Extraordinary stability of surfactant‐free bubbles suspended in ultrasound

Gravity‐induced drainage is one of the main destabilizing mechanisms for soap bubbles and foams. Here we show that solely through acoustic levitation without introducing any chemical stabilizers, liquid drainage in the bubble film can be completely inhibited, therefore leading to a significant enhancement of bubble lifetime by more than two orders of magnitude and enabling the bubble to survive puncturing by a needle. Based on sound simulation and force analysis, it has been found that acoustic radiation force, exerted on both the inner and outer surfaces of the levitated bubble, acts in opposite directions, thus providing a squeezing effect to the bubble film. The hydrostatic pressure that induces drainage has been balanced by the acoustic radiation pressure exerted on both sides of the film, which is at the origin of the sound stabilization mechanism. This study provides new insights into the interplay between sound and soap bubbles or films, thus stimulating a wide range of fundamental research concerning bubble films and expanding their applications in bio/chemical reactors. In this study, we report that, for the first time, extraordinary stability of liquid bubbles can be obtained from acoustic levitation. The lifetime of bubbles can be prolonged by more than two orders of magnitude owing to the stabilization by the sound field, for instance, a pure water bubble can remain unburst for about 8 min. This suggests the bubble can be stabilized without the presence of any surfactant or solid particles, thus preventing contamination by these materials. The bubbles can be kept both stable and clean, which is highly desired for the applications of these bubbles as bio/chemical reactors.