A novel method for sub-micron particle detection in clean liquids

A novel approach to the detection of liquid-borne submicrometer particles (extendible to ultra-clean liquids) is described. The key concept is to coax the submicrometer particles to soft cavitate and then detect the ensuing transient bubble activity acoustically rather than the particle itself (which has only a weak scattering signature). The method, therefore, relies on facilitating acoustic microcavitation through acoustic coaxing. Acoustic microcavitation is brought about by low megahertz acoustic fields giving rise to micrometer-size bubbles that live a few microseconds. Liquid-borne microparticles do not, ordinarily, cause any cavitation when exposed to strong sound fields (of 1 MHz). If, however, a very weak, high-frequency auxiliary acoustic field (e.g., 30 MHz) is added to this sound field, cavitation by the microparticles is readily facilitated. This technique of facilitating cavitation is referred to as "acoustic coaxing". Results of preliminary experiments indicate that even smooth spherical microparticles can be coaxed to cause cavitation. An explanation of the "acoustic coaxing effect" is offered. The physics seems not to be limited by the smallness of microparticles (effect possible up to 50 nm). This novel method based on acoustic coaxing of microcavitation promises to be a good basis for on-line, real time monitoring of liquid-borne submicronic particulates. This method is not limited to small sensing volumes and, unlike optical methods, it has an intrinsic, location specific, signal enhancement at the source particle.