Acoustic Drainage

Ofer Manor, Technion, Haifa, Israel
Amihai Horesh, Technion, Haifa, Israel
Matvey Morozov, Technion, Haifa, Israel

The drainage of a liquid film, separating solid or fluid particles at close proximity, is a common mechanism in chemical processing. Many times the rate of drainage determines the efficiency of the process employed. Examples are found in floatation processes in which bubbles attach to solid particles in water, coating processes in which particles, suspended in a volatile carrier liquid, must come in contact with a solid surface, and when applying cosmetic or medical lotions in which the active ingredient is contained in an emulsion drop and must traverse an inert carrier liquid in order to contact the skin.

We use experiment and theory to demonstrate a vibration in the form of a MHz-frequency surface acoustic wave (SAW) may reduce the time of drainage. As a model system we consider the drainage of a liquid film between a bubble and a solid surface, employing a microfluidic setup of a reduced geometry akin to the Helle Shaw cell geometry. We contain a long air bubble in silicon oil within a channel and monitor the micron thick oil film, separating the bubble from the solid surfaces of the channel. In the absence of vibration the film drains at a characteristic time of 1000 s. However, in the presence of vibration we find the characteristic drainage time reduces to 10 s.

We extract the spatiotemporal dynamics of the draining oil film by using light microscopy to detect light fringes of equal chromatic order (FECO). Using this technique we observe that the vibration supports the transport of oil mass in the film. We further construct a corresponding theory, where we scrutinize the acoustic flow in the draining oil film, invoked by the vibration. The quantitative comparison between experiment and theory highlights the fast drainage is governed by a balance between capillary and acoustic stresses.