Physics of Bubble-Propelled Microrockets

TL;DR

This study uses numerical simulations to analyze the chemical and hydrodynamic processes in bubble-propelled microrockets, identifying optimal designs for maximum propulsion efficiency based on geometry and physicochemical parameters.

Contribution

It introduces a comprehensive numerical model that couples chemical diffusion and hydrodynamics to predict bubble dynamics and microrocket propulsion, optimizing design parameters.

Findings

Hydrodynamics influence the distance traveled per cycle.

Chemistry determines bubble ejection frequency.

Optimal shape and size maximize swimming velocity.

Abstract

A popular method to induce synthetic propulsion at the microscale is to use the forces created by surface-produced gas bubbles inside the asymmetric body of a catalytic swimmer (referred to in the literature as microrocket). Gas bubbles nucleate and grow within the catalytic swimmer and migrate toward one of its opening under the effect of asymmetric geometric confinement, thus generating a net hydrodynamic force which propels the device. In this paper we use numerical simulations to develop a joint chemical (diffusive) and hydrodynamic (Stokes) analysis of the bubble growth within a conical catalytic microrocket and of the associated bubble and microrocket motion. Our computational model allows us to solve for the bubble dynamics over one full bubble cycle ranging from its nucleation to its exiting the conical rocket and therefore to identify the propulsion characteristics as function of all design parameters, including geometry and chemical activity of the motor, surface tension phenomena, and all physicochemical constants. Our results suggest that hydrodynamics and chemistry partially decouple in the motion of the bubbles, with hydrodynamics determining the distance travelled by the microrocket over each cycle while chemistry setting the bubble ejection frequency. Our numerical model finally allows us to identify an optimal microrocket shape and size for which the swimming velocity (distance travelled per cycle duration) is maximized.