Jiwoo Hong

Effects of Drop Size and Viscosity on Spreading Dynamics in DC Electrowetting

This study investigates the effects of drop size and viscosity on spreading dynamics, including response time, maximum velocity, and spreading pattern transition, in response to various DC voltages, based on both experiment and theoretical modeling. It is experimentally found that both switching time (i.e., time to reach maximum wetted radius) and settling time (i.e., time to reach equilibrium radius) are proportional to 1.5th power of the effective base radius. It is also found that the maximum velocity is slightly dependent on drop size but linearly proportional to the electrowetting number. The viscosity effect on drop spreading is investigated by observing spreading patterns with respect to applied voltages, and the critical viscosity at which a spreading pattern changes from under- to overdamped response is obtained. Theoretical models with contact angle hysteresis predict the spreading dynamics of drops with low and high viscosities fairly well. By fitting the theoretical models to experimental results, we obtain the friction coefficient, which is nearly proportional to 0.6th power of viscosity and is rarely influenced by applied voltage and drop size. Finally, we find that drop viscosity has a weak effect on maximum velocity but not a clear one on contact line friction.

Electrowetting-Induced Droplet Detachment from Hydrophobic Surfaces

Detachment of droplets from solid surfaces is a basic and crucial process in practical applications such as heat transfer and digital microfluidics. In this study, electrowetting actuations with square pulse signals are employed to detach droplets from a hydrophobic surface. The threshold voltage for droplet detachment is obtained both experimentally and theoretically to find that it is almost constant for various droplet volumes ranging from 0.4 to 10 μL. It is also found that droplets can be detached more easily when the width of applied pulse is well-matched to the spreading time (i.e., the time to reach the maximum spread diameter). When the droplet is actuated by a double square pulse, the threshold voltage is reduced by ∼20% from that for a single square pulse actuation. Finally, by introducing an interdigitated electrode system, it is demonstrated that droplets can be detached from the solid bottom surface without using a top needle electrode.

Three-dimensional digital microfluidic manipulation of droplets in oil medium.

We here develop a three-dimensional DMF (3D DMF) platform with patterned electrodes submerged in an oil medium to provide fundamental solutions to the technical limitations of 2D DMF platforms and water–air systems. 3D droplet manipulation on patterned electrodes is demonstrated by programmably controlling electrical signals. We also demonstrate the formation of precipitates on the 3D DMF platform through the reaction of different chemical samples. A droplet containing precipitates, hanging on the top electrode, can be manipulated without adhesion of precipitates to the solid surface. This method could be a good alternative strategy to alleviate the existing problems of 2D DMF systems such as cross-contamination and solute adsorption. In addition, we ascertain the feasibility of temperature-controlled chemical reaction on the 3D DMF platform by introducing a simple heating process. To demonstrate applicability of the 3D DMF system to 3D biological process, we examine the 3D manipulation of droplets containing mouse fibroblasts in the 3D DMF platform. Finally, we show detachment of droplets wrapped by a flexible thin film by adopting the electro-elasto-capillarity (EEC). The employment of the EEC may offer a strong potential in the development of 3D DMF platforms for drug encapsulation and actuation of microelectromechanical devices.

Size-Selective Sliding of Sessile Drops on a Slightly Inclined Plane Using Low-Frequency AC Electrowetting

When placed on an inclined solid plane, drops often stick to the solid surface due to pinning forces caused by contact angle hysteresis. When the drop size or the planes incline angle is small, the drop is difficult to slide due to a decrease in gravitational force. Here we demonstrate that small drops (0.4-9 μL) on a slightly inclined plane (~12°, Teflon and parylene-C surface) can be mobilized through patterned electrodes by applying low-frequency ac electrowetting under 400 Hz (110-180 V(rms)), which has a mechanism different from that of the high-frequency ac method that induces sliding by reducing contact angle hysteresis. We attribute the sliding motion of our method to a combination of contact angle hysteresis and interfacial oscillation driven by ac electrowetting instead of the minimization of contact angle hysteresis at a high frequency. We investigated the effects of ac frequency on the sliding motion and terminal sliding of drops; the terminal sliding velocity is greatest at resonance frequency. Varying the electrowetting number (0.21-0.56) at a fixed frequency (40 Hz) for 5 μL drops, we found an empirical relationship between the electrowetting number and the terminal sliding velocity. Using the relationship between the drop size and ac frequency, we can selectively slide drops of a specific size or merge two drops along an inclined plane. This simple method will help with constructing microfluidic platforms with sorting, merging, transporting, and mixing of drops without a programmable control of electrical signals. Also, this method has a potential in heat transfer applications because heat removal capacity can be enhanced significantly through drop oscillation.

Fast Electrically Driven Capillary Rise Using Overdrive Voltage.

Enhancement of response speed (or reduction of response time) is crucial for the commercialization of devices based on electrowetting (EW), such as liquid lenses and reflective displays, and presents one of the main challenges in EW research studies. We demonstrate here that an overdrive EW actuation gives rise to a faster rise of a liquid column between parallel electrodes, compared to a DC EW actuation. Here, DC actuation is actually a simple applied step function, and overdrive is an applied step followed by reduction to a lower voltage. Transient behaviors and response time (i.e., the time required to reach the equilibrium height) of the rising liquid column are explored under different DC and overdrive EW actuations. When the liquid column rises up to a target height by means of an overdrive EW, the response time is reduced to as low as 1/6 of the response time using DC EW. We develop a theoretical model to simulate the EW-driven capillary rise by combining the kinetic equation of capillary flow (i.e., Lucas-Washburn equation) and the dynamic contact angle model considering contact line friction, contact angle hysteresis, contact angle saturation, and the EW effect. This theoretical model accurately predicts the outcome to within a ± 5% error in regard to the rising behaviors of the liquid column with a low viscosity, under both DC EW and overdrive actuation conditions, except for the early stage (

Smart self-cleaning cover glass for automotive miniature cameras

We systematically study removal of sessile and pendant droplets with different volumes on an inclined plane tilted at various angles as well as on a horizontal plane by electrowetting (EW) actuation. In absence of an applied voltage, sliding angles (i.e., the minimum inclination angle for the onset of the sliding motion) are experimentally measured as a function of droplet volume. In addition, threshold voltages (i.e., the minimum electrical voltage for the onset of the sliding motion) of sliding droplets are found with respect to different droplet volumes and inclination angles when DC voltages are sequentially applied to the electrodes. The threshold voltage is found to decrease as the droplet volume and inclination angle increase because of the increase in gravitational force. Furthermore, we show that AC EW can be employed to remove droplets on inclined planes, because AC EW-driven interfacial oscillation helps overcome the pinning force caused by contact angle hysteresis. Finally, based on these results, we design a smart self-cleaning cover glass for miniature cameras assisted by EW. It can remove water droplets in a wide range of sizes to allow the cameras lens to get clean at any time. The proposed cover glass can perform a fast cleaning operation with effective energy consumption as well as can be easily integrated onto any device due to its simple design structure.

Novel energy harvesting using acoustically oscillating microbubbles

When a bubble hanging on a piezocantilever is excited by an acoustic wave around its resonant frequency, it oscillates and simultaneously generates cavitational microstreaming around it. The microstreaming bends the piezocantilever with fine vibration, resulting in electric power generation from the piezocantilever. In this study, we explore the dynamic behaviors of an acoustically oscillating bubble on the flexible substrate as well as demonstrate applicability of the proposed system to practical applications such as energy harvesting and acoustic wave sensors. First, the effects of an applied frequency and bubble size on the dynamic characteristics of an acoustically oscillating bubble, such as maximum amplitude and resonant frequency, are experimentally investigated. The amplitude of an oscillating bubble is maximized at its resonant frequency, which is inversely proportional to its size. In addition, electrical voltage generated by a piezocantilever attaching with an oscillating bubble is measured at different applied frequencies, bubble sizes, and distances between the bubble and piezoactuator. The results show that the generated voltage is strongly affected by the applied frequency and is inversely proportional to the bubble size and the distance between the bubble and piezoactuator. Finally, the output voltage is almost linearly proportional to the number of bubbles.