Bounce, burst, merge, or pass: Impact behaviors of droplets on bubbles

When falling droplets hit a surface, they may splash wildly, rebound, spread out gently, or even generate a bubble on the surface. These bubbles can become landing surfaces for subsequent droplets, leading to a distinct set of impact behaviors that vary with droplet viscosity and velocity.

To better understand these behaviors, Xurui Zhang and Hao Chang captured soap droplet-bubble impacts using high-speed cameras to develop a model to describe impact dynamics.

“The impact of droplets on bubbles plays a key role in anti-bubble production, droplet manipulation, and separation technology,” said author Xurui Zhang.

The duo described how four impact behaviors — bouncing, bubble bursting, coalescence, and passing — correlate with droplet viscosity and bubble size. They found that slow-moving droplets, irrespective of viscosity, bounce off bubbles like a trampoline. In contrast, fast-moving droplets pop large bubbles upon impact.

When it comes to small bubbles, though, only fast-moving, low-viscosity droplets cause bursting. Highly viscous droplets suppress this behavior, allowing coalescence and passing to dominate.

Coalescing droplets slowly drip into the bubble, leaving behind a bead on the bubble’s surface. The vertical retraction of the bubble sliding up along the droplet’s sides follows a three-phase Taylor-Culick retraction, in which it releases energy as it retracts.

A simple energy balance model with negligible viscous dissipation characterizes transitions between impact behaviors.

Zhang and Chang hope to further investigate “passing” and how properties of the soap bubble influence impact behavior.

“We aim to establish a physical model to quantitatively predict and interpret impact behavior,” said Zhang. “We hope our study can complement the existing theory of droplet impact dynamics with new insights, which have direct implications for industrial processes, such as pesticide spraying, fuel injection, and spray coating.”

Source: “Impact of viscous droplets on soap bubbles: coalescence suppresses bubble bursting in viscous regime,” by Xurui Zhang and Hao Chang, Physics of Fluids (2025). The article can be accessed at https://doi.org/10.1063/5.0300616 .