Directed Motion of Colloidal Particles via Chemical Reactions: Osmotic Propulsion
<p>Recent experiments showing reaction-driven propulsion at nanoscales have appeared as a possible mechanism for the transport of particles that may help us to not only understand chemo-mechanical transduction in biological systems, but also to create novel artificial motors that mimic living...
| Summary: | <p>Recent experiments showing reaction-driven propulsion at nanoscales have appeared as a possible mechanism for the transport of particles that may help us to not only understand chemo-mechanical transduction in biological systems, but also to create novel artificial motors that mimic living organisms and which can be harnessed to perform desired tasks. Reaction-driven propulsion consists of the generation of a localized potential gradient by an on-board surface chemical reaction. In this study, we propose and investigate a model for self-propulsion of a colloidal particle --- the osmotic motor --- immersed in a dispersion of ``bath" particles. The non-equilibrium concentration of bath particles induced by a surface chemical reaction creates an osmotic pressure imbalance on the motor causing it to move. The departure of the bath particle concentration distribution from equilibrium is governed by the Damkohler number Da --- the ratio of the speed of reaction to that of diffusion --- which is employed to calculate the driving force on the motor, and from which the self-induced osmotic velocity is determined via application of Stokes drag law. To illustrate the significant physics in osmotic propulsion, a first-order surface reaction on a portion of the motor's surface is assumed, for the most part, in this work. The implications of these features for different bath particle concentrations and motor sizes are discussed. Furthermore, we investigate the role played by the distribution of reactions on the motor's surface. Different responses are expected depending on the amount of reactive surface in the limiting behaviors of the reaction speed. Lastly, we consider a motor with constant production of particles on a hemisphere as a model that resembles actin-based motility of biological cells and organelles.</p>
<p>This research demonstrates that such an osmotic motor is possible and addresses such questions as: How fast can the motor move? How large of a force can it generate? What is the efficiency of such an osmotic motor? All motor behaviors discussed in this work are shown, after appropriate scaling, to be in good agreement with Brownian dynamics simulations.</p>
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