Oscillatory Dynamics of Single Bubbles and Agglomeration in a Sound Field in Microgravity

A dual-frequency acoustic levitator containing water was developed for studying bubble and drop dynamics in low gravity. It was flown on USML-1 where it was used in the Glovebox facility. High frequency (21 or 63 kHz) ultrasonic waves were modulated by low frequencies to excite shape oscillations on bubbles and oil drops ultrasonically trapped in the water. Bubble diameters were typically close to 1 cm or larger. When such large bubbles are acoustically trapped on the Earth, the acoustic radiation pressure needed to overcome buoyancy tends to shitl the natural frequency for quadrupole (n = 2) oscillations above the prediction of Lamb's equation. In low gravity, a much weaker trapping force was used and measurements of n = 2 and 3 mode frequencies were closer to the ideal case. Other video observations in low gravity include: (i) the transient reappearance of a bulge where a small bubble has coalesced with a large one, (io observations of the dynamics of bubbles coated by oil indicating that shape oscillations can shifl a coated bubble away from the oil-water interface of the coating giving a centering of the core, and (iii) the agglomeration of bubbles induced by the sound field. INTRODUCTION A. Background and Motivation Acoustic standing waves have long been used as a method for trapping very small bubbles in liquids in a normal gravitational field (lg). Early descriptions of this trapping are summarized in Refs. 1 and 2 and most applications of the technique to the study of bubble dynamics concern the trapping by acoustic radiation pressure of very small bubbles (typically e100 pm in radius) at the pressure antinode of the standing wave.3 For reasons described in Ref. 1 and reviewed in Sec. I, bubbles larger than a critical size determined by the acoustic frequency are pushed away from pressure antinodes and are attracted to the pressure nodes of a standing wave. This attraction to pressure nodes (due to the radiation pressure or average force on a bubble) is important to the experiments to be summarized since we are primarily concerned with bubbles larger than the aforementioned critical size associated with the Joint '%+I Science Review for USML-1 and USMP-1 with the Microgravity Measurement Group, September 22-24, 1993, Iluntsville, Alabamq USA. 673 https://ntrs.nasa.gov/search.jsp?R=20030075808 2017-10-28T13:45:55+00:00Z