Interfacial phenomena in all-aqueous systems : dewetting, phase separation, fingering instability, and interfacial emulsification

Abstract

Aqueous two-phase systems (ATPSs), also called as all-aqueous systems, are formed by phase separation of an aqueous solution containing two incompatible additives, such as two incompatible polymers above critical concentrations. Due to the major components of ATPSs are water, the interface between the two immiscible phases possesses some peculiar properties, such as a wide interface (order of 10 nm), an ultralow and a broad range of interfacial tension (less than 1 µN/m to 1 mN/m), as well as the permeability to small molecules (like water and ions). In addition, the all-water nature also enables ATPSs to be excellent platforms in various biomedical applications such as particle synthesis and artificial cell mimicking. Therefore, inspired by these peculiar properties, this thesis explores a wide range of problems involving the interfacial phenomena that highlight the properties of the all-aqueous interface. In Chapter 4, we systematically study the transformation dynamics of non-equilibrium double emulsions towards their equilibrium state. Two immiscible aqueous phases with a wide range of interfacial tension and viscosity are introduced to form the unstable double droplets in microfluidic channels. After systematically studying the dynamics of these unstable droplets, we conclude a universal scaling law to predict the dewetting velocity of the shell during the transformation of unstable double emulsions. In Chapter 5, by utilizing the permeability of the all-aqueous interface, we successfully achieve multi-layered (up to 4 layers) all-aqueous emulsion droplets from the phase separation of single-layered emulsion droplets in microfluidic channels. In particular, a phase diagram illustrating the complexity of resulting droplet is constructed, where the layer and size of resulting complex droplets are successfully predicted from this phase diagram. In Chapter 6, we report that fingers emerging at the non-equilibrium all-aqueous interface in a vertical Hele-Shaw cell can spontaneously break into an array of droplets. A phase diagram with a wide parameter space where finger breaking occurs or not is concluded. This spontaneous breakup of interfacial fingers suggests a new way to fabricate all-aqueous emulsion droplets. In Chapter 7, by combining spontaneous fragmentation and phase separation of surfactant-laden all-aqueous films on another aqueous substrate, we propose an interfacial emulsification strategy with potential up-scalability to rapidly produce small and uniform all-aqueous droplets. In particular, using the resulting droplets as templates, we successfully fabricate all-aqueous double emulsion droplets and hydrogel microparticles. To conclude, the main focus of this thesis is utilizing the special properties of ATPSs to study some fundamental interfacial phenomena, including dewetting dynamics in double emulsion droplets, mass-transfer induced phase-separation, fingering instability at the non-equilibrium all-aqueous interface, and interfacial emulsification of thin aqueous films. We believe that the above studies would help add to the understanding of interfacial dynamics as well as inspire some novel applications within ATPSs.