Simulation of diffusio-phoretic motion of colloidal particle suppressed by bound solutes within adsorption shell
Colloidal particle submerged in a non-equilibrium fluid with a concentration gradient of solutes experiences diffusio-phoresis. Such directional transport originates from the driving forces that exert on the fluid in a microscopic boundary layer surrounding the colloid. Based on a simple model of spherical colloid fixed in a concentration gradient of solutes, molecular dynamics simulations are performed to determine the interaction parameters that maximise the diffusio-phoretic mobility, which cannot be properly measured by conventional continuum theory. The diffusio-phoretic mobility is found to depend non-monotonically on the strength of the interaction between the colloid and solutes, due to the presence of bound solutes within adsorption shell that cannot contribute to diffusio-phoresis. The results also show that the phoretic mobility depends sensitively on the density of solutes in bulk, due to the uneven distribution of excess particles surrounding the colloid at a microscopic level. The simulations suggest that diffusio-phoresis may in principle be applied to the selective transport, separation and purification for colloidal systems. By substituting the spherical colloid with other realistic macromolecules, the model could provide results that are quantitatively comparable with experiments.