A Numerical Study on the Thermocapillary Migration of Droplet under Microgravity with Periodic Thermal Boundaries by Front Tracking Method

This paper mainly studied the thermocapillary migration of deformable droplets induced by periodic temperature boundary under microgravity conditions. The Finite-Difference/Front-Tracking (FD/FT) Method was used to solve the Navier-Stokes equation coupled with the energy equation, and the Continuum Surface Force (CSF) model was used to simplify the surface tension of the phase interface. The results showed that the maximum droplet migration velocity increased with the increase of temperature amplitude. And the droplet cycle period became shorter with the increase of temperature angular frequency. In the 1/4 cycle, the initial movement time of droplet decreased with the increase of temperature phase. If the phase was reversed, the initial movement direction of the droplet changed. With the increase of Reynolds number (Re), the droplet tended to maintain its motion inertia.

A simulation study of the forward and backward thermocapillary migration of fluids in a microchannel

In this study, the forward and backward thermocapillary migration of fluids in a microchannel is numerically investigated. Both the upper wall and the lower wall of the microchannel are set to be an ambient temperature. Two 40mW heat sources activated periodically are placed on the left side and the right side of the droplet in a microchannel. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet. The isotherms inside the droplet are extremely distorted by the thermocapillary convection. The forward and backward thermocapillary migration results in the net thermocapillary momentum which drives a water droplet moves from the hot side of the open channel to the cold side. The temperature gradient at the free interface on the side of acting heat source is always smaller than that on the cold side. The actuation velocity of the liquid droplet first increases significantly, and then decreases continuously for various interval times. The dynamic contact angle of a water droplet is strongly affected by the forward and backward oil flow motion and the net thermocapillary momentum inside the droplet. It is alternated due to the pressure difference acting on the free interface between two immiscible fluids during actuation process.

Numerical investigation of the thermocapillary migration of a water droplet in a microchannel by applying a heat source

The migration of a small droplet has been developed during the last two decades due to its applications in industry and high technology such as MEMS and NEMS devices, Lap-On-a- chip, transportation of fluids and so on. There have many studies on this topic in which the energy source as a driving force for the moving of a droplet is quite a difference like heating, magnetics, pressure, electric, laser, and so on. In this study, the numerical computation is used to investigate the transient thermocapillary migration of a water droplet in a micro-channel under the effect of heating source. For tracking the evolution of the free interface between two immiscible fluids, we employed the finite element method with the two-phase level set technique to solve the Navier-Stokes equations and continuity equation coupled with the energy equation. Both the upper wall and the bottom wall of the microchannel are set to be ambient temperature. 40mW heat source is placed at a distance of 1 mm from the initial position of a water droplet. When the heat source is turned on, a pair of asymmetric thermocapillary convection vortices is formed inside the droplet, and the thermocapillary on the receding side is smaller than that on the advancing side. The temperature gradient inside the droplet increases quickly at the initial times and then decreases versus time. Therefore, the actuation velocity of the water droplet first increases significantly and then decreases continuously. Furthermore, the results also indicate that the dynamic contact angle is strongly affected by the oil flow motion and the net thermocapillary momentum inside the droplet. The advancing contact angle is always larger than the receding contact angle during the actuation process.