Motion of a droplet through microfluidic ratchets.

We report numerical and experimental studies on a droplet moving through an array of mirofluidic ratchets. Droplets are formed at a T junction and subsequently forced through microfluidic ratchets in the form of diffuser/nozzle structures. At the same flow rates of the continuous and the dispersed phases, the velocity of the droplet is determined by the viscosity of the continuous phase and the interfacial tension between the two phases. Both numerical and experimental results show that the velocity of the droplet increases with increasing capillary number. The droplet velocity is higher than the mean velocity of the fluid system and increases with increasing viscosity of the continuous phase or decreasing interfacial tension. In all experiments, the droplet moves faster in the diffuser direction than in the nozzle direction. Our findings allow the development of a measurement approach for interfacial tension. The rectification characteristics can be used for the development of micropumps for multiphase systems.