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 μm height; during their lifetime which is shorter than 14 μs they expand to a maximum radius of up to Rmax = 38 μm. Their motion is recorded with high-speed photography at up to 500000 frames s−1. The separation at which equally sized bubbles are created, d, is varied from d = 46–140 μm which results into a non-dimensional stand-off distance, γ = d/(2Rmax), 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 μs), 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/d2.
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