Abstract:
Dielectrophoretic levitation of bubbles in an insulating liquid appears to be a novel way of studying acoustic radiation from transient bubble volume pulsations. Single bubbles were trapped in a spatially varying dc electric field designed to give stable levitation. A few cycles of a weaker oscillating field were superposed on the levitation field. When the modulation frequency was adjusted for the electric field to excite the monopole resonance, a transient acoustic signature was detected by an electrically shielded hydrophone in the surrounding oil bath. One mechanism for monopole excitation is that the energy of the bubble is increased by the electric field giving a positive electric pressure contribution proportional to the square of the field [P. L. Marston and R. E. Apfel, Phys. Lett. 60A, 225--226 (1977)]. The same result follows from a surface average of the Maxwell stress using the Minkowski form of the stress tensor. Additional monopole coupling mechanisms may include a Helmholtz dielectric stress contribution (mitigated by the electrostrictive liquid response) and vertical oscillations of the bubble's centroid in the hydrostatic pressure field. [Work supported by the Office of Naval Research.]