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.

Cavitation is the appearance and action of gaseous voids (bubbles) in fast liquid flows (e.g. close to ship propellers) or in intense ultrasonic fields. Bubbles form, expand and collapse, usually with after bounces. A strong collapse of bubbles can lead to extreme conditions inside the bubble and to severe pressure waves and shocks in the liquid. Close to boundaries, as e.g. solid boundaries, the collapsing bubble deforms. Usually fast liquid jets form, that pierce the bubble. Further phenomena related to collapse and jet formation are, e.g., bubble splitting or the formation of vortices in the liquid. Collapsing bubbles can erode the solid material. Well-controlled bubble oscillations, on the other hand, can be utilized to clean surfaces. The phenomena accompanying bubble collapse depend on the parameters of the setting, like, e.g., bubble size, liquid properties or geometry. There are several mechanisms leading to the formation of jets during the collapse phase of a bubble. Of particular interest here is a type of thin, very fast jet. Its formation is attributed to self-impact of liquid, that rapidly rushes inward during bubble implosion. For bubbles close to flat solid boundaries, this phenomenon only has been discovered in recent years. We find, that this type of jet is not an isolated phenomenon, being realized only under very special conditions, but rather appears frequently. It is found for bubbles collapsing close to non-trivial geometries (as e.g. the top of a pillar) and for bubbles with a deformed initial shape. It persists for a wide range of liquid viscosities. While this fast jet for millimeter sized bubbles in water might not be the main actor for surface erosion, fast jets from smaller bubbles or in more viscous liquids can have an effect. Fast jet formation also carries over to acoustically excited bubbles oscillating at a solid boundary. Bubble dynamics close to a solid under acoustic excitation, in general, is involved. Bubbles reveal intriguing shapes. Repeated jet formation and bubble splitting can be observed. Depending on parameters, bubbles can detach from the solid at some stage and jets directed away from the solid wall can form, which could have beneficial effects, e.g., for transporting material away from the surface. Results were obtained by means of a combination of numerical and experimental work. The experimental work has been undertaken by our cooperation partner at the Georg-August-University Göttingen (Germany). Results of the project are relevant for improving our understanding on cavitation erosion mechanisms but also on mechanisms for cleaning surfaces. A wide range of technical and also medical applications may profit from the findings.