Optimization and Constraints in Sonolithography
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Optimization and Constraints in Sonolithography Paul Campbell1, and Paul Prentice2 1 Carnegie Laboratory for Physics, University of Dundee, Dundee, DD1 4HN, United Kingdom 2 University of Dundee, Dundee, DD1 4HN, United Kingdom 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 a positive sense 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 could be designed so as to afford, for the first time, a level of spatio-temporal control over cavitation processes, and thus exploit the extreme energy-focussing effects that can arise. Specifically, we looked at controlling individual 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 substrate, etching the surface in a controllable manner: we have named this process ‘sonolithography’. Attempting to scale up the process to activate multiple bubbles in parallel is not straightforward however, as we demonstrate herein for the simplest case of two bubbles insonated together. Here, the action of secondary radiation forces exerts significant influence over the activated microbubbles, which acts to direct energy away from the target lithography area. We discuss the salient aspects of these preliminary observations. INTRODUCTION Commercial ultrasound contrast agents (UCA) are normally constituted as a suspension of microbubbles. The microbubbles themselves are usually comprised of lipid or polymer shells encapsulating gas cores, and exhibiting diameters within the range of 2-5 micrometers. UCA were originally developed to enhance echogenicity during sonography of the vascular system, and thus display an intrinsic physical characteristic to scatter incident ultrasound energy strongly. More recently, it has been realized that UCA may also be exploited as useful vehicles for localized molecular delivery, a characteristic that it is hoped can be transformed into a viable clinical tool for non-invasive surgical procedures. In general, upon exposure to ultrasound (US) pressure fluctuations, UCA microbubbles will respo
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