Authors: Shan Li
Source: Open Scholarship Institutional Repository (Washington University in St. Louis) (article, 2021-10-13)
Field: Materials Science — Surfaces, Coatings and Films

Investigation of Microdroplet Generation, Morphological Evolution, and Applications Under Quasi-steady and Dynamic States Microscale droplets are commonly encountered in the fields of materials processing, thermal fluids, and biology. While these droplets are naturally occurring, recent advances in microfabrication have enabled researchers to harness their enhanced transport characteristics for numerous laboratory and industrial applications from controlled chemical synthesis to inkjet printing and thermal management. Smaller droplets have larger specific surface area and a greater perimeter-to-area ratio when resting on a surface (i.e., sessile), which accelerates processes occurring at droplet surfaces like evaporation, chemical reaction, or combustion. The demand for microdroplets with smaller and more uniform sizes has motivated investigation of how such droplets can be created using robust and repeatable methods.Here, I present a comprehensive study of the generation, morphological evolution, and the potential implementations of droplets formed under quasi-steady (sessile) and dynamic (spray) states. Sessile droplets were generated and retained by pumping water into hollow micropillar structures with different cross-sectional shapes. The sharp edges of the hollow micropillar structures create an energy barrier that hinders the further advance of the three-phase (liquid-solid-gas) contact line, accompanied by an increase in the contact angle. This feature hinders the formation of flat liquid films on substrates, thus providing a longer triple phase contact line per unit area and potentially enabling the use of these structures in heat transfer devices. The shape evolution and evaporation rate of microdroplets pinned on micropillars with circular, square, and triangular cross-sections are investigated. For dynamic droplet studies, sprays comprising high velocity microdroplet clusters are generated using a micromachined nozzle array driven at ultrasonic frequencies by vibration of piezoelectric actuators, breaking the surface tension energy to create droplets. The microscopic evolution of the droplet profile, velocity, diameter, and ejection modes were investigated to evaluate atomization performance. Pinned sessile microdroplets and fine sprays with uniform droplet sizes benefit different applications. For example, the steady droplet profile of pinned sessile microdroplets allows controllable evaporative cooling, while fine sprays are widely used for inkjet printing, combustion, and materials synthesis, among others. In this project, the heat transfer performance of droplets pinned on micropillars with different cross-sections was investigated, and the overall ejection behavior from nozzle microarrays was characterized. The dissertation work therefore consists of three components: (1) design and fabrication of hollow micropillars and arrays of microscopic nozzles/orifices, (2) investigation of the shape evolution of pinned sessile microdroplets and free droplet clusters in sprays, and (3) characterization of both the heat transfer performance of pinned microdroplets and the overall ejection behavior. Results elucidate the complex physics governing two different droplet formation regimes, while also directly applying observations to two emerging implementations of microdroplets. Computer science ; Biological system ; Biology Surface Modification and Superhydrophobicity ; Polymer Surface Interaction Studies ; Block Copolymer Self-Assembly