Axisymmetric bubble collapse in a quiescent liquid pool. II. Experimental study

We present an experimental study of the detachment of a gas bubble growing quasistatically at constant flow rate conditions from a vertical nozzle placed at the bottom of a quiescent pool of water. In particular, we focus on the dynamics of the necking process and on its dependence on both the Bond and Weber numbers, respectively, defined as Bo=rho ga(2)/sigma, and We(Q)=rho Q(2)/(pi(2)a(3)sigma). Here, a, rho, sigma, g, and Q are the inner radius of the nozzle, the liquid density, the gas-liquid surface tension, the gravitational acceleration, and the gas flow rate. Our experimental data indicate that the collapse process is not only driven by capillarity but also by the liquid hydrostatic pressure. Good agreement is achieved between the measurements of the collapse time and that given by the scaling proposed as t(c)=t(sigma)/1+12(1/3)Bo(2/3) where t(sigma)=(rho a(3)/sigma)(1/2) is the capillary time, valid in the limit We(Q)-> 0. In addition, the details of the final instants previous to pinch-off have been analyzed by recording the time evolution of both the bubble neck radius, R-0, and the axial curvature at the minimum radius, 2r(1), using a high speed digital video camera and an appropriate set of microscopic lenses. We find that the dimensionless, asymptotic law, recently obtained for the inviscid pinch-off of a bubble, given by tau proportional to R-0(2) exp[-ln(R-0(2))], is never achieved down to about 20 mu m. However, the experimental results are accurately reproduced by a pair of two-dimensional Rayleigh-type equations that include liquid inertia as well as surface tension effects. (c) 2008 American Institute of Physics.