Chang-Jin “Cj” Kim

Characterization of Nontoxic Liquid-Metal Alloy Galinstan for Applications in Microdevices

We have obtained interfacial properties of Galinstan, a nontoxic liquid-metal alloy, to help replace mercury in miniature devices. To prevent formation of an oxide skin that severely hinders the fluidic behavior of small Galinstan droplets and leads to inaccurate property data, we performed our experiments in a nitrogen-filled glove box. It was found that only if never exposed to oxygen levels above 1 part per million (ppm) would Galinstan droplets behave like a liquid. Two key properties were then investigated: contact angles and surface tension. Advancing and receding contact angles of Galinstan were measured from sessile droplets on various materials: for example, 146.8 and 121.5, respectively, on glass. Surface tension was measured by the pendant-drop method to be 534.6 10.7 mN/m. All the measurements were done in nitrogen at 28 with oxygen and moisture levels below 0.5 ppm. To help design droplet-based microfluidic devices, we tested the response of Galinstan to electrowetting-on-dielectric actuation.

VALVELESS PUMPING USING TRAVERSING VAPOR BUBBLES IN MICROCHANNELS

Pumping of fluids in microchannels using the movement of a single or multiple vapor bubble(s) is proposed, analyzed, and demonstrated. The pumping mechanism requires no micromechanical moving parts for actuation by utilizing asymmetric heating which creates a variation in vapor pressure and surface tension due to the heater-induced temperature gradient along the channel. A heat and mass transfer analysis was performed to understand the pumping mechanism and estimate the pumping capability of the micropumping device. To verify the concept and our analysis, a pumping device with a transparent microchannel with a hydraulic diameter of 3.4 μm was fabricated on a silicon wafer using surface micromachining. Experimental results with the first generation device have shown pumping of isopropanol at velocities as high as 160 μm/s (0.5 nl/min flow rate) with a pressure head of approximately 800 Pa.

Equilibrium behavior of sessile drops under surface tension, applied external fields, and material variations

This article describes the equilibrium shape of a liquiddrop under applied fields such as gravity and electrical fields, taking into account material properties such as dielectric constants, resistivities, and surface tension coefficients. The analysis is based on an energy minimization framework. A rigorous and exact link is provided between the energy function corresponding to any given physical phenomena, and the resulting shape and size dependent force term in Young’s equation. In particular, the framework shows that a physical effect, such as capacitive energy storage in the liquid, will lead to 1/R “line-tension”-type terms if and only if the energy of the effect is proportional to the radius of the liquiddrop: E∝R. The effect of applied electric fields on shape change is analyzed. It is shown that a dielectric solid and a perfectly conducting liquid are all that is needed to exactly recover the Young–Lippmann equation. A dielectric liquid on a conducting solid gives rise to line tension terms. Finally, a slightly resistiveliquid on top of a dielectric, highly resistive solid gives rise to contact angle saturation and accurately matches the experimental data that we observe in our electro-wetting-on-dielectric devices.

Maximizing the giant liquid slip on superhydrophobic microstructures by nanostructuring their sidewalls.

In an effort to maximize the liquid slip on superhydrophobic surfaces, we investigate the role of the nanoscale roughness on microscale structures by developing well-defined micro-nano hierarchical structures. The nonwetting stability and slip length on the dual-scale micro-nano structures are measured and compared with those on single-scale micro-smooth structures. A force balance between a liquid pressure and a surface tension indicates that hydrophobic nanostructures on the sidewall of microposts or microgrates would expand the range of the nonwetted state. When a higher gas fraction or a larger pitch can be tested without wetting, a larger slip length is expected on the microstructures. An ideal dual-scale structure is described that isolates the role of the nanostructures, and a fabrication technique is developed to achieve such a microstructure-smooth tops and nanostructured sidewalls. The tests confirm such micro-nano structures allow a nonwetted state at a higher gas fraction or a larger pitch than the previous micro-smooth structures. As a result, we achieve the maximum slip length of approximately 400 microm on the dual-scale structures, an increase of approximately 100% over the previous maximum reported on the single-scale (i.e., micro-smooth) structures. The study ameliorates our understanding of the role of each scale on hierarchical structures for a wetting transition and a liquid slip. The resulting giant slip is large enough to influence many fluidic applications, even in macroscale.

All-electronic droplet generation on-chip with real-time feedback control for EWOD digital microfluidics.

Electrowetting-on-dielectric (EWOD) actuation enables digital (or droplet) microfluidics where small packets of liquids are manipulated on a two-dimensional surface. Due to its mechanical simplicity and low energy consumption, EWOD holds particular promise for portable systems. To improve volume precision of the droplets, which is desired for quantitative applications such as biochemical assays, existing practices would require near-perfect device fabrication and operation conditions unless the droplets are generated under feedback control by an extra pump setup off of the chip. In this paper, we develop an all-electronic (i.e., no ancillary pumping) real-time feedback control of on-chip droplet generation. A fast voltage modulation, capacitance sensing, and discrete-time PID feedback controller are integrated on the operating electronic board. A significant improvement is obtained in the droplet volume uniformity, compared with an open loop control as well as the previous feedback control employing an external pump. Furthermore, this new capability empowers users to prescribe the droplet volume even below the previously considered minimum, allowing, for example, 1 : x (x < 1) mixing, in comparison to the previously considered n : m mixing (i.e., n and m unit droplets).

Droplet Actuation by Electrowetting-on-Dielectric (EWOD): A Review

Abstract This paper reviews publications that have fortified our understanding of the electrowetting-on-dielectric (EWOD) actuation mechanism. Over the last decade, growing interest in EWOD has led to a wide range of scientific and technological investigations motivated by its applicability in microfluidics, especially for droplet-based optical and lab-on-a-chip systems. At this point in time, we believe that it is helpful to summarize the observations, insights, and modeling techniques that have led to the current picture showing how forces act on liquid droplets and how droplets respond in EWOD microfluidic devices. We discuss the basic physics of EWOD and explain the mechanical response of a droplet using free-body diagrams. It is our hope that this review will inspire new research approaches and help design useful devices.

Towards digital microfluidic circuits: creating, transporting, cutting and merging liquid droplets by electrowetting-based actuation

This paper reports a breakthrough in our quest for digital microfluidic circuits - full completion of all four fundamental microfluidic operations: (1) creating, (2) transporting, (3) cutting, and (4) merging of liquid droplets, based on electrowetting-on-dielectric (EWOD) actuation. All the operations were achieved with 25 V/sub DC/ lower than EWOD actuation voltages previously reported. We also report conditions to reduce the driving voltage even further and conditions to drive a droplet as fast as 250 mm/s.

Micro-chemical synthesis of molecular probes on an electronic microfluidic device

We have developed an all-electronic digital microfluidic device for microscale chemical synthesis in organic solvents, operated by electrowetting-on-dielectric (EWOD). As an example of the principles, we demonstrate the multistep synthesis of [18F]FDG, the most common radiotracer for positron emission tomography (PET), with high and reliable radio-fluorination efficiency of [18F]FTAG (88 ± 7%, n = 11) and quantitative hydrolysis to [18F]FDG (> 95%, n = 11). We furthermore show that batches of purified [18F]FDG can successfully be used for PET imaging in mice and that they pass typical quality control requirements for human use (including radiochemical purity, residual solvents, Kryptofix, chemical purity, and pH). We report statistical repeatability of the radiosynthesis rather than best-case results, demonstrating the robustness of the EWOD microfluidic platform. Exhibiting high compatibility with organic solvents and the ability to carry out sophisticated actuation and sensing of reaction droplets, EWOD is a unique platform for performing diverse microscale chemical syntheses in small volumes, including multistep processes with intermediate solvent-exchange steps.

Microscale Liquid-Metal Switches—A Review

Microelectromechanical systems (MEMS) have constituted an active R&D area over the last two to three decades, with one of the earliest application topics being microswitches. Typical designs involve actuation of microscale flexural elements (e.g., beams and membranes) to make a short or an opening in the transmission (signal) line. However, the problem of reliability of these switches persisted due to the presence of a solid-solid contact. Inspired by the regular mercury switches that use liquid-solid contact to solve the problems, several researchers have been exploring the use of liquid metal (LM) in developing microscale switches. Over time, the following two different approaches have evolved: LM-wetted microswitches and LM-actuated microswitches. In this paper, we summarize the progress of both approaches over the last decade by reporting a series of LM microswitches, each with the mechanism, fabrication, and performance. In addition, the properties of various LMs and LM alloys and the issues of fabrication and packaging involving LM are presented to help understand the reported developments as well as to assist in designing future LM microswitches.

Electrowetting on dielectric-based microfluidics for integrated lipid bilayer formation and measurement

We present a microfluidic platform for the formation and electrical measurement of lipid bilayer membranes. Using electrowetting on dielectric (EWOD), two or more aqueous droplets surrounded by a lipid-containing organic phase were manipulated into contact to form a lipid bilayer at their interface. Thin-film Ag/AgCl electrodes integrated into the device enabled electrical measurement of membrane formation and the incorporation of gramicidin channels of two bilayers in parallel.