(418a) Self-Propelling Bubbles

In many natural and technological multi-phase systems, bubbles are exposed to surface-active contaminants (surfactants) that may cover the whole or part of the bubble interface. A partial coverage of the interface may result in a spontaneous self-propulsion mechanism, which is yet poorly understood. Self-propulsion is the main feature of active matter, therefore a better understanding of the physics of self-propulsion is relevant to systems able to transduce surface energy into movement, such as swimming microorganisms and autophoretic particles. Controlled active motion is also relevant to the propulsion of drops and bubbles in sensors and microfluidic devices. Here, the flow physics leading to the self-propulsion of partially covered microbubbles is examined numerically. We present high-fidelity direct numerical simulations that simultaneously solves the full system of partial differential equations governing the free-surface flow physics and the surfactant transport on the deforming bubble interface. Our focus is on characterizing the self-propulsion regimes generated by the presence of surface-active species, and the influence of surfactant activity and surface coverage on the active bubble motion. Results show in microscopic detail how surface tension gradients (Marangoni stresses) induced by the uneven interfacial coverage produce spontaneous hydrodynamics flows (Marangoni flows) on the surrounding liquid, leading to bubble motion. Results also establish the influence of both surfactant activity and interfacial coverage on total displacement and average bubble velocity at the macroscale.