a1 School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
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.
(Received January 03 2009)
(Revised May 13 2009)