by Professor John F. Brady
Abstract: The design of nanoengines that can convert stored chemical energy into motion is a key challenge of nanotechnology, especially for engines that can operate autonomously. Recent experiments have demonstrated that it is possible to power the motion of nanoscale and microscale objects by using surface catalytic reactions â€“ so-called catalytic nanomotors. The precise mechanism responsible for this motion is not known, although a number of ideas have been put forth. Here, a very simple mechanism is proposed: osmotic propulsion. A surface chemical reaction creates local concentration gradients of the reactive and product species which generate a net osmotic force on the motor. The concentration distributions are governed by the ratio between the surface reaction velocity and the diffusion velocity of the reactants. For slow reactions the reaction velocity determines the self-propulsion. When surface reaction dominates over diffusion the osmotic velocity cannot exceed the diffusive speed of the reactants. The implications of these features for different reactant concentrations and motor sizes are discussed. Theoretical predictions are compared with Brownian Dynamics simulations. Through osmotic propulsion the motor is able to harness the ever-present random thermal motion via a chemical reaction to achieve directed autonomous motion.
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