Thermocapillary control of microfluidic transport with a stationary cyclic heat source

In this paper, a new method of cyclic flow control with an external heat source is developed for thermocapillary pumping of a micro-droplet in a closed microchannel. Unlike past studies with a moving heat source at the receding edge of the droplet, this new technique involves a stationary cyclic heat source embedded within an adjoining silicon substrate. Thermocapillary pumping of the micro-droplet is examined numerically (finite-volume method) and theoretically (slug-flow approximation). In contrast to past studies, this paper considers the solution of the full Navier-Stokes and energy equations within the droplet. Additionally, temperature boundary conditions for a stationary heat source are applied at the interface between the substrate and its surroundings, rather than along the microchannel wall. The finite-volume formulation is developed with a sliding grid in the liquid phase and an adaptive grid along the compressed air and substrate regions. Coupled pressure and velocity boundary conditions are applied along the droplet/air interface. Numerical predictions suggest that cyclic droplet displacement can be obtained with the stationary heat source. Close agreement between numerical and theoretical predictions has provided useful validation of the formulations.