Orientational dynamics of a heated Janus particle

Abstract

Using large scale molecular dynamics simulations we study the orientational dynamics of a heated Janus particle which exhibits self-propulsion. The asymmetry in the microscopic interaction of the colloid with the solvent is implemented by choosing different wetting parameters for the two halves of the sphere. Consequently a gradient in temperature is created across the poles of the sphere due to different interfacial resistance across the solid-fluid boundary in the two hemispheres. It is this self-created temperature gradient which leads to a self-propulsion along the direction of the symmetry axis. In this article, we look at the orientational dynamics of such a system, as well as the subsequent enhancement of the translational diffusivity at late times. The orientational correlation of the symmetry axis is measured from the simulation and provides a direct access to the rotational diffusion constant. The heating leads to an increase in the rotational diffusivity of the colloid. We quantify this increase in rotational diffusion against the temperature difference across the poles of the Janus sphere as well as the average surface temperature difference from the ambient fluid.We show that later quantification is better and results in a data collapse for different choices of the microscopic interaction. The average propulsion velocity is also measured for different choices of the wetting parameter. The directionality of self-propulsion changes depending on the microscopic interaction. We show that whenever the attractive interaction of the colloid with the solvent is switched off, the phoretic mobility changes sign. Further, the propulsion velocity is zero for heating below a certain threshold value. Finally, we combine the measured propulsion velocity and the rotational diffusion time to estimate the enhancement in the long time diffusion coefficient of the particle.