THE DYNAMICS OF ULTRASONICALLY LEVITATED DROPS IN AN ELECTRIC-FIELD

Ultrasonic and electrostatic levitation techniques have allowed the ex perimental investigation of the nonlinear oscillatory dynamics of free droplets with diameter between 0.1 and 0.4 cm. The measurement of the resonance frequencies of the first three normal modes of large amplit ude shape oscillations in an electric field of varying magnitude has b een carried out with and without surface charges for weakly conducting liquids in air. These oscillations of nonspherical levitated drops ha ve been driven by either modulating the ultrasonic field or by using a time-varying electric field, and the free decay from the oscillatory state has been recorded. A decrease in the resonance frequency of the driven fundamental quadrupole mode has been measured for increasing ob late deformation in the absence of an electric field. Similarly, a dec rease in this frequency has also been found for increasing DC electric field magnitude. A soft nonlinearity exists in the amplitude dependen ce of the resonant mode frequencies for freely decaying as well as ult rasonically and electrically driven uncharged drops. This decrease in resonance frequency is accentuated by the presence of free surface cha rge on the drop. Subharmonic resonance excitation has been observed fo r drops in a time-varying electric field, and hysteresis exists for re sonant modes driven to large amplitude. Mode coupling from lower-order resonances to higher-order modes has been found to be very weak, even for fairly large amplitude shape oscillations. Most of these results are in general agreement with predictions from recent analytical and n umerical investigations. (C) 1996 American Institute of Physics.