Impact of viscous droplets on soap bubbles: Coalescence suppresses bubble bursting in viscous regime

The impact of a viscous droplet on various surfaces is a fundamental physical process in nature and numerous technological fields. Despite the significant progress on understanding the impact dynamics of viscous droplets on solid surfaces and liquid films, the impact of viscous droplets on bubbles, especially the influence of droplet viscosity, is still poorly understood. Here, the impact dynamics of water-glycerol droplets with different viscosity on soap bubbles has been experimentally studied by high-speed camera. Similar to the water droplets impacting on soap bubbles, the droplet with higher viscosity rebounds from the soap bubble at low Weber number and passes through the large soap bubble at high Weber number. At high Weber number with small soap bubbles, the coalescence of the soap bubble with the highly viscous droplet mostly occurs, which is different from the “bubble bursting” for the impact of water droplets on soap bubbles reported previously. With the negligible viscous dissipation of droplets on soap bubbles, a general energy balance model is established to capture the transition of different impact behaviors for both low-viscosity and high-viscosity droplets impacting on soap bubbles. Increasing the viscosity of the droplet is found to suppress the bursting of small soap bubbles while promote the coalescence of the droplet with the soap bubble, and even the “passing” of the droplet. The dependence of the impact outcome on the viscosity of the droplet in the regime of small soap bubbles and high Weber number can be interpreted by the three-phase Taylor-Culick retraction.