Capillarity-Driven Droplet Ejection

Abstract

Drop Towers provide brief terrestrial access to microgravity environments. When used for capillary fluidics research, a drop tower allows for unique control over an experiment's initial conditions, which enables, enhances, or otherwise improves the study of capillary phenomena at significantly larger length scales than can normally be achieved on the ground. This thesis provides a historical context for the introduction of a new, highly accessible, 2.1s tower design used for capillary research and presents a variety of demonstrative experimental results for purely capillarity-driven flows leading to bubble ingestion, sinking flows, multiphase flows, and droplet ejections. The focus of this thesis is paid to capillarity-driven droplet ejection including historical significance, mathematical models, criteria for ejection and experimental validation. A scale analysis provides a single parameter Su+ which is used to predict the flow velocity at the base of the nozzle. By simplifying the flow in the nozzle we identify two criteria for auto-ejection, the nozzle must be `short' and the velocity of the flow must be sufficient to invert the liquid meniscus and overpower surface tension at the nozzle tip such that We⁺ > 12. Drop tower experiments are conducted and compared to analytical predictions using a regimemap. This thesis also includes results from experiments experiments conducted in a stationary ground-based laboratory and aboard the International Space Station which clearly demonstrate droplet ejection in regimes from transient liquid jets to large isolated drops. Droplets generated in a microgravity environment are 106 times larger than 1g₀ counter-parts.