Dynamics of a Laser-induced Bubble Near a Convex Free Surface

This paper experimentally documents the behavior of a laser-induced bubble near a convex paraboloidal free surface with different vertical radius of curvatures, generated in a rotating cylindrical water flask. The shape of the free surface plays a key role on bubble behaviors when bubble collapses. Due to the strong influence of the free surface, the bubble forms a microjet outwards the free surface when it collapses. And the nondimensional bubble collapse time, T_c/T_c’, is smaller than 1, where T_c is the bubble collapse time and T_c’ is the Rayleigh time of bubble collapse with an equivalent maximum radius. As the relative distance between the bubble center and the free surface, h/R_max, decreases, T_c/T_c’ decreases, which is different with the bubble behavior near a flat rigid boundary, where h is the distance between the bubble center and the flat free surface or the convex free surface center, R_max is bubble equivalent maximum radius. Compared with the bubble dynamics near a flat free surface, the bubble near a convex free surface presents a milder collapse behavior. T_c/T_c’ increases with the nondimensional radius of curvature of the free surface center, r_s/R_max, decreases, where r_s is the vertical radius of curvature of the free surface, which is different from the bubble behavior near a curved rigid boundary. It implies that the convex shape of free surface reduces the Kelvin impulse, and the strength of the microjet when bubble collapses decreases with a decreasing r_s/R_max. Meanwhile the convex free surface provides a focusing mechanism to the splash on the free surface when bubble collapses. The splash gets a higher velocity when r_s/R_max decreases.