Thermocapillary actuation of droplets on a microfluidic chip

In this paper, we report a digital liquid transporting chip which can manipulate droplets precisely based on thermal Marangoni force induced by thermal gradient. Detailed mathematical discussion shows that threshold force for mobilization is a function of droplet size and liquid parameters, and droplet velocity after depinning is a function of applied thermal gradient, droplet size and liquid parameters. The device with Ti as micro-heater, Au film as pad, Cr/Au as controlling electrode arrays, high quality SiO ₂ film as dielectric layer on glass substrate is fabricated by silicon bulk process and employs fluorocarbon polymer as hydrophobic layer to functionalize the surface to be hydrophobic so as to facilitate droplet transportation. Test results show that transportation velocity of 3 l DI water and silicone oil can reach 0.1 mm/s and 1 mm/s respectively at a voltage of 7 V. Driving voltage, transportation velocity of droplets and effect of contact angle hysteresis are discussed and the results are in agreement with theoretical results. The results provide some practical guidelines for the design of microfluidic chips based on thermocapillary actuation. In this study, we demonstrated that thermocapillary actuation of droplets is tunable and the chip can find great application in lab-on-a-chip due to its simple structure and low-cost fabrication process.