Sang Kug Chung

Droplet manipulation and microparticle sampling on perforated microfilter membranes

This paper describes droplet manipulation and microparticle sampling where droplets driven by the electrowetting-on-dielectric (EWOD) principle are transported on perforated microfilter membranes and pick up microparticles in their path. Three designs of microfilter membranes that have different hole shapes (rounded rectangle versus circle holes), sizes (≥6 µm) and opening area ratios (5, 9 and 20%) are microfabricated and tested along with a commercial membrane (rounded rectangle holes of 60 µm × 128 µm, 17% opening area ratio). All the tested perforated membranes are embedded with a linear array of electrodes for EWOD actuation. Reversible EWOD actuations and droplet transportations are successfully achieved on the perforated filter membranes. Particle sampling is examined against glass particles (8 µm diameter and ~14° contact angle) and polystyrene particles (8 µm diameter and ~66° contact angle). It is demonstrated that as droplets are moved on the microfilter surfaces by EWOD actuation, they efficiently pick up the microparticles in their path, showing high sampling efficiencies: over 95% for the glass particles and over 85% for the polystyrene particles. This particle sampling method uses a small liquid volume (microliters or smaller) and has the capability of fully automatic handling. Thus, it is expected to be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological and chemical analyses.

On-chip manipulation of objects using mobile oscillating bubbles

This paper describes a new on-chip manipulation method for handling millimeter- and micron-sized objects using oscillating mobile bubbles. It is found that acoustically excited oscillating bubbles can attract and capture neighboring objects. A variety of objects, including hydrophilic glass beads (80 µm), polystyrene beads (100 µm), a fish egg (~1 mm) and a live water flea (~1 mm), are successfully captured. The capturing performance is characterized using 80 µm hydrophilic glass particles while varying the acoustic excitation frequency and amplitude. The oscillation amplitude of the bubbles is quantified using high-speed images. At the natural frequencies of the bubbles the capturing range is highest. The capturing range increases as the oscillation amplitude increases. It is also found that while the bubbles are in lateral motion the capturing force is strong enough to hold the captured objects. By integrating acoustic excitation with electrowetting-on-dielectric (EWOD) bubble transportation, it is demonstrated that oscillating mobile bubbles can capture, carry and release neighboring objects on a chip. This new manipulation method may provide an efficient tool for handling millimeter- as well as micron-sized objects such as biological cells.

Micropumping by an Acoustically Excited Oscillating Bubble for Automated Implantable Microfluidic Devices

When a gaseous bubble in liquid is excited by acoustic waves, it oscillates (expands and shrinks) at the wave frequencies and generates strong vortical flows around it, the so-called cavitational microstreaming. This article describes the development of a micropumping principle using cavitational microstreaming. The key idea is to place a capillary tube vertically above an oscillating bubble to collect the upward microstreaming flow. When the bubble is excited at its resonance frequency, it oscillates with surface undulations (surface wave mode) and pumps water through the tube. The performance of this pumping mechanism is experimentally studied using millimeter and microscale bubbles. The flow rate and generated pressure are measured in a variety of conditions. The measured results indicate that the present pump falls into the category of moderate-flow-rate and low-pressure type pumps. The present pump operates without physical connections or electrical wiring to the bubbles, implicating potential applications as implantable micropumps in many lab-on-a-chip type systems.

Electrowetting propulsion of water-floating objects

This letter describes a propulsion principle along with experimental verification of this principle by which an air-to-water interface vertically oscillated by ac electrowetting generates a quasisteady, “streaming” flow that can be utilized to propel water-floating objects. This propulsion does not require any mechanical moving parts. Using a centimeter-sized boat whose outer surfaces were covered with microfabricated electrowetting electrodes, linear, and rotational motions of the boat were achieved up to maximum speeds of 5 mm/s and 20 rpm, respectively. By combining the above two motions, the boat was successfully propelled and steered along a curvilinear pathline. A potential application of this principle is to propel and maneuver various water-floating mini/microrobots and boats used for water/air quality monitoring or surveillance/security purposes.

On-chip creation and elimination of microbubbles for a micro-object manipulator

This paper describes on-chip microbubble creation and elimination using an electrolytic process integrated with electrowetting on dielectrics (EWOD) actuations. These bubble operation units are developed for the microbubble-based micro-object manipulator in which EWOD-actuated microbubbles can be used not only as micro-object carriers but also as flow amplifiers using ultrasonic excitations. By applying voltages between the anode and cathode, microbubbles of oxygen, hydrogen and their mixture ranging from several microns to several hundred microns in diameter are on-chip created electrolytically. EWOD actuation is used to detach and transport the bubbles from the creation site. On-chip bubble elimination is accomplished by using the reverse electrolytic reaction. A platinum electrode, which acts as a catalyst in the reverse electrolytic reaction, substantially increases the elimination rate of microbubbles, by an order of magnitude, as compared to a Teflon-covered Si surface. The catalytic reaction is effective in eliminating hydrogen, oxygen and mixture bubbles. Interestingly, the Teflon-coated Pt electrodes show a higher elimination rate than the bare Pt electrode. This is attributed to a larger contact area with the Teflon-covered electrode (hydrophobic surface) than with the bare Pt electrode (hydrophilic surface). For the Teflon-covered electrodes, the bubble elimination rate increases with thinner Teflon coating. Integrated bubble operations—creation, transportation and elimination—are realized on a single chip in which coating of the Pt electrode with a Teflon layer allows transporting of bubbles into the elimination site. Finally, as a proof of concept for the micro-object manipulator, it is demonstrated that EWOD-actuated bubbles can push micron- and millimeter-sized objects and release them at a different place. (Some figures in this article are in colour only in the electronic version)

On-demand magnetic manipulation of liquid metal in microfluidic channels for electrical switching applications

We report magnetic-field-driven on-demand manipulation of liquid metal in microfluidic channels filled with base or acid. The liquid metal was coated with iron (Fe) particles and treated with hydrochloric acid to have strong bonding strength with the Fe particles. The magnetic liquid metal slug inserted in the microchannel is manipulated, merged, and separated. In addition, corresponding to the repositioning of an external magnet, the liquid metal slug can be readily moved in microfluidic channels with different angles (>90°) and cross-linked channels in any direction. We demonstrated the functionality of the liquid metal in the microfluidic channel for electrical switching applications by manipulation of the liquid metal, resulting in the sequential turning on of light emitting diodes (LEDs).

Micro bubble fluidics by EWOD and ultrasonic excitation for micro bubble tweezers

Recently, we envisioned so called micro bubble tweezers where EWOD (electrowetting-on-dielectric) actuated bubbles can manipulate micro objects such as biological cells by pushing or pulling them. Besides, oscillating (shrinking and expanding) bubbles in the presence of ultrasonic wave act as a to deliver drugs and molecules into the cells. In this paper, as a great stride in our quest for micro bubble tweezers, we present (1) full realization of two critical bubble operations (generating of bubbles in an on-chip and on-demand manner and splitting of single bubbles) and (2) two possible applications of mobile bubbles oscillating under acoustic excitation (a mobile vortex generator and micro particle carrier).

Drug perfusion enhancement in tissue model by steady streaming induced by oscillating microbubbles

Drug delivery into neurological tissue is challenging because of the low tissue permeability. Ultrasound incorporating microbubbles has been applied to enhance drug delivery into these tissues, but the effects of a streaming flow by microbubble oscillation on drug perfusion have not been elucidated. In order to clarify the physical effects of steady streaming on drug delivery, an experimental study on dye perfusion into a tissue model was performed using microbubbles excited by acoustic waves. The surface concentration and penetration length of the drug were increased by 12% and 13%, respectively, with streaming flow. The mass of dye perfused into a tissue phantom for 30s was increased by about 20% in the phantom with oscillating bubbles. A computational model that considers fluid structure interaction for streaming flow fields induced by oscillating bubbles was developed, and mass transfer of the drug into the porous tissue model was analyzed. The computed flow fields agreed with the theoretical solutions, and the dye concentration distribution in the tissue agreed well with the experimental data. The computational results showed that steady streaming with a streaming velocity of a few millimeters per second promotes mass transfer into a tissue.

Manipulation of Micro/Mini-objects by AC-Electrowetting-Actuated Oscillating Bubbles: Capturing, Carrying and Releasing

Abstract This paper describes a new principle and experimental results of object manipulation using oscillating bubbles. AC-electrowetting is applied to oscillate an air bubble in a water medium in order to capture or repel neighboring micro/mini-objects. A series of experiments show capturing of various objects including glass beads and fish eggs, which is highly dependent on the oscillation frequency/amplitude and the density difference between the object and medium. In particular, it is shown that oscillating bubbles generate a repelling force when the density of neighboring objects is much lower than that of the medium. This result partially verifies the Nyborg/Miller’s prediction based on the radiation force, implying that the radiation force is responsible for capturing and repelling neighboring objects. In addition, the capturing force is indirectly measured by exposing the oscillating bubble along with the captured glass beads to the stream in a mini-channel. By measuring the maximum speed of the channel flow that makes all captured glass beads separated from the oscillating bubble, the capturing force is inferred based on the Stokes approximation. Finally, capturing and releasing are integrated with bubble transportation. AC-electrowetting not only oscillates a bubble, but also laterally transports the oscillating bubble on a 2-D surface. Using this scheme, a series of operations (capturing, carrying, and releasing) are achieved on a chip by sole AC-electrowetting without any acoustic excitations for bubble oscillation. This new manipulation method may provide an efficient tool for handling micro/mini-objects including biological cells.

Underwater propulsion using AC-electrowetting-actuated oscillating bubbles for swimming robots

This paper describes development and experimental verifications of a novel underwater propulsion technique using AC-electrowetting-actuated oscillating bubbles. To prove the concept of propulsion, an air bubble (300 µm dia. or 1.5 mm dia.) is installed on the tip of a metal rod covered with electrowetting dielectric layers. When an AC-electrowetting signal is applied between the metal rod and water medium, the bubble oscillates at the frequency of the applied signal and generates a steady streaming flow around the bubble. The streaming flow in turn generates a reaction force to the metal rod, resulting in propelling of the metal rod. The similar propulsion principle is applied to a centimeter-sized object. To the sidewalls of the object, a pair of microfabricated electrowetting electrodes with air bubbles is diagonally attached to generate a torque. When an electrowetting signal is transferred by wired or wirelessly to the electrodes, the object is rotated. These results experimentally prove that oscillating bubbles propel underwater objects, the size of which ranges from a few hundred microns to centimeters. Unlike the conventional propulsion, this technique does not need any moving solid parts, possibly providing a simple and efficient propulsion mechanism for robots swimming inside human body in applications of bio-surgery, bio-sensing and drug delivery.