Coalescence-induced jumping of bubbles in shear flow in microgravity

Bubble removal from a solid surface is of significant importance to many technical processes and applications. In addition to the conventional buoyancy-aided bubble removal, there is also a passive strategy to remove bubbles from a solid surface via coalescence. However, likewise several processes, the coalescence-induced removal of bubbles from the solid surface is masked by the dominant buoyancy, hence, difficult to observe in terrestrial conditions. Microgravity condition offers a unique opportunity to investigate such phenomenon in great detail that can significantly improve our fundamental understanding. In this work, we report coalescence-induced jumping of isolated vapor bubbles from the heated substrate during shear flow in microgravity condition. We show that, similar to the coalescence-induced jumping droplets, when two bubbles coalesce, the resulting big coalesced bubble jumps from the substrate due to the conversion of excess surface energy into the translational kinetic energy, which provides the requisite initial velocity for jumping. Jumping of bubbles over a wide range of bubble size (post-coalescence radius asymptotic to 0.9 - 3.4 mm) is observed. Bubbles oscillate continuously while rising through certain height post-coalescence. We perform force balance and scaling analysis to develop a model to predict the maximum jumping height of bubbles. We show that the jumping height is strongly related to the bubble size and the non-dimensional Ohnesorge number, which captures the role of fluid properties governing the coalescence. The physical insight presented in this work has implication for the design of energy systems and microfluidic devices for the earth and space-based applications.