| Abstract
| - The effect that surface-active solutes, such as aliphatic alcohols and sodium dodecyl sulfate(SDS), have on the extent of bubble coalescence in liquids under different sonication conditions has beeninvestigated by measuring the volume change of the solution following a period of sonication. In general,the adsorption of surface-active solutes onto the bubble surface retards bubble coalescence. Within thelimitations of the measurement method and the systems studied, bubble coalescence does not appear tobe dependent on the applied acoustic power. Also, varying the applied acoustic frequency has a minimaleffect on the extent of bubble coalescence in systems where long-range electrostatic repulsion betweenbubbles, imparted by the adsorbed surface-active solutes, dominates. However, when short-range stericrepulsion (or other short-range repulsive forces) is the primary factor in inhibiting bubble coalescence, thedependence on the applied acoustic frequency becomes apparent, with less coalescence inhibition at higherfrequencies. It is also concluded that SDS does not reach an equilibrium adsorption level at the bubble/solution interface under the sonication conditions used. On the basis of this conclusion, a method is proposedfor estimating nonequilibrium surface excess values for solutes that do not fully equilibrate with the bubble/solution interface during sonication. For the case of SDS in the presence of excess NaCl, the method wasfurther employed to estimate the maximum lifetime of bubbles in a multibubble field. It was concluded thatan acoustic bubble in a multibubble field has a finite lifetime, and that this lifetime decreases with increasingapplied frequency, ranging from up to 0.35 ± 0.05 ms for 213 kHz to 0.10 ± 0.05 ms for 1062 kHz. Theseestimated lifetimes equate to a bubble in a multibubble field undergoing an upper limit of 50−200 oscillationsover its lifetime for applied ultrasound frequencies between 200 kHz and 1 MHz.
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