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Title: Electrovariable liquid interfaces for optical applications : structure and dynamics
Author: Marinescu, Monica
ISNI:       0000 0004 2715 2936
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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Due to their versatility, liquid/liquid interfaces are currently of great interest for a variety of optical applications, such as variable-focus lenses, microfluidics, optical switches and display elements. This thesis explores the properties of the electrified interface between two immiscible electrolytic solutions (ITIES ) in two types of systems. The ITIES formed between a droplet and the surrounding liquid, supported on an electrode, has been shown to provide a viable ultra-low-voltage alternative to currently used systems. Due to the electrowetting effect, significant changes in the contact angle are obtained as the result of small variations in the applied voltage, 1 V. On real electrodes, chemical and physical inhomogeneities were found to lead to significant hysteresis of the contact angle variation. Pulsed voltage was used to reduce hysteresis and increase the interval of angles reached. A theoretical model of the droplet dynamics is developed by analogy to the harmonic oscillator. The predicted time-dependent motion of the system rationalises the behaviour observed experimentally, and helps in the design of viable systems. The developed model could also provide a new framework for the study of the role of friction in wetting dynamics. Beside changing its shape, another way to functionalise the ITIES is by allowing nanoparticles to spontaneously assemble at the interface. Once localised, their arrangement greatly influences the response of the system to incident light. An external voltage can control the response time and the structure of this assembly, as well as make it reversible. Such systems are of great interest for controllable optics and sensing applications. There are few independent characterisation methods of populated interfaces. A model is developed for the capacitance of an ITIES populated by metal nanoparticles. Model predictions allow to extract important information from capacitance measurements on the structure of the adsorbed layer, promoting this method as a quantitative characterisation tool. The Faraday rotation from a monolayer of dielectric nanoparticles at an ITIES is calculated in a dipolar approximation. The particle density is assumed to be reversibly controlled by an applied potential. The difference between the angle of rotation caused by particles dissolved in the bulk and that caused by a monolayer arrangement is predicted to be large enough to make low-energy switchable Faraday isolators possible.
Supervisor: Kornyshev, Alexei Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral