Dewetting-mediated Interfacial Transport at Extremes

Abstract

Surface and interface are not only the starting points of interfacial transport, but also the intersection points of various physical, chemical, and biological processes. Tailoring surface and interface to yield preferential mass, energy, and momentum transport is of significant importance for practical applications ranging from power plants, environment to agriculture, biology, and more. Despite exciting progresses in the manifestation of wetting theories and functional surfaces such as non-wetting surfaces and directional transport surfaces, to date, our fundamental understandings in maximizing transport efficiency predominately rely on classical liquid wetting mechanism, which facilitates the conversion of capillary energy to other forms of energy (such as kinetic energy) by maximizing interactions between liquid and solid/liquid. However, such a mechanism is inherently limited by undesired interfacial effects exemplified by wetting collapse and surface pinning effect, which dwarfs the efficiency of transport. Besides, unlike normal sets of working conditions, the wetting mechanism can be further compromised under extreme conditions associated with limited capillary energy yet high dissipations, such as viscous liquid flow, droplet transport under total wetting regime, solid transport across interfaces at capillary length scale, and so on.

Contrary to wetting mechanism, the dewetting mechanism favors minimal liquid-solid/liquid contacts yet demonstrate uncompromised or even stronger liquid kinetics, which is a promising strategy in alleviating the interfacial effect imposed by liquid-solid/liquid interactions. Despite its practical applications in coating and microfabrication technologies, to date, the implementation of dewetting mechanism to tailor the interfacial transport remains elusive because several scientific questions pertinent to practical applications of dewetting remain unsolved: 1. Is there a framework that integrate thermodynamic and hydrodynamic characteristics of dewetting? 2. How to maximize the conversion efficiency of capillary energy to kinetic energy through dewetting? 3. How to achieve liquid transport under total wetting state by harnessing dewetting? 4. Can dewetting be made possible between two hard, nondeformable solids, and how to harness this for solid transport? 5. Can hydrodynamics of liquid flow and transport be spatiotemporally controlled through dewetting?

This dissertation aims to provide a comprehensive set of dewetting-mediated strategies to address the challenges associated with transport of condensed matter at extreme conditions. In particular, in Chapter 1, we propose a thermodynamic and hydrodynamic framework for dewetting-mediated transport, which serves as a versatile toolbox for the guidance of dewetting-mediated transport techniques in the following chapters. In Chapter 2, we propose a heterogeneous slippery surface that allows spontaneous and directional transport of viscous liquids with minimal surface tensions, featuring passive transport of oils with viscosities exceeding 10000 mPa·s^-1. Such a surface allows droplet to dewet asymmetrically and establish a large Laplace pressure gradient along it, which maximizes the release of capillary energy to kinetic energy by maintaining a minimal viscous dissipation. In Chapter 3, moving from passive dewetting to active dewetting, we reported a previously undiscovered acousto-dewetting effect that overcomes the undesired viscous dissipations imposed by total wetting of liquids, achieving residue-free transport of liquids on a super-hydrophilic surface. We show that acousto-dewetting originates from the intricate interplay between highly confined ultrasound and liquid dynamics, which can be capitalized for the development of droplet microfluidics working in both in vitro and in vivo environments, a task that is otherwise impossible in traditional wetting-based microfluidics.

Traditional insights hold that dewetting between two solids can only be made possible when one of the solids is soft. In contrast, in Chapter 4, we show that dewetting can be achieved between two rigid solids. This is achieved by introducing a chamber of air, which could remove the surface water on an object while imposing minimal stress to it. Based on this, we developed a capillary tweezer that can be used to perform both dry- and wet-adhesion tasks otherwise impossible by using the wetting mechanism.

Moving from two-dimensional surfaces to three-dimensional domains, in Chapter 5, we demonstrate that dewetting can also occur in 3D cellular porous networks, featuring a degree of controllability to liquid flow. In particular, we show that hydrodynamics of turbulent flow can be spatiotemporally programmed when fluid is subject to dewetting in a cellular porous network, which is in contrast to traditional microfluidics that necessitates a laminar flow with a high controllability. By harnessing this, we developed a dip-and-extract fabrication technique for efficient and scalable fabrication of 3D objects. Different from traditional fabrication technique, our fabrication allows fabrication of multi-materials with distinct or even conflicting interfacial properties. As a key result, we fabricated a compound eye with complex optical microchannels, a task has been proven challenging for wetting based methods. We envision the dewetting-mediated interfacial transport techniques could provide new insights for tailoring the interfacial transport, providing an alternative for interfacial transport at extreme conditions.

Date of Award	7 Apr 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Steven WANG (Supervisor) & Zuankai Wang (External Co-Supervisor)