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Chemical Swimming
The design of nanoengines that can convert stored chemical energy into motion is an
important 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 – socalled
catalytic nanomotors. The precise mechanism(s) responsible for this motion
is(are) still debated, although a number of ideas have been put forth. Here, a very simple
mechanism is discussed : A surface chemical reaction creates local concentration
gradients of the reactant (the fuel) and product species. As these species diffuse in an
attempt to re-establish equilibrium, they entrain the motor causing it to move. This
process can be viewed either as osmotic propulsion or as self-diffusiophoresis – or more
figuratively as ‘chemical swimming.’ The concentration distributions are governed by
the ratio of the surface reaction velocity to the diffusion velocity of the reactants and/or
products. For slow reactions the reaction velocity determines the self-propulsion. When
surface reaction dominates over diffusion the motor velocity cannot exceed the diffusive
speed of the reactants. The implications of these features for different reactant
concentrations and motor sizes are discussed and the predictions are compared with
Brownian dynamics simulations. We also show that chemically active particles can
attract or repel each other through long-range ‘Coulomb-like’ interactions. And
suspensions of active particles can exhibit Debye-like screening and phase behaviors
analogous to those of a one-component plasma.
Informations contextuelles :
Séminaire du laboratoire Gulliver
Contact : mathilde.reyssat@espci.fr