Nature-Inspired Surface Engineering for Efficient Atmospheric Water Harvesting

Atmospheric water harvesting is a sustainable solution to global water shortage, which requires high efficiency, high durability, low cost, and environmentally friendly water collectors. In this paper, we report a novel water collector design based on a nature-inspired hybrid superhydrophilic/superhydrophobic aluminum surface. The surface is fabricated by combining laser and chemical treatments. We achieve a 163° contrast in contact angles between the superhydrophilic pattern and the superhydrophobic background. Such a unique superhydrophilic/superhydrophobic combination presents a self-pumped mechanism, providing the hybrid collector with highly efficient water harvesting performance. Based on simulations and experimental measurements, the water harvesting rate of the repeating units of the pattern was optimized, and the corresponding hybrid collector achieves a water harvesting rate of 0.85 kg m–2 h–1. Additionally, our hybrid collector also exhibits good stability, flexibility, as well as thermal conductivity and hence shows great potential for practical application.


S2
Cross-sectional profiles of fs-laser ablated Al with scan number of (f) 1, (g) 2, (h) 3, (i) 4, and (j) 5. It should be noted that in the elevation maps, different colors from blue to red only represent the relative elevation from low to high, not the absolute value.   The expression for water harvesting rate (R) where m receive , t and A are the mass of water received by the balance, the time used and the apparent surface area of the collector, respectively.
Clausius-Clapeyron equation describing the relationship between pressure (p) and temperature (T) of given substance (water) can be written as where R w is the specific gas constant for water vapor and L is the latent heat of evaporation for water. Assuming L is also a constant so that we can integrate Equation S3a and obtain an equation that relates two different states. In our case, the state at dew point T D and a state at ambient temperature T, where p and p D are the vapor pressure given temperature T and dew point T D , respectively. By substituting p/p D with relative humidity (RH, in percentage) according to the definition of the relative humidity (the ratio of partial water vapor pressure p w and saturation water vapor pressure p s ), dew point can be expressed as a function of T and RH.
According to the definition of specific humidity q, it can be approximated as mixing ratio ω. Since RH = ω/ω s , q can be expressed using RH, we have Since saturation water vapor mixing ratio ω s is the ratio of the mass of water in saturated air and mass of dry air, by combining the ideal gas law, where p 0 is the sea-level atmospheric pressure (one standard atmosphere of 101,325 Pa was used in this paper), and molar mass of dry air (M d ) and molar mass of water vapor (M w ) are 28.96 g mol -1 and 18.02 g mol -1 , respectively.
Integrating and rearranging Equation S2a and replace p with p s , we will obtain the expression of p s at any given temperature T where T 0 is a reference temperature (here the triple point temperature of water, 273.16 °C is used) and p s,0 is the saturation vapor pressure at T 0 (611.2 Pa). Therefore, T D as a function of T and RH can be simplified as Equation 4 in the main manuscript.
The density of humid air (ρ) with volume V made up of dry air and water vapor with masses of m d and m w respectively, can be expressed as follow using the ideal gas law, where R d is the specific gas constant for dry air and equals to 1000R 0 /M d ( is the universal gas constant with the value of 8.3145 J mol -1 K -1 ). By definition of the relative humidity, By combining Equation S3c, we can get the expression of ρ at given T and RH.
The Kelvin equation governs equilibrium systems involving meniscus and is used to describe the phenomenon of capillary condensation. where p v is the equilibrium vapor pressure, is the mean curvature of the meniscus, γ is the interfacial surface tension, V m is the molar volume of the liquid.
Consider a conical pore with a half apex angle of θ. If a meniscus can form with zero angle of contact with the wall of the pore, when the radius of curvature r reaches its maximum, the corresponding maximum volume of condensed water V r in the pore will be 1 = 3 (1 -sin ) 2 sin Equation S6