Acoustic Surface Cavitation

Merely the presence of compressible entities, known as bubbles, greatly enriches the physical phenomena encountered when introducing ultrasound in a liquid. Mediated by the response of these bubbles, the otherwise diffuse and relatively low energy density of the acoustic field can induce strong, localized liquid motion, high internal temperatures and pressures as well as secondary acoustic emissions. In turn, these effects give rise to considerable stresses exerted on nearby objects and molecular dissociation of the bubble constituents. These unique characteristics of acoustic cavitation enable a wide variety of applications, notably ultrasonic cleaning and sonochemistry. Scientific knowledge of cavitation bubbles predominantly comprises the dynamics and effects of pre-existing spherical bubbles in an infinite liquid medium. Much less is known about the origin of cavitation bubbles or their behavior when in close proximity to a solid surface. A disparity that can be attributed to the experimental difficulties posed by the microscopic length- and timescales combined with the rapid and unpredictable motion which characterizes cavitation bubbles. Still, for successful application of acoustically driven bubbles, thorough understanding of their coming into existence as well as their interaction with solid surfaces is of great importance. In this thesis both these aspects are investigated. An emphasis is placed on the role of microbubbles entrapped inside artificially created micropits. This is motivated by two reasons. First of all, acoustic cavitation bubbles are believed to originate mostly from stable microscopic volumes of gas entrapped in naturally occurring crevices inside a solid object. An artificially created micropit thus constitutes a model for such a crevice. Secondly, modern micromachining techniques enable precise control over the dimensions and locations of such micropits. This greatly facilitates the experimental study of acoustically driven micropit bubbles and the various aspects of the therewith induced surface acoustic cavitation.