Hydrophilic reentrant SLIPS enabled flow separation for rapid water harvesting

Water harvesting from air is desired for decentralized water supply wherever water is needed. When water vapor is condensed as droplets on a surface the unremoved droplets act as thermal barriers. A surface that can provide continual droplet-free areas for nucleation is favorable for condensation water harvesting. Here, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface (SLIPS) that rapidly removes droplets with diameters above 50 μm. The slippery reentrant channels lock the liquid columns inside and transport them to the end of each channel. We demonstrate that the liquid columns can harvest the droplets on top of the hydrophilic reentrant SLIPS at a high droplet removal frequency of 130 Hz/mm². The sustainable flow separation without flooding increases the water harvesting rate by 110% compared to the state-of-the-art hydrophilic flat SLIPS. Such a flow-separation condensation approach paves a way for water harvesting.

Condensation is a common phase-change phenomenon that is widely used in water harvesting (1–5). Dropwise condensation can form discrete droplets on a low-surface-energy substrate and promote the heat transfer coefficient up to 10 times higher than filmwise condensation due to the rapid removal of condensates (6, 7). However, the displacement of air inside the structures can lead to a higher pinning force on the droplets, resulting in flooding on the superhydrophobic surface (8, 9). By replacing the air trapped inside the surface structures with liquid lubricant, researchers developed the slippery liquid-infused porous surface (SLIPS) to further promote heat transfer performance (1, 10–13). When condensed droplets grow to a certain diameter, they shed off a vertical surface due to the negligible contact angle hysteresis (12). To accelerate the droplet removal, directional droplet movements were developed to regulate the removal of water droplets on SLIPS (2, 5). However, these surfaces rely on gravity to remove large droplets that remain on the surface as thermal barriers before shedding during condensation.Therefore, surfaces with partially hydrophilic and partially hydrophobic domains were developed to remove droplets from the condensing surface to the structures underneath. Such an amphiphilic surface has a hydrophobic top with a hydrophilic porous structure underneath (14, 15). During the condensation, droplets on the hydrophobic top are absorbed by the wetted hydrophilic structures underneath. However, the liquid film within the hydrophilic structures leads to partial or complete flooding at elevated heat fluxes due to the high pinning forces (9, 16, 17). Unlike the radiative cooling-induced dew harvesting in nature, droplets are directly condensed on the reentrant SLIPS by conduction and convection (18), where condensed droplets act as thermal barriers (19–21). Based on the condensation models (6, 22–24), each droplet contributes to the total thermal resistance, while smaller droplets have a lower thermal resistance. When a surface is only covered by small droplets (e.g., at the beginning of the condensation), the heat transfer coefficient is higher than that of a surface with larger droplets (25). Thus, rapid droplet nucleation and removal are desired to achieve a high heat transfer coefficient. Microchannels are applied to transport condensates as the flow resistances can be much smaller than those of micropillars. Hydrophilic slippery channels with or without biphilic coatings are used to enhance water harvesting, but they failed to separate the vapor and liquid as droplets emerged out of the channels or completely wetted the surfaces (1, 26–28). To suppress condensates from emerging out of the channel, the liquid column must be locked inside. The reentrant channels show a potential to lock the liquid inside due to their overhang structures. With a surface engineering approach, the reentrant structure has been modified to keep highly nonwetting liquids inside (29). However, the reentrant structures are wetted by liquids during condensation as the condensates are pinned inside the channels (30). Thus, the surface will transition to a complete wetting state at elevated heat fluxes.To address those challenges, on a hydrophilic reentrant SLIPS, we present a flow-separation condensation mode that sustains rapid droplet removal on top and transports liquid columns underneath. Our approach is to make reentrant channels with hydrophilic and slippery boundary lubrication. Once droplets are condensed on the top hydrophilic slippery surface, they are removed immediately to the reentrant channels underneath, resulting in sustainable flow separation. Such a new condensation mode could significantly reduce the thermal resistance by rapidly removing droplets with diameters above 50 µm, leading to an exceptional water harvesting rate. This work not only proposes a flow separation approach to enhance water harvesting but also provides a universal concept to design surfaces for condensation heat transfer.