An acousto-gravitational balance in climbing films of water and oil for separating liquid mixtures

We present the climb of water menisci and silicon oil films up a vertical acoustic actuator. The experimental system is comprised of a vertical 20MHz-frequency surface acoustic wave (SAW) device whose edge is submerged in a reservoir of liquid. Upon the application of power, the SAW propagates down the device toward the liquid reservoir. We monitor the response of the liquid to the SAW. Partially wetting water and surfactant solutions naturally satisfy a finite three phase contact angle between the liquid, vapor, and the solid substrate of the device, which gives rise to curved liquid menisci in the absence of SAW excitation. In the presence of the SAW, the menisci rise up the device to obtain steady state heights above the level of liquid in the reservoir. In contrast, silicon oil supports a vanishing contact angle with the SAW device, which gives rise to flat films of oil. In the presence of the SAW, silicon oil films are found to continuously climb up the device at a constant velocity, which is similar to the corresponding velocity on a horizontal SAW device. We use the difference in the response of oil and water to separate liquid mixtures.