Stabilization of a capillary bridge in air beyond the Rayleigh–Plateau limit in low gravity using acoustic radiation pressure

Abstract

In the absence of gravity, cylindrical capillary bridges consisting of liquid between two circular supports naturally become unstable and break when the length of the bridge exceeds its circumference. This limit is known as the Rayleigh–Plateau limit. The stability of liquid bridges is important to various technologies even in normal gravity. In experiments performed aboard NASA’s low gravity KC-135 aircraft, it was found that acoustic radiation pressure can be used to stabilize capillary bridges against breakup. Capillary bridges composed of a mixture of water and glycerol were deployed in a 21 kHz ultrasonic standing wave in air. The bridges were extended to a length about 11% beyond the natural limit, and the extended bridges broke immediately when the ultrasound was turned off. In contrast with previous work [M. J. Marr-Lyon et al., J. Fluid Mech. 351, 345–357 (1997)], this stabilization method does not use active feedback; the stabilization is a passive effect of the sound field. The acoustic wavelength is chosen such that the radiation pressure due to the sound field is a function of the local bridge radius, such that areas of larger radius are squeezed, and areas of smaller radius are expanded. [Work supported by NASA.]