Ultrasound cavitation-induced shear stress on rigid and elastic surfaces

• Applicants: Dr. Robert Mettin; Professor Dr. Claus-Dieter Ohl
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505703537

Interest in bubble-induced shear stress is motivated by a variety of technological, chemical, and biomedical applications where this effect is used. Often acoustic cavitation bubbles are involved, and ultrasonic cleaning, micromixing of liquids, intensification of chemical reactions, or heat-exchange processes are examples of such applications in the engineering field. In the biomedical field, ultrasound-mediated drug delivery, ultrasound-induced blood-brain barrier opening, bacteria lysis, or disinfection are examples of bubble-mediated bioeffects. During decades research works mainly focused on the violent mechanisms resulting from cavitation bubble collapses, including shockwave emissions and the generation of microjets. Recent sensitive applications have demonstrated that more weakly oscillating bubbles may also produce significant mechanical effects on rigid or elastic surfaces through the generation of shear stress. This shear stress results from the liquid flows created in vicinity of the oscillating bubbles. Up to now, the influence and modification of surfaces by bubble-induced shear stress has mostly been investigated qualitatively. The quantitative measurement of shear stress, as well as the potential control of the force exerted by an oscillating or a collapsing bubble near rigid and elastic surfaces, remain challenging. The CaviStress project consequently focuses on the quantification of bubble-induced shear stress, through theoretical, numerical and experimental investigations of the interplay between a cavitation bubble and an in-vicinity interface. The main objective of the project is the control and optimization of wall-near stresses induced by oscillating or collapsing bubbles, and its application in two different fields: (i) the cleaning of solid surfaces, and (ii) the molecular uptake of biological cells. We investigate theoretically and numerically the shear stress induced by oscillating and collapsing bubbles both in bulk fluid, and near rigid and elastic walls. The bubble-induced liquid flows are derived theoretically. The fundamental findings are compared to controlled experiments, from the single bubble case to a realistic multibubble streamer where turbulence and mixing occur. Once the liquid flows are characterized, the shear stress is theoretically and numerically quantified. Experimental investigation of the impact of shear stress on rigid walls focuses on its scaling dependence, thus allowing to identify parameter ranges where damage-free cleaning of sensible surfaces is feasible. In parallel, experimental studies of the shear stress on elastic walls focus on the penetration of molecules into biological cells by evaluating the cell poration efficiency from well controlled oscillating or collapsing bubbles. The expected quantification and differentiation of the bubble-induced mechanical effects pave the way towards improved ultrasound based procedures for cleaning and drug delivery through bubbles.

• DFG Programme: Research Grants
• International Connection: France
• Partner Organisation: Agence Nationale de la Recherche / The French National Research Agency
• Cooperation Partners: Professor Dr. Claude Inserra; Professor Dr. Cyril Mauger