Complex dynamics of cavitation bubbles near objects

This project aims to elucidate the mechanisms how imploding bubbles can erode hardest materials and clean surfaces. Cavitation is the appearance and action of gaseous voids (bubbles) in fast liquid flows or intense ultrasonic fields. It is well- known that the strong collapse of bubbles is the relevant process for energy concentration. Cavitation bubble collapse can lead to strong heating, chemical reactions and plasma inside the bubble, and to severe pressure waves and shocks in the liquid. However, bubble implosions near objects are remarkably complicated events that are not yet completely understood. The collapse can be accompanied by strong bubble deformation, splitting, rapid jet flows through the bubble, and vortex generation. All these phenomena can sensitively depend on the geometry of the solid surface and bubble characteristics. The standard jet flows directed towards a solid surface typically have a speed of the order of 100m/s. However, in previous work we have shown that bubbles expanding and collapsing directly at a solid can develop extremely fast jet flows that are faster by a factor of ten (1000m/s), implying a high relevance for erosion and cleaning. We expect that these peculiar liquid jets can occur under a variety of conditions, e.g. for bubble collapse in various geometries. Furthermore we hypothesize equally involved and partly unknown bubble dynamics for acoustically excited bubbles at a solid surface. Accordingly, we will explore and characterize bubble collapse in several geometric settings, and investigate the behavior of bubbles driven by a sound field for a range of acoustic and geometric parameters. To clarify these scientific questions on bubble dynamics, a combination of experimental and numerical studies are conducted. The experimental work will be undertaken by our cooperation partner at the Georg-August-University Göttingen (Germany). Experimental techniques comprise nucleation of individual bubbles by focused laser pulses and high-speed imaging of bubble shape and shock waves. Bubbles are placed near objects of various geometries, and additionally sound fields can be applied. In numerical simulations the bubble evolution is computed by solving appropriate equations for a gas bubble in a liquid with the help of a computer program. The numerical studies allow to compute details that are not resolved by the experimental method, as e.g. providing information on the bubble interior during jet formation. Results on the complex behavior of collapsing and acoustically driven bubbles at objects will lead to a better understanding of the action of cavitation, directly linked to better control and optimization of numerous technical and medical applications.