Colloidal bubble propulsion mediated through viscous flows

TL;DR

This paper develops a theoretical framework combining diffusive and hydrodynamic principles to explain bubble propulsion in catalytic colloids, identifying key parameters and flow behaviors influencing their movement.

Contribution

It introduces a combined diffusive and hydrodynamic model for bubble growth near colloids, explaining propulsion mechanisms and environmental influences.

Findings

Identifies two key dimensionless groups governing bubble dynamics.

Calculates flow fields analytically for different boundary conditions.

Suggests experimental behaviors may arise from setup peculiarities rather than fundamental physics.

Abstract

Bubble-propelled catalytic colloids stand out as a uniquely efficient design for artificial controllable micromachines, but so far lack a general theoretical framework that explains the physics of their propulsion. Here we develop a combined diffusive and hydrodynamic theory of bubble growth near a spherical catalytic colloid, that allows us to explain the underlying mechanism and the influence of environmental and material parameters. We identify two dimensionless groups, related to colloidal activity and the background fluid, that govern a saddle-node bifurcation of the bubble growth dynamics, and calculate the generated flows analytically for both slip and no slip boundary conditions on the bubble. We finish with a discussion of the assumptions and predictions of our model in the context of existing experimental results, and conclude that some of the observed behaviour, notably the ratchet-like gait, stems from peculiarities of the experimental setup rather than fundamental physics of the propulsive mechanism.