Electro-driven interfacial phenomena of droplets, emulsions and liquid marbles

Abstract

The interplay of electric stress and surface tension enriches the interfacial phenomena of droplets, emulsions and liquid marbles. In this thesis, we mainly focus on how the electric stress affects the generation, coalescence/non-coalescence and mixing of liquid droplets as well as liquid droplets with stabilizers. By exploring the science behind these phenomena, we have also extended some approaches to characterize the robustness/ stability of the emulsions and liquid marbles. Chapter 2 focuses on the phenomena of liquid/liquid interfaces in response to an electric field. We studied the dynamics of two pinned droplets under the influence of an applied electric stress. We found that a sufficiently strong field can induce the contact of droplets, leading to three types of droplets behaviors: “coalescence”, “fuse-and-split” and “periodic non-coalescence”. We characterized these dynamic regimes in a phase diagram and explored the physics behind them. This work provides the rationale for understanding the interplay between surface tension and electric stresses that governs the behavior of charged droplets. In chapter 3, we studied the responses of surfactant-stabilized liquid/liquid interfaces to a DC/AC electric field. We found that two contacting droplets stabilized by surfactants can be forced to coalesce upon a critical DC voltage. The critical voltage changes with the type, concentration of surfactants and temperature. Based on this, we devised an approach to probe the threshold disjoining pressure which can help predict the emulsion stability quantitatively. In an AC electric field, we found a critical frequency below which the surfactants-stabilized droplets coalesce immediately. Such critical frequency is highly sensitive to the amplitude of the AC voltage, viscosity of the fluids, concentration and nature of the surfactants. In chapter 4, we studied the generation of liquid droplets utilizing an electric stress; this is a study of liquid/air interface in an electric field. We developed a robust and reliable approach to fabricate multi-compartment particles by combining the microfluidic nozzles with the electrospray ionization. The multi-compartment particles fabricated using this technique can be applied for various applications. For instance, by encapsulating different types of cells, or biological cell factors in each compartment of the particles, our particles can be potentially used for cell studies and co-culture of different cells. In chapter 5, we studied the liquid/air interface stabilized by particles. We focused on the mechanical robustness, electro-coalescence and electro-mixing of the liquid marbles. We first devised a new approach to measure a critical pressure for characterizing the robustness of liquid marbles. Using this technique, we systematically investigated the effect of type and size of stabilizing particles as well as the chemical nature of the liquid droplet on the robustness of liquid marbles. Moreover, we found that a chain of liquid marbles can be forced to coalesce when charged by a sufficiently large voltage. Also, following the electro-coalescence, the electric stress can induce a strong convective flow which can mix the fluids in liquid marbles rapidly and efficiently. Our approach based on electro-assisted manipulation of liquids marbles is especially useful for chemical and biomedical reactions involving multiple reagents and steps.