How bubbles (can) clean

摘要

When bubbles expand and collapse close to boundaries, a shear flow is generated which is able to remove particles from the surface, thus locally clean it. Here, we demonstrate experimentally with a flow visualization technique that the strongest forcing of particles occurs during a very brief time interval of the bubble oscillation period. During this interval a jet flow impacts and spreads radially along the surface. We argue that this flow pattern is responsible for the removal of particles. It is a well known fact that in ultrasonic cleaning bubbles and not the sound field itself is responsible for removal of particulates from surfaces [1]. With the invention of ultrasonic cleaning at frequencies in the megahertz range (megasonics) [2] it was thought that the importance of bubbles is reduced; yet it showed the opposite [3]. Although, it is commonly accepted that bubbles are cleaning, the mechanism and more specifically the fluid dynamics of cleaning has not been revealed. Here, we demonstrate an experimental approach and report the first results