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Dynamics of electrowetting: spreading dynamics and oscillation-induced droplet transport

Dynamics of electrowetting: spreading dynamics and oscillation-induced droplet transport
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Electrowetting (EW) controls wettability of a drop on an insulator-coated electrode surface by applying DC or AC electrical voltages externally. EW has many advantages, such as fast and precise control of contact angle with a low driving voltage. Thus, EW has attracted considerable attention in both industry and academia, and many practical applications have been developed, including digital microfluidics, liquid lenses, micro switches, optical valves and mirrors, and reflective displays. These practical applications should require accurate, quick, and stable positioning of the contact line (CL), which has been one of the main challenges in EW research. In this study, the effects of drop size and viscosity on the DC EW-driven spreading characteristics of drops (e.g., response time, spreading pattern transition, and maximum velocity) in air are investigated both experimentally and theoretically. Both switching time (i.e., time to reach maximum wetted radius) and settling time (i.e., time to reach equilibrium radius) are found to be proportional to 1.5th power of the effective base radius. The effect of drop viscosity on drop spreading is investigated by observing spreading patterns with respect to applied voltage, and the critical viscosity where a spreading pattern changes from under- to over-damped response is obtained. By fitting the theoretical models to the experimental results, the friction coefficient is found to be strongly correlated to drop viscosity and be rarely influenced by applied voltage and drop size. In addition, EW-based devices have been frequently used in water/oil systems, because oil medium reduces contact angle hysteresis (CAH, i.e., difference between advancing and receding contact angles) and prevents drop evaporation. However, the viscosity of oil medium acts as a resistance force, thereby limiting the speed of transporting drops. The entrapment of the oil film between a drop and a solid surface also degrades the performance of EW-based devices in a water/oil system. To resolve these problems practically as well as to extend the aforementioned research academically, the effects of oil viscosity and drop size on the spreading behavior of a drop submerged in oil under various DC EW actuations are investigated. Settling time is found to be linearly proportional to the radius of the spherical drop and oil viscosity. The oil entrapment process and the instability of the entrapped oil film are also investigated by observing the bottom part of the spreading drops submerged in oil. The size of the oil drops generated by oil-film instability decreases as the applied voltage increases. However, it is rarely affected by oil viscosity. AC EW-driven drop oscillation method (or resonance oscillation) has been used in many practical applications, such as manipulation of cells and particles inside drops, measurement of surface degradation, and transporting (or detachment) of drops without any complicated electrical control. The resonance frequency of an oscillating drop is determined by its mass and surface tension, whereas drop viscosity damps its oscillation. However, most previous studies focused on exploring the oscillation of levitated drops. Here, the effects of drop viscosity on the oscillation dynamics of a sessile drop, such as resonance frequency and oscillation amplitude, are investigated based on both experiments and theoretical modeling. Drop viscosity rarely affects resonance frequency, but has strong influence on the oscillation amplitude and peak width of the resonance frequency. In addition, drop oscillation in the resonance mode is no longer observed, when drop viscosity is larger than the critical value, which increases with applied AC voltage. AC EW-induced drop oscillation method can be used to overcome CAH and mobilize small drops sticking on a solid surface. It enables drops to slide down a patterned electrode substrate with a small angle of inclination by applying low-frequency AC EW. The effects of AC frequency on the sliding velocity of drops are investigated. As a result, the sliding velocity is found to be maximized at resonance frequency. By using the unique dependence of drop motion on its volume and the applied AC frequency, a drop of a specific size is selectively slid on the inclined substrate. Similarly, a drop constrained between two non-parallel electrodes moves into a narrow gap via AC EW-driven drop oscillation. When AC frequency is below 100 Hz, drops move toward the narrow gap by repeated wetting and de-wetting. At frequencies higher than 10 kHz, however, drops move slowly in the same direction with weak oscillation and then suddenly penetrate the narrow gap. Both of the drop sliding on an inclined plate and the drop transporting between two non-parallel plates result from a combination effects of initially asymmetric contact angle of a drop, CAH, and interfacial oscillation driven by AC EW.
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