|Ph.D Student||Horesh Amihai|
|Subject||Acoustic Flow and Interfaces|
|Department||Department of Chemical Engineering||Supervisor||Dr. Ofer Manor|
|Full Thesis text|
In this thesis, we have studied the contribution of MHz-frequency Rayleigh surface acoustic waves (SAWs), in the solid substrate of a microchannel or a flat surface, to the dynamics of thin liquid films. We concentrated on two types of physical systems: During most of my thesis, we have studied the contributions of SAW to the stability of bubbles in microchannels as a simple model for the stability of soft matter particles in microfluidic systems. In addition, we have studied the contribution of SAWs to the dynamic wetting of solids by fully wetting and partially wetting liquids against the action of gravity.
In detail, we used monochromatic light to illuminate bubbles in rectangular microchannels, filled with silicone oil or water to investigate bubble stability in the presence of SAWs. The monochromatic light causes the appearance of light fringes along with the air/liquid interface of the bubbles and the liquid, which we used for analyzing the dynamics of the micron-thick liquid film separating the bubble from the solid substrate of the channel. we showed that the SAWs may contribute in opposing manners to the stability of the liquid film and hence to the stability of the bubbles. Below a power threshold, the SAWs destabilize the thin liquid film, rendering the attachment and pinning of the bubbles to the channel wall. Above a power threshold, the SAWs stabilize the thin liquid film, inhibiting the attachment and pinning of the bubbles to the channel wall.
The viscous penetration of the SAW into the liquid imposes a convective drift of mass, redistributing the fluid in the film against capillary resistance and producing a net drift of liquid in and out of the film. Below the power threshold, the acoustic-capillary balance in the film enhances the drift of liquid out of the film, which results in film thinning and bubble destabilization. Above the power threshold, the acoustic-capillary balance in the film enhances the drift of liquid into of the film until it is balanced by capillary stress, which eventually results in steady film thickness and bubble stabilization.
We further studied the contributions of acoustical and gravitational stresses to the dynamic and static wetting states of liquid atop a vertical substrate, as a model for acoustically enhanced coating. The SAWs gave rise to the continuous climb of films of oil or to the finite climb of the menisci of mixtures of water and a surfactant atop the substrate. The measured rate of oil climb was found to be solely determined by a balance between acoustical and viscous stresses in the liquid, due to acoustic resonance in the film that balances gravitational contributions. However, we found that the curved, partially wetting, water menisci do not support acoustical resonance. Hence, the water menisci were found to climb over the solid substrate to a finite height, which is determined from an interplay between acoustical, capillary, and gravitational stresses.