Coarsening Droplet: Meniscus-mediated Spontaneous Droplet Climbing and Its Applications

Inspired by nature, fundamental investigations of droplet directional movement grow explosively in recent years, aiming to generate energy, solve water shortage, and protect the environment. To move the droplets, strategies were developed based on changing the property of the droplet/substrate, and the substrate geometry. However, it is challenging to remove submicrometer droplets from the surface, which limits the potential applications of water harvesting. In this dissertation, I present a new spontaneous droplet movement on the hydrophilic slippery liquidinfused porous surface (SLIPS), named coarsening droplet. The coarsening effect can rapidly remove droplets with a diameter less than 20 μm. As a mechanism, low interfacial tension of hydrophilic SLIPS enables droplet climbing, while a high oil surface tension provides the driven force. By applying the coarsening droplet for water harvesting, it re-evaluated the classical condensation model since 1973, which neglected droplet disappearance (e.g., removed from the surface) that can enhance the heat transfer. Our new model elucidates the comprehensive heat transfer process, giving rise to a clear guideline of surface design for dropwise condensation. To further apply our new condensation theory of rapid droplet removal, I present a vapor-liquid separation surface to further enhance the water harvesting. By separating the water vapor with condensed droplets, the surface is always fresh to new water nucleation and has a higher droplet disappearance frequency with smaller droplets. In this dissertation, the coarsening droplet is focused on the fundamental study to show the importance of droplet removal. As SLIPS is not durable for water harvesting due to the loss of lubricants, I present a quasi-liquid surface (QLS) by tethering flexible polymer on various solid substrates to solve the durability issue of SLIPS. QLS shows excellent durability during water harvesting experiments, which could be further applied to industrial applications. In this dissertation, Chapter 1, I introduced the fundamental and current progress of droplet directional removal. In Chapter 2, I reported the coarsening droplet and investigate the mechanism of the coarsening droplet as surface tension force. In Chapter 3, I apply the coarsening droplet for water harvesting. The self-propelled coarsening droplet on hydrophilic SLIPS shows rapid removal of condensed submicrometer droplets regardless of surface orientations, showing a promising approach in water harvesting. In Chapter 4, I re-evaluate the condensation model on a hydrophilic slippery liquid-infused surface. I propose a modified condensation model by considering the droplet coverage ratio and removal frequency, which can precisely predict the heat flux. In Chapter 5 I further enhance the water harvesting by achieving vapor-liquid separation on T-shape structures. In Chapter 6, a quasi-liquid surface (QLS) is investigated to solve the durability issue of SLIPS. In Chapter 7, I summarized all my contributions and novelties.