Detuning/dephasing instabilities in acoustic levitation and Rayleigh disk systems Free

The equation of motion for an acoustically levitated sample or Rayleigh disk in a resonant cavity is equivalent to a mass on a spring with a variable spring constant. The variation in the spring constant for a fixed drive frequency line with sample position (disk angle) [M. Barmatz, J. L. Allen, and M. Gaspar, J. Acoust. Soc. Am. 73, 725–732 (1983)]. The acoustic restoring force (disk torque) and sample displacement (disk angle) become dephased due to the nonzero response time (Q/πf) of the cavity. Due to this retardation, the modulated acoustic force (disk torque) can do either positive or negative work on the sample, leading to instability or superstability (enhanced damping). Simple calculations of the instability threshold and initial growth rate will be presented along with more detailed computer calculations. A video tape illustrating the instability in a ground‐based levitation system will also be shown. [Work supported by NASA.]