I investigated a meniscus-mediated spontaneous droplet climbing and rapid coalescence on hydrophilic SLIPS (slippery liquid-infused surface), which I characterized as the coarsening effect. The self-generated oil menisci surrounding water droplets can provide a bridge for tiny droplets with sizes < 20 µm to climb toward and coalesce with the larger droplets spontaneously. Thus, the coarsening effect results in rapid droplet coalescence and removal, showing a small water coverage area (< 30%) and significantly reducing the thermal resistance. The dew harvesting rate on hydrophilic SLIPS is at least 200% higher than state-of-the-art surfaces. Based on the coarsening effect, I developed a dynamic heat transfer model for dropwise condensation to consider rapid droplet disappearance(2). The dynamic model provides a theoretical foundation to design surfaces for condensation (i.e., high‐frequency droplet disappearance), benefitting the design of water and energy systems

Based on the droplet removal theory of the coarsening droplet, any droplet with a diameter larger than 50 μm should be removed from the surface immediately after condensing to further enhance water harvesting. Thus, I designed a hydrophilic reentrant SLIPS that rapidly removes condensed droplets (Fig. 1B). The hydrophilic reentrant SLIPS is created using microchannels and isotropic etching to produce undercuts. When droplets are condensed on the top of the channel, they are removed immediately to the reentrant channels underneath, resulting in sustainable flow separation. Thus, flow separation sustains a water coverage ratio down to 20% and a droplet removal frequency of 130 Hz/mm2. The water harvesting rate is 110% higher than hydrophilic flat SLIPS. 

Since SLIPS has limitations during high heat flux condensation, We designed a microchannel-elevated micromembrane (MEM) to achieve rapid droplet removal for steam condensation. The MEM structure consists of a hydrophobic mesh as the condensing surface and hydrophobic microchannels for rapid liquid removal. As each liquid column grows and contacts the hydrophobic mesh, the condensed droplets on the mesh are removed, which separates the vapor and liquid flow. MEM could sustain phase separation at a heat flux of 1,000 kW/m2 at a subcooling of 10 K without apparent failure, achieving an HTC 300% higher than the dropwise condensation on a hydrophobic flat surface. 

Surfaces with special wettability (i.e., beetle-inspired patterned wettability) to promote vapor-to-liquid condensation are another approach to enhance HTC, but current solutions suffer from low HTC due to inefficient droplet removal. We develop a biphilic QLS (quasi-liquid surface) to push the limit of condensation performance. The biphilic striped QLS consists of PEGylated and siloxane polymers as hydrophilic and hydrophobic patterns, showing a highly slippery interface. The hydrophilic stripes improved the overall nucleation, and the slippery property allows large droplets to sweep for rapid droplet removal, showing a 170% higher heat transfer coefficient in steam condensation than the conventional beetle-inspired surface.