Abstract
Recently it was reported that the interface mobility of bubbles and emulsion droplets can have a dramatic effect not only on the characteristic coalescence times but also on the way that bubbles and droplets bounce back after collision (Vakarelski, I. U.; Yang, F.; Tian, Y. S.; Li, E. Q.; Chan D. Y. C.; Thoroddsen, S. T. Sci. Adv.2019, 5, eaaw4292). Experiments with free-rising bubbles in a pure perfluorocarbon liquid showed that collisions involving mobile interfaces result in a stronger series of rebounds before the eventual rapid coalescence. Here we examine this effect for the case of pure water. We compare the bounce of millimeter-sized free-rising bubbles from a pure water–air interface with the bounce from a water–air interface on which a Langmuir monolayer of arachidic acid molecules has been deposited. The Langmuir monolayer surface concentration is kept low enough not to affect the water surface tension but high enough to fully immobilize the interface due to Marangoni stress effects. Bubbles were found to bounce much stronger (up to a factor of 1.8 increase in the rebounding distance) from the clean water interface compared to the water interface with the Langmuir monolayer. These experiments confirm that mobile surfaces enhance bouncing and at the same time demonstrate that the pure water–air interfaces behave as mobile fluid interfaces in our system. A complementary finding in our study is that the ethanol–air interface behaves as a robust mobile liquid interface. The experimental findings are supported by numerical simulations of the bubble bouncing from both mobile and immobile fluid interfaces.
Highlights
The dynamic interaction between deformable bubbles and droplets are of continued research interest because of their involvement in numerous industrial applications, naturally occurring phenomena, and biological processes.[1−8] The outcome of the collision between two bubbles or droplets is primarily determined by the hydrodynamic interaction that acts from separation distances comparable to the size of bubbles and droplets down to the submicrometer thicknesses of the thin liquid film formed between the two colliding surfaces
First we evaluate the mobility of freely rising bubbles in water and ethanol over the range of bubble sizes investigated
The terminal rise velocity, UT, in each case depends on the bubble Reynolds number, Re = ρDU/μ, where ρ is the density of the liquid, μ is the liquid shear viscosity, D is the bubble diameter, and U is the velocity
Summary
The dynamic interaction between deformable bubbles and droplets are of continued research interest because of their involvement in numerous industrial applications, naturally occurring phenomena, and biological processes.[1−8] The outcome of the collision between two bubbles or droplets is primarily determined by the hydrodynamic interaction that acts from separation distances comparable to the size of bubbles and droplets down to the submicrometer thicknesses of the thin liquid film formed between the two colliding surfaces. Along a liquid−solid interface the liquid molecules are immobile and the fluid velocity is zero, or what is known as the no-slip boundary condition. In practical situations the adsorption of surface active molecules at the gas−liquid or liquid−liquid interface can effectively immobilize the interface due to Marangoni stress related effect and make it behave as a solid interface.[12−15] For a gas−liquid interface, because of the large difference between the gas and liquid viscosities, the interface cannot sustain any significant tangential stress, which is referred to as fully mobile or a free-slip liquid interface. For a clean liquid - liquid interface the mobility is determined by the ratio of the liquids viscosities. A droplet with much lower viscosity than the Received: March 10, 2020 Revised: May 7, 2020 Published: May 7, 2020
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