| dc.description.abstract | Contactless handing of drops in air is an important technique for various basic researches and applications requiring high purity and less contamination. In recent years, acoustic levitation has attracted a great deal of interests due to its less restriction in materials. By using the acoustic radiation force, the drop can be suspended around the pressure node of an intensive acoustic standing wave. However, the stability of acoustic levitation is still one of the problem comparing with other techniques like electrostatic or magnetic levitation.
In this work, the stability of mm sized water drops suspended in a single-node acoustic levitation system is investigated experimentally. The unstable self-excited oscillations, including quasi-periodic oscillation, chaotic oscillation, and shedding of the drop by decreasing the acoustic wave intensity below a threshold,
are observed. The perturbation of the local acoustic field made by the drop motion in a small cavity could be a possible source for generating an effective negative damping force to sustain the growing amplitude of the oscillation and leads to the unstable of the levitation.
The self-excited modes are determined by the interaction between surface tension, gravity, drop inertia, acoustic intensities, and mode numbers. The large drop shows self-excited quasiperiodic motion formed by a vertical oscillation mode and a zonal-like shape oscillation mode with growing coupling and amplitude, until falling to the ground. For small drops, chaotic shape oscillations are composed of several interacting spectrum-broadened sectorial modes and parametrically induced zonal modes. The high amplitude shape oscillation leads to droplet shedding at highly stretched lobes. | en_US |