Optimization and Constraints in Sonolithography

Abstract

The violent interaction between pressure driven cavitation nuclei and nearby rigid substrates is usually a troublesome occurrence, giving rise to damage, and often system failure in hydraulic systems. However, the extreme nature of the phenomenon can also be exploited in situations where deliberate wear or erosion of a material is desireable, such as with the application of shock wave lithotripsy to fragment kidney stones in a medical context. The purpose of the present study was to examine whether a system can be designed so as to afford a level of control over cavitation processes, so that other useful applications might arise. Specifically, we looked at controlling single cavitation nuclei, constituted by encapsulated microbubbles, in proximity to a nearby rigid substrate and activated by ultrasound. This was achieved using a novel optical trapping arrangement, which facilitated establishment of an arbitrary, stable, initial spatial configuration for a bubble system. Critically, exercising optical control in such a way meant that a microbubble could be isolated from a resident population during insonation thus ensuring that ‘cross-talk’ with the rest of the bubble population was minimised. We observed, using high speed microphotography at circa one million frames per second that fine microjets are issued from cavitation microbubbles, and these impact the nearby surface, creating indentations of controllable size. Specific applications may arise that exploit this action, which, in the general case we refer to as ‘sonolithography’. However, scaling up the process to activate multiple bubbles at once, may lead to complications arising via the action of secondary radiation forces. We discuss the salient aspects of our preliminary findings on this subject herein.