Experimental Study Of Droplet Oscillation And Evaporation Characteristics Under AC Electrowetting

Rapid advances in Micro-Electro-Mechanical Systems (MEMS)have provided a great opportunity for the development of microthermofluidic devices, such as microfluidic chip, microactuator,micromixer, micro-chemical reactor, micro fuel cell, micro heatexchanger and etc. The involved microscale heat and mass transferprocesses play an important role in the performance of these microthermofluidic devices. Particularly, microfluidic systems based on droplet,including micro-droplet mixer and micro droplet heat exchanger, havebeen being a hot research topic in engineering thermophysics and relatedinterdisciplines. However, the micro-droplet systems are not easy to bemanipulated by traditional mechanical methods owing to the scale effectof micro systems. In this thesis, we propose to enhance the internalmixing and evaporation of the droplet by alternate current (AC)electrowetting on a dielectric (EWOD). The micro-electrodes anddielectric layers are fabricated on the silicon chips using MEMStechnology to make EWOD chips with coplanar-electrodes configuration.Firstly, we investigated the droplet spreading and oscillation underDC/AC electrowetting. Subsequently, droplet evaporation characteristicswith or without electrowetting were studied experimentally. High speedCCD camera along with optical lens was used to record dropletoscillation and evaporation processes from both of the side and top views.To the best knowledge of the author, it is the first time for the related kindof studies.For DC electrowetting, it was found that the droplet experiences anunderdamped oscillation process before reaching the steady state. As toAC electrowetting, it was found that the droplet resonates at somespecific frequencies (i.e., resonance modes Pnwith n=2,4,6...), whichagree reasonably well with those predicted using a previous linear theoretical analysis. At resonance modes Pn, the droplet oscillatessymmetrically with n/2peaks appearing on the droplet surface. Thereexists a critical frequency between the adjacent resonance modes, atwhich the droplet oscillates symmetrically but very weakly, and theoscillation phase relationship between the contact line width and height ofthe droplet switches. At frequencies lower than this critical frequency, thecontact line will form lobes whose number increases with the order ofadjacent resonance mode and whose position alternates periodically inazimuthal direction through droplet spreading and receding (i.e.,post-resonance mode Pn,a). At frequency higher than the critical frequency,droplet oscillates asymmetrically with the transmission of ripples on thedroplet surface (i.e., pre-resonance mode Pn,b). It is believed theseasymmetric oscillations will produce more chaotic fluid flows inside thedroplet than symmetric oscillations and therefore can enhance the mixingefficiency in droplet-based microfluidics.For the droplet evaporation, we firstly conducted experiments ofdroplet evaporation on a hydrophobic surface under constant temperaturecondition without an electric field. Experimental results suggest that theevaporation mode is the so-called "constant contact angle" mode, whichis consistent with existing experimental results in previous literature.After this, sessile droplet evaporation on a surface with constanttemperature under AC electrowetting is also investigated experimentally.The frequency is kept constant and equal to the corresponding resonancefrequency of some specific resonance mode fnfor the initial dropletvolume. The transition from resonance modes Pnto Pm(2≤m<n) occursduring the evaporation process of the droplet. At the same time,before-resonance mode Pn,band after-resonance mode Pn-2,aare foundbetween two adjacent resonance modes Pnand Pn-2. Experimental resultsalso suggest that voltage is the dominant factor that affects theevaporation rate. For a constant frequency, droplet evaporation rateincreases drastically with the increase of voltage. While for a constantvoltage, the relation between the frequency and the evaporation rate isnon-monotonic. The frequency has little effect on evaporation rate andthis effect decreases with the increasing voltage. Our work on droplet evaporation enhancement using electrowetting is believed to be of greatimportance and will provide new insight into the cooling of chips.