Nonequilibrium Dynamics of Transient Autoelectrophoresis and Effect of Surface Heterogeneity.

Nonuniform proton flux around a reactive Janus particle as a result of zone selective heterogeneous surface reaction leads to the formation of asymmetric electrical double layers (EDLs) which assists in generating a proximate electric field dipole around the Janus particle to initiate autoelectrophoretic migration. To estimate the force of the autoelectrophoretic motion of such Janus particles, a mathematical model is set up taking Poisson-Nernst-Plank (PNP) equations coupled with the Navier-Stokes (NS) equations with appropriate boundary conditions. To track the actual motion of these particles, we employ moving deforming mesh and fluid-structure interactions (fsi) of COMSOL Multiphysics while a finite element method is deployed for solving the set of modeled equations. At the outset, transient genesis of the electric field around the particle owing to the nonuniform proton flux has been explored. We further explore the detailed unsteady particle dynamics of the autoelectrophoretic motion with the help of fluid structure interaction physics. It has been observed that the concept of perfect ionic equilibrium in autoelectrophoretic motion is hard to achieve. The autoelectrophoretic particle undergoes continuous change in terms of the ionic concentration around it, speed of the particle, and the transient electric field gradient across the particle. The parametric variation of proton flux reveals that at a relatively lower proton flux a quasi-equilibrium state can be achieved, whereas for higher proton flux the phenomenon can be a pure nonequilibrium case. This parametric study has been done to support the transient dynamics. It has also been shown that the presence of chemical heterogeneity on the particle surface can alter the dynamics of the particle significantly, and the chemical heterogeneity can be used as a tool to control directionality and tuning speed of autoelectrophoretic motion.