Experimental And Numerical Study On Droplet Electrowetting Dynamics And Electrothermal Flow

The microfluidic chip, which can integrate many operations(including sample-getting, mixing, diluting, sample separation, as well asreaction etc.) into a single chip with an area of only several squarecentimeters, has the advantages of automatic, fast, and low reagentconsumption compared with conventional lab, and will bring out arevolutionary change in the scientific field of bio-chemical analysis andmedical diagnosis in the near future.Microfluidic chips can be catergorized into two types according toflow pattern, i.e., continuous flow micro-chips where the continuous flowin microchannel is controlled by the elements of micropump, microvalveand so on, and droplet-based micro-chips, where the discrete microdroplet are driven directly by the varieties of forces acting on it. Theflows of fluid, the prerequisite for the functioning of the microfluidicchips, are mostly driven by DC or AC electrical field at present. Both ofDC or AC techniques involve the electrohydrodynamic phenomenon ofelectroosmotic flow near the wall, the electrothermal effect in the bulkfluid, and the electrowetting in near triple-phase contact line in EWOD(Electrowetting on a Dielectric) digital microchips. In this thesis, based on the principles of electrohydrodynamics, some important problemsabout continuous flow and digital droplets in microfluidic chips arestudied by using the methods of experimental observation and numericalsimulation. The main contents of the thesis are summarized as follows:(1)Experimental research on the manipulation of micro-droplet onsplit-electrodes single plate EWOD chips. spilt-electrodes EWOD chipswere designed and fabricated using MEMS techniques, and theexperimental observations on the droplet transport, merging process andoscillation were carried out, with the emphasis on the variation ofdynamic contact angle and dynamic contact line, which is confirmed toplay a key role in EWOD droplet dynamics. Moreover, a special kind ofasymmetric droplet oscillation was found at fist time during theexperiment.(2) The development of a fully coupled electrothermal flow modeland its application in the numerical simulation of an electrothermalmicropump. An electrically-hydrodynamically-thermally coupledelectrothermal model was derived using electrohydrodynamic theories tocorrect the error of using traditional electrothermal flow model in thecases of large temperature rise of fluid. The comparisons in the numericalsimulation of a electrothermal micropump verified the correctness of thefully coupled model. Meanwhile, an effective way of improving thevelocity of electrothermal flow in micropump was discovered. (3) Numerical simulation research of the rotation flow within thedroplet in-needle-plane structure of EWOD chip. Previous researchershave found a vortex of inner flow when high frequency AC is used indroplet EWOD. Though this phenomenon has been explained due toelectrothermal flow and certified by the numerical simulation using theclassic electrothermal model, the results between experiment andnumerical calculation do not agree with each other with respect tomaximum temperature rise, flow velocity, and the peak frequency atwhich the max velocity appears. In this thesis, numerical analysis wascarried out using the fully coupled model to investigate the inner flow ofdroplet and was compared with the result of the classic model andexperiments. It turns out that the fully coupled model can predict theelectrothermal more correctly. Besides, the effects of the size and locationof needle electrode on electrothermal flow were also analyzed.The results and numerical model in this thesis not only can givemore insight on the EWOD dynamics and the mechanism ofelectrothermal flow, but also can provide guidance on the practicalapplication of microfluidic chips.