Non-spherical osmotic motor: chemical sailing

Abstract

The behaviour of a non-spherical osmotic motor – an axisymmetric catalytic particle self-propelling in a dilute dispersion of reactant particles – is considered. In contrast to a conventional osmotic motor that creates differences in concentration, and hence in osmotic pressure, due to asymmetry in reaction rate along its surface (e.g. a Janus particle with reactive and non-reactive patches), a non-spherical particle is able to move even with uniform chemical activity on its surface. For small departures from a sphere the velocity of self-propulsion is proportional to the square of the non-sphericity or distortion of the particle shape. It is shown that the inclusion of hydrodynamic interactions (HI) may drastically change the self-propulsion. Except for very slow chemical reactions, even the direction of self-propulsion changes with and without HI. Numerical calculations at finite non-sphericity suggest that the maximum velocity of self-propulsion is obtained by a sail-like motor shape, leading to the name 'chemical sailing'. Moreover, no saturation in the speed of propulsion is found; the motor velocity increases as the area of this 'sail' grows and its thickness decreases. The self-propulsion of a non-spherical particle releasing products of a chemical reaction – a constant flux motor – is also considered.

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