UC Santa Barbara

The question my PhD research aims to answer is how light-responsive molecular switches can be used to develop accessible and versatile actuating materials through the control of fluid properties. Light is one of the most abundant natural resources and presents an opportunity to develop materials that can be controlled remotely, precisely and through both intensity and wavelength. Moreover, light-driven processes, particularly when leveraging visible light, provide clean, accessible methods by which scientific advancement can harness an abundant resource to do work; this is important now more than ever as we seek alternative energy sources to mitigate consumption of natural resources contributing to the global climate crisis.My research aims to use photoswitches to control fluids by harnessing changes in polarity and absorbance for changes in interfacial tension and temperature, respectively. To accomplish this, we focus on the molecular photoswitches donor–acceptor Stenhouse adducts (DASAs) and spiropyran. Before we can engineer such actuators, it is crucial that we understand the photophysical properties that drive large changes in interfacial tension and temperature in order to most effectively leverage these properties. First, we developed and synthesized a library of photo-responsive surfactants and provide a comparative study on the influence of polarity change and photophysical effects on wetting at a multiphase interface. Following, we exploit the large change afforded by the photo-responsive surfactants for the depinning of multiphase fluid in microgravity. In this effort, we work to address the challenges associated with boiling in microgravity conditions. Second, we leverage the photothermal effect to drive self-regulating Rayleigh-Bénard convection and phase change actuation. DASA is especially promising for raising the temperature of a solution due to its negative photochromism and its sensitivity to the environment. We exploit the switching and large change in absorption afforded by DASA molecules upon photoswitching to generate thermal gradients in solution which lead to convection. This convection can be turned on/off based on factors that control the extent of bleaching under irradiation. Furthermore, by tuning the environment using additives, we instill another knob by which we may control the switching kinetics to generate or dissipate heat within a fluid system. Through an understanding of these effects, we work towards the goal of developing actuators that could oscillate under constant irradiation. The integration of chemistry and engineering principles presented in this work aims to guide future research in both photochemistry and soft materials fields through a consideration of factors that affect and optimize photoswitching within fluid systems to lay foundations for the development of photo-responsive actuator systems.