arrangement of a surface are two critical factors that influence the dewing process, and this review will concentrate on a water harvesting surface whose microstructure and wettability arrangement have been modified simultaneously. The preparation, mechanism and dew productivity performed in the laboratory of the above mentioned material will be introduced in the next chapters.

Particles or reagents like modified SiC, Cu₂O, TiO₂ or ZrO₂, polyurethane, or sodium alginate are commonly used components that act as hydrophilic or hydrophobic spots on the surface of substrates which are commonly made of polydimethylsiloxane, polyurethane, Cu, or Al sheets. The contour of the above mentioned particle or microsphere is convex and the amplitude from top of the particle to the surface where the particle is embedded is around 5–50 μm. Spraying, pouring and then heating at around 333–353 K by ovens are usual fabrication flows in which the particle solution makes contact and eventually bonds with the substrate (Wang et al. 2019a, 2019b, 2020; Feng et al. 2020; Nguyen et al. 2020; Wang et al. 2021a, 2021b). Weaving is another usual method to produce water harvesting materials, where textile fibers of different wettabilities form bulging cross-bedded hydrophilic or hydrophobic spots (Yu et al. 2021).

The concepts of condensation acceleration of the above mentioned amphiphilic/amphiphobic or alternating hydrophilic and hydrophobic water harvesting surfaces are also diverse. Most of them still adopt the dew harvesting principle as shown in Figure 2. However, some literature still put forward new investigations about the influence of convexity of bumps on droplet growth rate and new dew collection mode in which multi-layer is adopted to increase the opportunity of droplet shedding. Although the Gibbs' free energy barrier of condensation on concave surfaces is lower than those on convex and flat surfaces, surfaces with convex contour still produce a higher vapor concentration gradient between the atmosphere and droplet surface (Medici et al. 2014; Park et al. 2016). Most water harvesting surfaces prepared by the physical blending method have a large scale of bumps which have an amplitude of about 5–50 μm. Droplets located on these convex bulging profit will not only form a high moisture diffusion rate which accelerates the droplet growth but also form a high frequency of inter-droplet collision. Therefore, benefiting from the synergistic effect of condensation enhancement shown by prepared materials and a simple processing flow, physical blending has become a popular method in water harvesting material preparation.

In order to enhance the amount of dew collected, a multi-layer mode has been designed for thin water harvesting surfaces (Wang et al. 2019b, 2020). The concept of multi-layer mode is simple: thin films of condensation surface have been set up in a vertical direction and in a side by side arrangement, and the distance from layer to layer has been set up for about 1–2 mm. After dews appear, small drops begin to move along the wettability gradient and then collide with droplets lying on hydrophilic sections. Large droplets detach from the surface or ‘jump’ to the adjacent layer which increases the possibility of inter-droplet fusion. The ‘jumping’ of small dew drops from layer to layer is caused by energy conservation. After two small droplets mix together, the coalesced droplet releases surface energy due to the reduction of surface area. The released surface energy is to overcome the energy consumed due to friction loss and the energy needed by separating the liquid-solid, i.e., the droplet-substrate interface from each other, as the coalesced droplet leaves the layer. The rest of the energy is transformed into kinetic energy that makes the droplet ‘jump’ a short distance. To sum up, the jumping of small droplets found in multi-layer mode of water harvesting materials is worthy of attention in the field of dew droplet collection.

Dip-coating, hydrothermal method, UV irradiation, or even heat treatment are common techniques of chemical modification to prepare amphiphilic/amphiphobic or alternating hydrophilic and hydrophobic surfaces with structured patterns. The process of modification usually carries out on copper mesh or textile, reagents that regulate the wettability are CuO, TiO₂, ZnO, Fe₂O₃, or other monomers. The diversity of polymer monomers is a unique advantage of chemical modification, however, the difficulty of obtaining a long range ordered microstructure is the shortage of this method as well (Ai-Khayat et al. 2017; Zhong et al. 2019; Guo & Guo 2020; Zhang et al. 2020; Lyu et al. 2021). A typical process flow of preparing such water harvesting surfaces by chemical modification is for instance as follows (Lyu et al. 2021): a piece of polyester textile was dipped into a mixture consisting of H₂N-PDMS-NH₂, tetraethyl orthosilicate, and triethoxy (3-glycidyloxypropyl)silane at room temperature for 10 min. After drying in an environment containing 2 mL of HCl for 9 h, the textile was then moved into an oven at 293 K for 12 h. Eventually, a hybrid textile was obtained with NH₃ hydrophilic sites and hydrophobic polymer chains.

Compared with chemical modification, the etching method has the potential to produce long range ordered micromorphology while maintaining the expected wettability arrangement at the same time. Reactive or deep reactive ion etching (RIE/DRIE) and plasma dry etching are commonly used processes. Micromorphologies manufactured by etching are usually conical array, conical groove, honeycomb array, and cylindrical array. The amplitude of the microstructure (tiny cone or cylinder) is 5–50 μm away from the surface ground level. The purpose of manufacturing the above mentioned microstructures is to offer sites which realize dew droplets movement driven by the imbalance force caused by contact angle difference (Bingi & Murukeshan 2017; Brindhu & Padmanabhan 2021; Hirai et al. 2017; Wang et al. 2021b). For example, while preparing a water harvesting surface with long range ordered triangle array (Brindhu & Padmanabhan 2021), a polystyrene (PS) monolayer served as the mask was first deposited onto a silicon substrate using evaporation-induced convective self assembly technique followed by sintering at 372 K for 1 min. Then, Au was deposited onto a promoting layer of Ti using sputter deposition. The PS monolayer was removed by using adhesive tape. The whole surface was then dipped into a 16-mercaptohexadecanoic acid (MHA) solution and a 1-octadecanethiol (ODT) solution, respectively, in order to form bondings between MHA and Au spots as well as between ODT and Si substrate.

Unfortunately, few cases have been found which introduced research on the preparation of hybrid water harvesting materials by nanosphere lithography. At the moment, materials fabricated by such method have been applied in the field of optics, photovoltaic, cancer therapy and drug delivery (Jia et al. 2020; Lee et al. 2021; Welbourne et al. 2021; Mai et al. 2022). But, at present, it is noteworthy that the utilization of gas–liquid interface deposition for the formation of a long range ordered Polystyrene microsphere monolayer on a silicon wafer has become one of the simplest and one of the most convenient techniques among other mask forming methods, and eventually, by adjusting the diameter of the Polystyrene microsphere, the minimum dimension of each micro pit on silicon wafer can be reduced to about 1 μm or even less than 1 μm which is getting closer to the same scale of the critical radius of a stable water nucleus at room temperature (nano- or submicronmetre scale) (Wu et al. 2014; Wang et al. 2015; Kismann et al. 2021). Therefore, in order to realize the reduction of the energy barrier in gas–liquid transition, it is worthwhile to sequentially investigate a feasible, systematic workflow of nanosphere lithography for manufacturing alternating hydrophilic and hydrophobic surfaces with the array that consists of nano- or submicronmetre scale of pit.

Micro-milling, laser lithography, and 3D printing have also become adoptable techniques for preparing hybrid surfaces with microstructure. It is reported that a hydrophobic Al alloy surface with a hydrophilic microdot array has been prepared by Micro-milling method, where an Al sheet was first electrochemically etched in a 0.1 mol/L NaCl solution and dipped in a 1 wt% ethanol solution with fluoroalkylsilane for 90 min. After that, an array consisting of micro cylindrical spots has been milled on the Al sheet by a milling system. However, limited by the strength of the drilling bit, the radius of each spot was only around 300 μm – hundreds of times the critical nucleation radius (Yang et al. 2016). As a result of that, hybrid surfaces with modified surface structures manufactured by micro-milling could not achieve a distinct reduction of the energy barrier in gas–liquid transition yet.

The principle of laser lithography is similar to wet etching, in which certain features on the surface of a metallic substrate are manufactured by high power laser, for example, a TiO₂ hierarchical microstructure with a certain wettability arrangement has been prepared, where a Ti sheet has been etched by a femtosecond laser system with a central wavelength of 1,030 nm, 800fs duration and a 400 kHz repetition rate. Then the modified Ti sheet has been hydrothermally treated in a 3 mol/L NaOH solution followed by a 593 K heating in an oven for 24 h. After that, the component of the Ti surface has been transformed to TiO₂ by a process in which the Ti sheet has been dipped into a 1 mol/L HCl solution for 10 min and then heated at 723 K for 1 h. Eventually, a superhydrophobic surface with hierarchical microstructure has been obtained by depositing the vapor of PDMS ((C₂H₆OSi)_n, Sylgard 184) solution onto the TiO₂ sheet (Lu et al. 2019). Another representative achievement done by Laser lithography is hybrid surfaces with grooves in which a glass substrate has been first covered by a hydrophobic coating. Then a pulsed laser with a 1,064 nm wavelength, 500 kHz pulse repetition and 200 ns pulse duration penetrated from the back side of the glass substrate. The part made of glass was ablated leaving a hybrid surface that consists of hydrophilic grooves and hydrophobic tips. The minimum width of each groove was 25 μm which was only about 50 times larger than the critical nucleation radius (Li et al. 2017; Lee et al. 2018).

3D printing, which has become popular in recent years can also be used for fabricating surfaces with modified microstructure. Inspired by the shape of cactus' leaves, the typical microstructure done by 3D printing is a set of cones pointed to different directions. The dew droplet will move from the tips of each cone to the bottom part of the cone under the gradient of Laplace pressure which is caused by the contact angle difference between the front and rear sides of a single dew droplet (Li et al. 2019a). The average radius of each cone was around 100 μm and the components of such materials were mostly polymer. Limited by the incompatibility between the molten hydrophilic polymer and a hydrophobic polymer, water harvesting surfaces with different wettabilities manufactured by 3D printing are still under development.

Besides surfaces prepared with physical blending, chemical modification, etching, and micromanufacturing, other efficient water harvesting materials such as metal organic frameworks (MOFs) also deserve to be mentioned (Pan et al. 2020). As a kind of water harvesting material, MOFs benefit from their high porosity and large specific surface area where water molecules are willing to be absorbed. As an example, the principle of a CaCl₂-decorated photothermal Fe-ferrocene MOFs for water molecules absorption has been introduced as follows (Hu et al. 2021b): CaCl₂ has been encapsulated into hollow iron-based Ferrocenyl microspheres (Fe-Fc-HCPs), which avoids the collapse of the channel between particles when vapor flows through and the leakage of deliquescent salt. In this material, CaCl₂ served as a typical desiccant and the hollow of Fe-Fc-HCPs enhances light absorption by increasing the optical reflection area. At night when the temperature becomes low and the humidity rises, the adsorption of water molecules occurs due to the adsorption effect of CaCl₂, while in the daytime when the temperature is getting higher and the humidity reduces, the desorption of water molecules occurs due to the photothermal heating effort of Fe-Fc-HCPs. Extra sources for heating the water molecules are not required because of the self-photothermal heating effect of Fe-Fc-HCPs charged by solar energy.

To summarize, a consideriable amount of methods are available for the preparation of alternating hydrophilic and hydrophobic surfaces with modified microstructure. Currently, whatever method is used, the prepared materials possess the following common characteristics: (1) most micromorphologies of current material are either hydrophobic bumps surrounded with a hydrophilic flat surface or an array of tiny conical grooves or bumps that trigger the droplet migrates from tip to bottom of the cone. Both structures are biomimetic, or more exactly, the ideas are based on the pattern on the back of Fog-basking beetles and the shape of cactus leaves, respectively. Other configurations of micromorphology are hydrophilic or hydrophobic cylindrical pits or band-shaped grooves surrounded by flat surfaces with counter wettability. The size of each pit or groove was more than 20 times larger than that of the critical nucleation radius. (2) The dew droplet migrates along the wettability gradient or geometrical gradient or ‘jumping’ due to the reduction of the surface energy – these are the most common mechanisms with which the water harvesting materials are adopted. The effect brought by these mechanisms is the increase of the droplet growth rate caused by inter-droplet collision and fusion and eventually, shedding of large droplets will occur frequently which increases the amount of dew water collected.

The severe climate conditions in desertification areas require a reliable functionality of water harvesting materials. In order to analyze the dew productivity of such materials and conditions including temperature and humidity in which an optimal dewing amount can be achieved, the maximum value of the dew collection rate and their experimental environments adopted by the representative literature mentioned in the previous chapter are summarized in Table 1. Meanwhile, as supplementary, dewing efficiencies and test environments of hybrid surfaces without modified microstructure have also been shown in this table. In this review, the maximum value of the dew collection rate (in g/cm²h⁻¹) has been chosen to characterize the dew productivity. The temperature and the humidity refer to the value of the cold end temperature which also represents the temperature of the sample and the relative humidity in the dewing test chamber, respectively.

According to the data shown in Table 1, several conclusions can be summarized: (1) At present, almost all contours of the microstructure adopted by hybrid surfaces are convex. Alternating hydrophilic and hydrophobic surfaces with cavity arrays are gaps that are worth to be studied. (2) The dimensions of the microstructure in most of the works are still larger than that of the critical nucleation radius (≈1 μm or smaller) which makes the reduction of the free energy barrier in gas–liquid transition inapparent. (3) After analyzing the experimental configurations adopted by the literatures listed in Table 1, two parameters that influence an accurate measurement of dew productivity have not been introduced in detail: firstly, when large droplets roll off, they sweep other droplets on the path, which causes a sudden unpredictable increase in droplet volume, due to the randomness of the volume and number of droplets be swept by each dewing test. Therefore, to measure the weight that represents the final dew production of each sample accurately, one should consider the impact of ‘inter-droplet sweeping’ on ‘net’ dew productivity which is caused only by surface features such as wettability or geometrical gradient. Secondly, information about droplets roll off and be collected from other non-measuring surfaces, such as from the frame that holds the sample, which would amplify the final value of dew production when ‘unexpected’ dew water is weighed was insufficient. Therefore, an analysis of reasons for the max. dew productivity fluctuation could be further investigated only when the above mentioned ‘distractors’ are eliminated or when they are accurately observed, i.e., the weight increment of dew water contributed by ‘sweeping’ and droplet falling from non-measuring surfaces could be measured. (4) The dew productivities of most hybrid surfaces are no more than 0.6 g/cm²h⁻¹ or slightly larger than 1 g/cm²h⁻¹. It is questionable whether such a tiny amount of dew water will match the demand of drought-endurable plants. Besides, almost all relative humidities in test chambers are higher than 80%. In order to compare the relative humidities in desertification areas with those in test chambers and analyze the applicability of the water harvesting material in desert areas, the climates in various types of desertification areas have been listed in Table 2.

According to the temperatures and relative humidities shown in Table 2, it is implied that in places suffering from desertification, the relative humidities are generally less than 70%: in FuYuan county (YunNan province, Southeast China) for instance, where the climate is not as dry as that of cities in North and Northwest China (NingXia, ShanXi or XinJiang province), the humidity reached 80% in July and came down to 46% in January. The humidity occasionally rose to nearly 100%, but remained most of the time less than 64%. In Umudike, Nigeria where the climate is also not as dry as that of cities in North Nigeria, the annual humidities (1983–2013) were still less than or slightly larger than 70%. In NingXia and ShanXi provinces (North China) the largest annual humidities before the year 2014 were less than 65%, respectively, while in QingHai-Tibetan Plateau (China), the humidity (from the year 2011 to 2018) rose up to 90% in June and came down to 38% in May, the annual value was about 65%. In Abu Dhabi, Dubai, and AlAin (the UAE), the largest annual humidities (from the year 1982 to 2009) were 65, 57, and 50%, respectively. Therefore, it is unusual for desertification areas to have an annual relative humidity of more than 80%. This is unfriendly to the application of the water harvesting materials described in the previous chapter. By comparison between the artificial test environments (seventh column in Table 1) and the natural environments (fifth column in Table 2), the relative humidities during dewing experiments for most of the materials are more than 80% while the relative humidities in most areas listed in Table 2 are less than 70%. Hence, due to the insufficient air humidity, it is questionable whether current water harvesting materials will still reach their optimal dew productivity when utilized in desertification areas.

In the previous section, we have shown that low environmental humidity is the bottleneck that limits the utilization of hybrid surfaces in the field of water harvesting in an outdoor environment. In this chapter, methods that will improve the applicability and increase the dewing amount of surfaces with modified morphology and wettability arrangement are introduced.

The key to dew formation in low humidity or a small subcooling environment is to minimize the Gibbs free energy barrier of gas–liquid transition when vapor nucleates on the cold surface. The latter is also called heterogeneous nucleation, and certain artificial surface properties such as wettability and microstructure arrangement will make dewing easier, i.e., dewing in an unsaturated air condition or dewing in a small subcooling condition. A hydrophilic surface and concave surface whose radius is as small as the critical radius of a stable water nucleus at a given temperature will both dramatically reduce the Gibbs free energy barrier of the gas–liquid transition. It has been demonstrated that the free energy barrier for the formation of a liquid nucleus on a flat hydrophilic surface could be 117 times smaller than that on the flat hydrophobic surface (Varanasi et al. 2009). The nucleation rate on a hydrophilic surface could be zillions of orders of magnitude (≈10¹²⁹ times) higher than that on the hydrophobic surface (Varanasi et al. 2009). It has also been proved that a distinct advantage of heterogeneous nucleation on a concave spherical surface of radius R over a flat surface occurs when 2R < 10r is satisfied, where r represents the critical radius of a stable water nucleus at a certain temperature. Conversely, the disadvantage of heterogeneous nucleation on a convex spherical surface of radius R over a flat surface also occurs when 2R < 10r is satisfied (Qian & Ma 2012). This phenomenon dissipates rapidly when 2R > 10r for both concave and convex surfaces, hence, only nanoscale or submicrometer scale surface features will really affect vapor nucleation. On the other hand, at the macroscopic level, the moisture concentration gradient above convex surfaces is larger than that above concave and flat surfaces, which means that dew droplets obtain a faster growth rate at the early stage of condensation on convex surfaces (Medici et al. 2014; Park et al. 2016).

In conclusion, surfaces with modified microstructure and wettability arrangement would have a superiority in heterogeneous nucleation in a low humidity environment, when further investigations carried out in the following research directions: (1) nanosphere lithography which is capable of fabricating concave array, in which the dimension of each cavity can be limited to nano- or submicron scale, should be adopted as one of the important methods of water harvesting surface preparation. (2) By utilizing nanosphere lithography, a hydrophobic surface with a hydrophilic spherical cavity array is an optimal option of water harvesting material, if the radius of a single spherical cavity can be reduced to the dimension that is close to the critical radius of a stable water nucleus at a certain temperature. (3) If preparing of nano- or submicron scale spherical cavity is infeasible, a hydrophobic surface with a hydrophilic spherical bump array is an optimal option for water harvesting material. It should be noted that the radius of hydrophilic particles embedded in a hydrophobic surface can not be equal to or less than the critical radius of a stable water nucleus at a certain temperature.

In this review, the possibility of trace quantity irrigation for providing fresh water to desert plants has been confirmed. According to our findings that the actual water consumption of some desert plants can be satisfied by the dew water produced from heterogeneous nucleation, the idea of preparing modified functional surfaces for accelerating the dew droplet growth rate during heterogeneous nucleation has been introduced.

Physical blending, chemical modification, etching, and micromanufacturing are reliable technologies for wettability and microfeature modification of water harvesting materials. At present, most of the existing materials prepared by the above mentioned methods possess a hybrid morphology in which bump array has a counter wettability to the plane section of the surface. By comparing the environmental conditions required by such materials in condensation experiments with climates in desertification areas, it is deduced that the dew productivities performed by current water harvesting materials in desertification regions characterzied by low relative humidity will not be as efficient as those shown in test chambers. The challenge that needs to be overcome and the potential of surfaces with modified wettability and microstructure in achieving in situ water harvesting in low humidity condition is reflected in the following two aspects: (1) reliable methods need to be developed and the process of nanosphere lithography needs to be improved in order to promote the feasibility of producing surfaces embedded with nano- or submicron scale cavity. (2) Hydrophobic surfaces embedded with nano- or submicron scale hydrophilic spherical cavity array prepared by nanosphere lithography can reduce the Gibbs free energy barrier for generating a stable water nucleus and will thus achieve gas–liquid transition in a small subcooling or in a low humidity environment.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.