Dynamics of three-phase systems: Bubbles, droplets and their interactions with surfaces
One of the most ubiquitous phenomena in daily life, boiling, remains surprisingly poorly understood. While boiling’s technical applications are widely employed, many of the fundamental dynamics of individual bubbles and their interactions with the surface are not well known, and lack of experimental measurements of the interaction with surfaces has led to the development of countless competing numerical models. Experimentally, bubbles provide a challenge because they are fast and stochastic, and obtaining accurate measurements of the dynamics, especially those beyond basic shape measurements, remains elusive. In this work, I have developed and implemented novel experimental techniques to illuminate the dynamics of both bubbles and droplets. I have designed a system which permits continuous locally high heat fluxes of up to 250 W/cm2 while still maintaining single bubble boiling. With this, a new spectra of bubbles has been measured, beginning with the classical picture of a bubble growing and gradually pinching off, but many more modes have been observed. Surprisingly a loose correlation is observed in which the lower energy mode bubbles are actually more prevalent at higher heating powers. When non-classical bubbles are observed their contribution to the overall heat transfer from the surface is unexpectedly high. Turning to the specific interactions with surfaces, we have observed a novel type of oscillation mode by heated bubbles sitting on a surface. As a surface becomes more hydrophilic, a new frequency-volume relation emerges which is currently not predicted by theory. Utilizing a novel thermoreflectance sensor which can enable high-speed single-shot measurements of temperature fields, I have identified different cooling mechanisms for the classical and non-classical bubbles. Due to the high heat fluxes accessible by our measurement, we have determined that even the non-classical bubbles cool the surface through substantially different pathways than had previously been observed. Building on work in which surface wettability under bubbles were modified by exposing a metal-oxide surface to ultraviolet illumination, I finally address the application of UV-responsive ZnO surfaces to colloidal particles for emulsion destabilization. Utilizing a holographic microscope, we have measured in situ the change of wettability of colloids sitting at an oil-water interface, providing a pathway to develop photoswitchable emulsions.
Pickering Emulsions, Surfaces, Boiling, Heat Transfer, Time-Resolved Measurements
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign