Physical Characteristics of Chemically Propelled Colloids

Understanding the transport properties of microorganisms in fluid is a fundamental problem in soft matter physics, and the dynamics of an active colloid in non-equilibrium statistical mechanics has recently attracted pioneering investigations into the design of artificial swimming robots at the microscale. A topical review of the remarkable discoveries in the field, both theoretically and experimentally, is first addressed. The mechanism of interfacial phoretic transport is used for an active colloid achieving autonomous propulsion by diffusiophoresis. A theoretical framework has been established to ascertain the generic properties of the active motion of such a self-propelled colloid, driven by the use of surface catalytic reactions. The kinetic route for the chemical reaction is considered as a two-step process, followed by quantitative procedures that examine the influence of fuel concentration and colloid size on the propulsion velocity. Specifically, both Janus and inhomogeneous colloids are studied, and their propulsion velocities rise linearly with the fuel concentration in a dilute solution and decay with the size scale in the small size limits. The theoretical results for a Janus sphere are consistent with the experimental observation. Furthermore, to what extent can the incorporation of advection into the diffusion be interpreted as a means of self-propulsion. An advection-diffusion model is constructed to compute the concentration distribution of the solute and propulsion velocity, aiming to explore the role played by the advection effects on the movement of Janus particles. The numerical results show that while Janus particle can achieve an autonomous propulsion at a small P\'eclet number, the accumulation of solute particles in the upstream of the colloid disappears at the large Pe limit. In conclusion, the major outcome of this work is the demonstration of the qualitative agreement with the present analysis of mobility to explain the observed size dependence. Lastly, the results suggest that a catalytic colloid powered by diffusiophoresis is a useful model for self-propulsion and indicate what still needs to be done to obtain a full understanding of the swimming characteristics of colloidal dispersions