Coalescence-Induced Jumping Bubbles during Pool Boiling

As micrometric dew droplets grow on a nanostructured superhydrophobic surface, coalescence events trigger a capillary-inertial jumping-droplet effect. Here, we explore how this concept of coalescence-induced jumping can be applied to vapor bubbles during pool boiling, as opposed to condensation. Micro-cavities were laser cut into a slender aluminum substrate, which facilitated clear side-view imaging of microbubbles coalescing at small superheats. Above a critical bubble diameter of ~100 µm, the surface energy released upon coalescence exceeded the work of adhesion of the pinning necks, enabling early bubble departure. In contrast, an isolated bubble departing by buoyancy alone was ~1 mm in size. In addition to the capillary-inertial jumping regime that is analogous to jumping-droplet condensation, we also observed a buoyant-inertial departure at larger sizes, where capillarity only serves to depin the necks and it is buoyancy that balances the inertia of lift-off.