Kwan Hyoung Kang

An electrohydrodynamic flow in ac electrowetting

In ac electrowetting, hydrodynamic flows occur within a droplet. Two distinct flow patterns were observed, depending on the frequency of the applied electrical signal. The flow at low-frequency range was explained in terms of shape oscillation and a steady streaming process in conjunction with contact line oscillation. The origin of the flow at high-frequency range has not yet been explained. We suggest that the high-frequency flow originated mainly from the electrothermal effect, in which electrical charge is generated due to the gradient of electrical conductivity and permittivity, which is induced by the Joule heating of fluid medium. To support our argument, we analyzed the flow field numerically while considering the electrical body force generated by the electrothermal effect. We visualized the flow pattern and measured the flow velocity inside the droplet. The numerical results show qualitative agreement with experimental results with respect to electric field and frequency dependence of flow velocity. The effects of induced-charge electro-osmosis, natural convection, and the Marangoni flow are discussed.

Effect of electric field on electrical conductivity of dielectric liquids mixed with polar additives: DC conductivity.

To understand the dynamics of dielectric-liquid-based colloidal systems, the effect of electric field on conductivity of dielectric liquids must be characterized. We measured dc conductivity of dielectric liquids mixed with polar additives, including surfactants, an alcohol, and an oil-soluble salt. The conductivities of the mixtures increased with the electric field and concentration of polar additives and were also affected by temperature. Conductivity increased with electric-field strength at very similar rates, almost irrespective of the kind of mixture. A simple formula derived from Onsager theory predicted the electrical conductivity under electric field fairly well.

Fast and reliable droplet transport on single-plate electrowetting on dielectrics using nonfloating switching method

In a droplet transport based on electrowetting on dielectrics, the parallel-plate configuration is more popular than the single-plate one because the droplet transport becomes increasingly difficult without cover plate. In spite of the improved transport performance, the parallel-plate configuration often limits the access to the peripheral components, requesting the removal of the cover plate, the single-plate configuration. We investigated the fundamental features of droplet transport for the single-plate configuration. We compared the performance of several switching methods with respect to maximum speed of successive transport without failure and suggested nonfloating switching method which is inherently free from the charge-residue problem and exerts greater force on a droplet than conventional switching methods. A simple theory is provided to understand the different results for the switching methods.

Jumping of a droplet on a superhydrophobic surface in AC electrowetting

Consistent droplet bouncing driven by AC electrowetting was achieved by introducing a superhydrophobic surface instead of conventional hydrophobic surfaces. A superhydrophobic surface is very effective to reduce interfacial energy barrier or adhesion, allowing complete detachment of a droplet from the substrate. While a fixed electric potential (100xa0Vrms) was applied, the shape deformation and the droplet bouncing were significantly influenced by the frequency of the AC electrowetting. Consistent droplet bouncing only occurred at very narrow frequency ranges (e.g., 30–31xa0Hz for 8xa0μL droplets), indicating that resonance dominates the droplet bouncing. Interestingly, the resonance was 1/2 sub-harmonics, where every other actuation was skipped, when the droplet was in the air. Theoretical evaluation of the resonant frequency based on the linear theory implies that the fundamental resonance between the AC electrowetting and the vertical vibration of the shape oscillation could be important to produce consistent droplet bouncing.Graphical Abstract

How the change of contact angle occurs for an evaporating droplet: effect of impurity and attached water films

On a hydrophobic surface, the contact angle of an evaporating droplet decreases with time and becomes much smaller than its receding contact angle; the rate of decrease is accelerated with time. When we use impurity-concentrated water produced by partial distillation, the decrease in contact angle is remarkably accelerated with time. In contrast, for purified water, the decrease in contact angle is significantly reduced and the start of stage 3 is delayed. These results indicate that the submicron-sized impurities cause the decrease in contact angle. Also, we found that a number of attached thin films of water are generated around the periphery of impure droplets. We derived the contact angle equation under the Cassie–Baxter model by regarding the water film as a heterogeneous layer. We compared the theoretical model to the experimental data, and these results suggest that the attached film should be considered as one of the direct causes for the large deviation from the Youngs angle of evaporating droplets.

Optoelectrofluidic field separation based on light-intensity gradients

Optoelectrofluidic field separation (OEFS) of particles under light -intensity gradient (LIG) is reported, where the LIG illumination on the photoconductive layer converts the short-ranged dielectrophoresis (DEP) force to the long-ranged one. The long-ranged DEP force can compete with the hydrodynamic force by alternating current electro-osmosis (ACEO) over the entire illumination area for realizing effective field separation of particles. In the OEFS system, the codirectional illumination and observation induce the levitation effect, compensating the attenuation of the DEP force under LIG illumination by slightly floating particles from the surface. Results of the field separation and concentration of diverse particle pairs (0.82-16 mum) are well demonstrated, and conditions determining the critical radius and effective particle manipulation are discussed. The OEFS with codirectional LIG strategy could be a promising particle manipulation method in many applications where a rapid manipulation of biological cells and particles over the entire working area are of interest.

A New Switching Method of Two-Dimensional EWOD-Based Droplet Translation on Single-Plate Configuration

In electrowetting-on-dielectric (EWOD), droplet can be transported using either parallel-plate configuration or single-plate configuration of electrodes. Single-plate configuration, i.e. without cover plate, is less popular due to its difficulty in successive translation of droplet and slow translation speed, in spite of its enhanced flexibility in interfacing with other components. This paper investigates the effect of switching method on droplet translation for the single-plate configuration. Droplet was transported on a two-dimensional four by four electrode array by EWOD actuation. In previous works, high voltage and ground condition were applied on two activating electrodes, maintaining other surrounding electrodes in floating state. Here, we propose a new switching method in which all the electrodes are grounded except one activating electrode. We compared the performance of the present switching method with conventional ones using the same electrode array. Performance of each method was experimentally examined with respect to maximum translation speed in which successive translation was possible. Our method showed much better performance under various experimental conditions. In addition, the present method does not necessitate the floating condition, which leads to the reduction of the number of switches and controllers to a half of conventional one.Copyright

Hydrodynamic Flows in Electrowetting

In this work, we found experimentally that there exist fairly strong fluid flows in AC electrowetting, which can be utilized as a means to mix the fluids in EWOD-based micro-devices. We visualized the internal flow. There may exist two distinct flow-generation mechanisms; one is the droplet oscillation, and the other is the electrohydrodynamic flow. The flow pattern is significantly dependent on the applied AC frequency. At low frequencies (represented here by 1 kHz), the center of the vortices is located somewhat randomly and the flow directs upward near the symmetric axis. At high frequencies (represented by 128 kHz), however, a pair of vortices having quite a regular structure is clearly visible and the flow directs downward near the symmetric axis. The flow patterns are strongly dependent on the position of the point electrode. The droplet surface undergoes a periodic oscillation (visualized by a high-speed camera) with a frequency exactly twice the frequency of applied electrical signal. The oscillating interface can generate a steady streaming. However the numerical results show that there exists no electric field at low AC frequencies. On the contrary, there exists quite a strong electric field inside the droplet at high frequencies. It means the electrohydrodynamic flow cannot be generated at the low frequency region, and the droplet oscillation might cause the flow generation at low frequencies. We also demonstrated the flow can be beneficially utilized as a mixing method.© 2008 ASME