Dynamics of laser-induced cavitation bubbles near elastic boundaries: Influence of the elastic modulus

The interaction of a laser-induced cavitation bubble with an elastic boundary is investigated experimentally by high-speed photography and acoustic measurements. The elastic material consists of a polyacrylamide (PAA) gel whose elastic properties can be controlled by modifying the water content of the sample. The elastic modulus, E, is varied between 0.017 MPa and 2.03 MPa, and the dimensionless bubble-boundary distance, γ, is for each value of E varied between γ = O and γ = 2.2. In this parameter space, jetting behaviour, jet velocity, bubble migration and bubble oscillation time are determined. The jetting behaviour varies between liquid jet formation towards or away from the elastic boundary, and formation of an annular jet which results in bubble splitting and the subsequent formation of two very fast axial liquid jets flowing in opposite directions. The liquid jet directed away from the boundary reaches a maximum velocity between 300 m s^-1 and 600 m s^-1 (depending on the elastic modulus of the sample) while the peak velocity of the jet directed towards the boundary ranges between 400 m s^-1 and 800 m s^-1 (velocity values averaged over 1 μs). Penetration of the elastic boundary by the liquid jet is observed for PAA samples with an intermediate elastic modulus between 0.12 and 0.4 MPa. In this same range of elastic moduli and for small Υ-values, PAA material is ejected into the surrounding liquid due to the elastic rebound of the sample surface that was deformed during bubble expansion and forms a PAA jet upon rebound. For stiffer boundaries, the bubble behaviour is mainly characterized by the formation of an axial liquid jet and bubble migration directed towards the boundary, as if the bubble were adjacent to a rigid wall. For softer samples, the bubble behaviour becomes similar to that in a liquid with infinite extent. During bubble collapse, however, material is torn off the PAA sample when bubbles are produced close to the boundary. We conclude that liquid jet penetration into the boundary, jet-like ejection of boundary material, and tensile-stress-induced deformations of the boundary during bubble collapse are the major mechanisms responsible for cavitation erosion and for cavitation-enhanced ablation of elastic materials as, for example, biological tissues.