Thermocapillary actuation and droplet manipulation in microfluidics

The researches on actuation and manipulation of micro-droplets in microchannels or on solid surfaces have been active in recent years. Based on thermocapillary effects, manipulations and temperature recycling of discrete droplets can be achieved in micro-devices, which can provide technical support for genomics, proteomics, drug delivery, etc., in biochemical and biomedical areas. It is of interests in both academic understandings and practical applications to investigate the thermal-driven actuation of liquid droplets inside capillaries and planar microchannels. This thesis presents a study on the actuation and manipulations of discrete droplet in microchannels based on thermocapillary effect. The major objective of the study is to investigate the transient and periodic motions of thermal-driven liquid droplet in both one dimensional and two dimensional microchannels. This is achieved by modeling the temperature variations inside the channels, the coupling of the temperature fields to the liquid surface tensions, and the dynamics of the liquid plugs/droplets under the actuation. Experiments are conducted, not only to verify the predicted results from the modeling, but also to provide further information on thermocapillary effects in microchannels. The feasibility of using thermocapillary effect to control the discrete droplets in micro-scale devices is examined. The research associated with this thesis has been carried out in three stages, and the results are accordingly documented in three parts. Firstly, the periodic thermocapillary actuation of a liquid plug in capillary is studied. An analytical model is developed to predict the temperature distributions along the capillary generated by alternated heating conditions. The temperature field is then coupled to the liquid plug through surface tensions at the plug ends. The motion of the liquid plug is predicted in terms of the displacement and velocity. In the experiments, wire heaters were used to create alternate heat fluxes at two ends of the working portion along the capillary tube. Silicone oil was injected into the tube by a precise syringe to form a micro liquid plug with certain length. The actual temperature distributions along the capillary tube were captured by a thermal-tracer camera against time, and the reciprocating motion trajectory of the plug was recorded by a CCD camera.DOCTOR OF PHILOSOPHY (MAE