Physics of Bubble‐Propelled Microrockets

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.

Magnetic Fields Enhanced the Performance of Tubular Dichalcogenide Micromotors at Low Hydrogen Peroxide Levels

“…[17] Nonetheless, for our system, diffusion-governed transport is favorable as the low flow speed in the catalyst surfacea llows for improved bubble nucleation and growth. [12] This effect also translates into am ean nucleation length X nucleation closer to the inlet opening, letting the bubbles grow bigger and, therefore, exertingabigger thrust. As seen in Figure 4D,t he effect on the speed is noticeably highera tl ow fuel levels.…”

“…As such, the conclusion of the study was that both hydrodynamic and chemical components can be decoupled. [12] As previously stated, scientists have tried to model the motion behavior of self-propelled micromotorsb ym odulating fuel concentration and dimensions. With the advance of materials for micromotor fabrication and new propulsion mechanisms, it is essential to develop new models to study such variables.…”

“…In this work, we studied the influence of externalm agnetic fields on the performance of TMD-based tubular micromotors containing Ni and Pt layers.T MDs provide al arge surface area, whicha long with their good electrical conductivity, facilitatet he growth of high surface inner catalytic layers. [13] The speed boost experienced by micromotors under the effect of applied magnetic fields is modeled by combining solid thrust models previously reported [7,10,12] with computational tools, hence giving aw ell-founded explanationb ased upon both experimental evidence and theoretical propulsion models. We will illustrate by numerical simulations how the application of magnetic fields results in an acceleration of the overall speed,m ainly due to ac hange in the flow regime from turbulentt ol aminar and to an increase in the capillaryf orce exertedb yt he generated bubbles.…”

“…The laminarf lux increases the fuel residence time close to the inner wall, which improves the contact of the fuel with the catalyst, resulting in highero xygen-expellingf requency. [12] In addition, bubble nucleationo ccurs closer to the inlet opening. The effect is more pronounced as the level of fuel decrease.…”

Viscous growth and rebound of a bubble near a rigid surface

“…In that case, the detailed evolution in time of the bubble radius does not have any influence on the geometric evolution of the fluid film and bubble position, which is solely determined by the radius of the bubble itself, hence the bubble radius can be substituted for time as a parameter for the problem (see also Ref. [10]).…”