Bubble bursting in a sessile droplet
We analyzed experimentally and numerically the bursting of a bubble within a sessile droplet. Our experiments show that both sessile droplet curvature and confinement enhance the energy focusing. In the low-viscosity regime, this effect results in thinner, faster Worthington jets. In the high-viscosity regime, droplets are ejected for values of the Laplace number (the Reynolds number based on the visco-capillary velocity) smaller than the threshold for a bubble in an infinite liquid bath. This is probably the major result of the present work. Numerical simulations show the critical role of the additional pressure gradient arising from the curvature of the sessile droplet interface. The resulting force drives the liquid towards the bottom of the cavity, compressing it and accelerating jet formation. In the low-viscosity limit, the bottom of the cavity becomes smoother before jet ejection. This effect resembles the energy-focusing enhancement that occurs in an infinite liquid bath at the critical Laplace number, where short-wavelength waves are damped by viscosity.