59th Annual Report on Research 2014

Bubble-solid interactions play important roles in understanding multiphase flows occurring in many industrial processes like petroleum purification processes. In this second year of the ACS award, we have studied various situations of bubble interacting with solids through an integrated experimental and theoretical approach; 1) Understand how a macroscopic bubble interacts with a particle in a thin fluid film, 2) understand the coupling between fluid interface and elastic body, and 3) characterize the dynamics of a particle penetrating through an interface.

Task 1) Bubble-particle interaction in a thin fluid (Hele-Shaw) cell [About to be submitted]

Hydrodynamic interactions of drops and bubbles with particles in viscous fluids are important in oil purification or separation processes. In this present work, we explore the fundamental mechanism of such complex processes by studying the collision of a single bubble with a fixed solid particle inside a Hele-Shaw cell.  Physical experiments show that an air bubble either splits or slides around the particle depending on the initial transverse offset between the bubble and particle centroids. A bubble slides around the particle until the offset is decreased below a critical value, in which case the bubble splits into two daughter bubbles. We are able to predict this slide-split transition using a theoretical model that compares the relative change in surface energy, gravitational potential and viscous dissipation in the two regimes.

Task 2) Capillary interaction with an elastic helix [Published in Soft Matter, 2014]

We published the results of a combined experimental and theoretical investigation of the capillary instability along an elastic helical thread bound within a fluid. Surface energy interacts with flexible bodies through inducing elastic deformation. In this study, we designed a scale-up experiment to produce an elastic helix that is sufficiently flexible to deform under the influence of surface tension. The flexible coil interacts with the fluid column, and stretches or compresses in response to capillary forces. Our theoretical model results from the minimization of both surface and elastic energies. Theoretical prediction shows the most unstable wavelength can be substantially increased by the influence of the helical coil, which is in a good agreement with experimental measurements.

Task 3) Particle shooting [In progress]

In this study, we investigate the dynamics of a solid particle moving from liquid to air through a liquid-air interface. The experimental setup consists of an air-piston system that shoots a solid particle into water towards the free surface from below. Experimental results indicate that the particle either penetrates or bounces back depending on the particle size, impact speed, and surface tension. In particular, the particle needs to overcome the resistive interfacial forces in order to penetrate through the liquid-air interface. This transition from bouncing to penetration regimes is captured theoretically by conducting a simple force balance and is further compared with experiments.

In this second year of the project, we have performed both experimental and computational approaches for bubble-particle dynamics. This ACS PRF helped us to understand the fundamental multiphase dynamics in fluid interfaces. In the following year, we will further focus on the computational work for further understanding our experimental observations. This award supported one graduate student and one undergraduate to conduct research. Also, the PI and students attended national meetings and conferences to disseminate our results to scientific communities.