The behaviour of drops in an acoustic levitator is simulated numerically. The ultrasound field is directed along the axis of gravity, the motion of the drop is supposed to be axisymmetric. The flow inside the drop is assumed inviscid (since the time intervals considered are short) and incompressible. First, as a test case, we consider a stationary ultrasound wave. We observe, as in previous experimental and theoretical works, that stable drop equilibrium shapes exist for acoustic Bond numbers up to a critical value. The critical value depends on the dimensionless wave number of the ultrasound. Beyond the critical value, we still observe equilibrium drop shapes, but they are not purely convex (i.e. ‘‘dog-bone’’ shaped) and found to be unstable. Next we modulate the ultrasound pressure level (SPL) with a frequency x 2 , which is comparable to the first few drop resonance frequencies, and a small modulation amplitude. Simulations and experiments are performed and compared; the agreement is very good. We further on investigate numerically the more general case of an arbitrary x 2 (still comparable to the first few drop resonance frequencies, yet). A very rich drop dynamics is obtained. We observe that a resonant drop break-up can be triggered by an appropriate choice of the modulation frequency. The drop then disintegrates although the acoustic Bond number remains below its critical value. |
