Interaction between two laser-induced cavitation bubbles in a quasi-two-dimensional geometry

We report on experimental and numerical studies of pairs of cavitation bubbles growing and collapsing close to each other in a narrow gap. The bubbles are generated with a pulsed and focused laser in a liquid-filled gap of 15 mu m height; during their lifetime which is shorter than 14 mu s they expand to a maximum radius of up to R-max = 38 mu m. Their motion is recorded with high-speed photography at up to 500 000 frames s(-1). The separation at which equally sized bubbles are created, d, is varied from d = 46-140 mu m which results into a non-dimensional stand-off distance, gamma = d/(2R(max)), from 0.65 to 2. For large separation the bubbles shrink almost radially symmetric; for smaller separation the bubbles repulse each other during expansion and during collapse move towards each other. At closer distances we find a flattening of the proximal bubbles walls. Interestingly, due to the short lifetime of the bubbles (<= 14 mu s), the radial and centroidal motion can be modelled successfully with a two-dimensional potential flow ansatz, i.e. neglecting viscosity. We derive the equations for arbitrary configurations of two-dimensional bubbles. The good agreement between model and experiments supports that the fluid dynamics is essentially a potential flow for the experimental conditions of this study. The interaction force (secondary Bjerknes force) is long ranged dropping off only with 1/d as compared to previously studied three-dimensional geometries where the force is proportional to 1/d(2).