To jump or not to jump: Adhesion and viscous dissipation dictate the detachment of coalescing wall-attached bubbles

TL;DR

This study combines experiments and simulations to understand how adhesion and viscous forces influence bubble detachment during coalescence on surfaces, revealing a critical adhesion energy threshold and a predictive energy model.

Contribution

It introduces a new global energy balance model for bubble coalescence outcomes and uncovers the role of viscous dissipation and adhesion in bubble detachment.

Findings

Critical adhesion energy threshold of ~15% of surface energy for detachment.

The bubble neck generally does not contact the surface during coalescence.

The developed model predicts coalescence outcomes across various conditions.

Abstract

Bubble coalescence can promote bubble departure at much smaller sizes compared to buoyancy. This can critically enhance the efficiency of gas-evolving electrochemical processes, such as water electrolysis. In this study, we integrate high-speed imaging experiments and direct numerical simulations to dissect how and under which conditions bubble coalescence on surfaces leads to detachment. Our transparent electrode experiments provide new insights into contact line dynamics, demonstrating that the bubble neck generally does not contact the surface during coalescence. We reveal that whether coalescence leads to bubble departure or not is determined by the balance between surface energy, adhesion forces, and viscous dissipation. For the previously unexplored regime at low effective Ohnesorge number, a measure of viscosity that incorporates the effect of asymmetry between the coalescing bubbles, we identify a critical dimensionless adhesion energy threshold of $\approx$15% of the released surface energy, below which bubbles typically detach. We develop a global energy balance model that successfully predicts coalescence outcomes across diverse experimental conditions.