Phase-field based lattice Boltzmann model for simulating thermocapillary flows

Abstract: This paper proposes a simple and accurate lattice Boltzmann model for simulating thermocapillary flows, which is able to deal with thermophysical parameters contrasts. In this model, two lattice Boltzmann equations are utilized to solve the conservative Allen-Cahn equation and the incompressible Navier-Stokes equations, while another lattice Boltzmann equation is used for solving the temperature field, where the collision term is delicately designed such that the influence of the thermophysical parameters contrasts is incorporated. In contrast to previous lattice Boltzmann model for thermocapillary flows, the most distinct feature of the current model is that the forcing term used in the present thermal lattice Boltzmann equation is not needed to calculate space derivatives of the heat capacitance or the order parameter, making the scheme much simpler and also possible to retain the main merits of the lattice Boltzmann method. The developed model is firstly validated by considering the thermocapillary flows in a heated microchannel with two superimposed planar fluids. It is then used to simulate thermocapillary migration of a two-dimensional deformable droplet, and its accuracy is once again consistent with the theoretical prediction when the Marangoni number approaches zero. Finally, we numerically study the motion of two recalcitrant bubbles in a two-dimensional channel where the relationship between surface tension and temperature is assumed to be a parabolic function. It is found that owing to the competing between the inertia and thermal effects, the bubbles are able to move against the liquid's bulk motion and towards areas with low surface tension.