Capillary dynamics of drops and bubbles: Splashing, wetting, electrocoalescence, inverse coarsening, and thin films.

摘要

Small drops and the thin-films of bubbles are similar in that the surface to volume ratio is large. Consequently, capillary forces, which result from changes in the surface energy, tend to dominate the drop and bubble dynamics. For example, capillarity is responsible for breaking up a liquid jet from a faucet in a sink into a stream of individual droplets, and for coalescing these droplets into a puddle at the bottom of the sink.;This dissertation identifies four situations in which a drop or a bubble exhibits unusual and perhaps counter-intuitive dynamics. The first example (Chapter 2) occurs when a drop impacts either an angled or moving dry, solid surface. Existing physical models attempt to predict the resulting dynamics, spreading or splashing, based on a variety of parameters. Yet it is unclear how these models would extend to include tangential velocity. Our high-speed experiments highlight a distinct third regime in which a fraction of the drop spreads while the other part splashes. The second example (Chapter 3) occurs when a drop contacts a wettable surface with a finite contact angle. Our high-speed experiments challenge the existing models by both showing that the spreading is inertially dominated and that the distance spread follows a power-law scaling in time where the exponent depends on the equilibrium contact angle. The third example (Chapter 4) occurs when two drops are drawn together in an electric field. When the voltage between the drops is low, the drops contact and coalesce. However, when the voltage is sufficiently high, the drops contact and then recoil. The fourth example (Chapter 5) occurs when a bubble on a liquid or solid surface ruptures. Foam coarsening theory would predict that the bubble vanishes when it pops, yet our experiments show that a ring of smaller bubbles is created from the retracting film. This inverse coarsening phenomena is a source of aerosols, and therefore may have implications for health and climate. This dissertation sets out to describe each of these four phenomena and develop plausible physical mechanisms and includes a novel computational approach (Chapter 6) to model retracting thin films.