Numerical investigation of the thermocapillary actuation behavior of a droplet in a microchannel

The thermocapillary actuation behavior of a silicone droplet in a microchannel is numerically investigated in the present paper. The finite element method with the two-phase level set technique, which is ideally suitable for tracking the interfaces between two immiscible fluids, is employed to solve the Navier-Stokes equations coupled with the energy equation. The lower wall of the microchannel is subjected to a uniform temperature gradient, while the upper one is either adiabatic or isothermal. The thermocapillary flow inside the droplet is significantly affected by the thermal condition of the upper wall. When the upper wall is set to be adiabatic, a pair of asymmetric thermocapillary convection vortices initially occurs inside the droplet but these turn into a sole thermocapillary vortex once enough time has passed. For the isothermal case, a pair of asymmetric thermocapillary convection vortices always appears inside the droplet. The droplet initially accelerates for both the adiabatic and isothermal cases. The droplet velocity then decreases dramatically for the adiabatic case while it decreases slowly for the isothermal one. The dynamic contact angle of the droplet in a microchannel is strongly affected by the passage of the air flow over the droplet which is induced by the thermocapillary convection and the presence of the upper wall. The actuation velocity is enhanced by a higher temperature gradient, a reduction of microchannel height and a smaller contact angle for both adiabatic and isothermal cases.