Interfacial dynamics of aqueous two-phase systems

Abstract

Microfluidic aqueous two-phase system (ATPS) has rich interfacial physics thanks to its unique properties. Two immiscible aqueous phases possess ultra-low interfacial tension which is typically orders of magnitude lower than common liquid-liquid and liquid-air systems. The interface is partially permeable, hence allows certain degree of diffusion and osmosis. In this thesis, I show that the aqueous-aqueous interfaces demonstrate interesting interfacial phenomena, namely jet with extensively long breakup length, jet surface highly susceptible to deformation under minute vibration, formation and rupture of interfacial finger under external perturbation, and new concentric interfaces formed under phase separation.

Attributed to the ultra-low interfacial tension, an aqueous-aqueous jet commonly exhibits a long jetting morphology and is not susceptible to spontaneous breakup. Investigating the interaction of the interfacial, inertial and viscous forces in ATPSs, I identify the criterion to set apart dripping and jetting phenomena in microchannel flow. At the dripping-to-jetting transition, the small interfacial force is balanced by the inertial and viscous forces, which are both minute due to the slow fluid speed. The inertial and viscous forces are comparable in magnitude and hence neither of them shall be neglected. This finding has application in tensiometry.

By imposing a minute sound wave, an aqueous-aqueous interface deforms. The slow interfacial dynamics gives a relevant timescale for the interface to develop and evolve into different morphologies in response to different applied sound waves. Hence, the interface can capture and visualize minute sound waves as interfacial ripples with the exact frequency, and sense the sound signal with different amplitudes and waveforms. With this approach, I develop a microfluidic acoustic sensor to record music. This technology has potential applications on infrasound sensing and fabrication of complex microfibers.

Upon stronger perturbation, interfacial fingers are developed on the aqueous-aqueous interface. Studying the evolution of the interfacial finger, from formation to decay or rupture, I find the interfacial finger breaks up with a length-to-width ratio specific to the Rayleigh-Plateau instability. Since the interfacial fingers are generated locally following each cycle of the perturbation, this study has implication on formation of aqueous-aqueous emulsions with small size, controlled frequency and high mono-dispersity in size.

In the presence of osmotic difference, an ATPS undergoes extensive diffusion, which can lead to cyclic phase separation of the dispersed phase. Consequently, new interfaces are generated and the droplet size changes. I investigate the formation and evolution of the concentric interfaces of ATPSs. The phase separation and coalescence of newly formed cores take place simultaneously. Through cyclic phase separation, a single emulsion transforms into a quadruple emulsion with a core-shell structure. Hence, this work has application on generation of multi-compartmental aqueous-aqueous emulsions.

I find that ATPSs demonstrate slow dynamics and minute forces at the dripping-to-jetting transition, high flexibility to form interfacial ripples and fingers, and cyclic formation of new interfaces due to mass transfer and phase separation. This thesis reveals various distinctive interfacial behaviors of ATPSs, enriching the understanding of liquid-liquid systems with ultra-low interfacial tension and partially permeable interface.