Electrowetting-on-dielectrics for manipulation of oil drops and gas bubbles in aqueous-shell compound drops

Bubble Manipulation Driven by Alternating Current Electrowetting: Oscillation Modes and Surface Detachment

In this paper, a millimeter-sized bubble in water pending on a substrate is manipulated by applying an alternating current (AC) electric field, known as electrowetting on dielectric. In this setup, standing waves on the bubble surface are observed. The amplitude of these waves varies with frequency, and three resonance peaks (21, 76, and 134 Hz) can be identified. By incorporating the nonlinear friction force for the contact line to an existing surface mode model, a significant improvement to explain the spectrum of the oscillations is obtained, predicting three peak positions, widths, and heights with good accuracy. We also show that bubble detachment correlates with the low-frequency resonance peak. It is found experimentally that if close enough to this peak, then bubbles at sufficiently high voltages are observed to detach from the substrate. This suggests that inertial effects can effectively promote bubble detachment. To confirm this hypothesis, the bubble dynamics is simulated with COMSOL using the full Navier-Stokes equation with a two-phase field and electrostatic stresses. It was found that the bubble experimental detachment process is quite well-reproduced in the simulation, confirming the role of fluid inertia for the detachment process. Given the nice correspondence between the experimental state diagrams and the theoretical modeling, this work contributes to identify a window for precise and reliable bubble manipulation by means of AC electrowetting.

Towards controlled bubble nucleation in microreactors for enhanced mass transport

The exact location of bubbles with respect to the catalytic layer impacts the microreactor performance. In this work, we propose to control bubble nucleation using micropits to maximize the microreactor efficiency.

An integrated microfluidic device for studying controllable gas embolism induced cellular responses

Gas embolism is the abnormal emergence of bubble in the vascular system, which can induce local ischemic symptoms. For studying the mechanism underlying gas embolism and revealing local ischemic diseases information, novel technique for analyzing cells response to bubble contact with high controllability is highly desired. In this paper, we present an integrated microfluidic device for the precise generation and control of microbubble based on the gas permeability of polydimethysiloxane (PDMS) to study the effect of bubble's mechanical contact on cells. Cell viability analysis demonstrated that short-term (<15 min) bubble contact was generally non-lethal to cultured endothelial cells. The significant increase in intracellular calcium of the microbubble-contacted cells and cell-to-cell propagation of calcium signal in the adjacent cells were observed during the process of bubble expansion. In addition, the analysis of intercellular calcium signal in the cells treated with suramin and octanol revealed that cell-released small nucleotides and gap junction played an important role in regulating the propagation of calcium wave triggered by bubble contact. Thus, our microfluidic method provides an effective platform for studying the effect of gas embolism on cultured adherent cells and can be further needed for anti-embolism drugs test.

On-chip organic synthesis enabled using an engine-and-cargo system in an electrowetting-on-dielectric digital microfluidic device

This paper presents a microfluidic chemical reaction using an electrowetting-on-dielectric (EWOD) digital microfluidic device. Despite a number of chemical/biological applications using EWOD digital microfluidic devices, their applications to organic reactions have been seriously limited because most of the common solvents used in synthetic organic chemistry are not compatible with EWOD devices. To address this unsolved issue, we first introduce a novel technique using an "engine-and-cargo" system that enables the use of non-movable fluids (e.g., organic solvents) on an EWOD device. With esterification as the model reaction, on-chip chemical reactions were successfully demonstrated. Conversion data obtained from on-chip reactions were used to characterize and optimize the reaction with regard to reaction kinetics, solvent screening, and catalyst loading. As the first step toward on-chip combinatorial synthesis, parallel esterification of three different alcohols was demonstrated. Results from this study clearly show that an EWOD digital microfluidic platform is a promising candidate for microscale chemical reactions.

Controllable manipulation of bubbles in water by using underwater superaerophobic graphene-oxide/gold-nanoparticle composite surfaces

Manipulation of bubbles in water is of great importance in the mineral industry, oil production and separation, wastewater treatments, boiling processes, biological cell incubations, microfluidics and miniature reactor technologies. Generally, bubbles in an aqueous environment are inclined to stick to the channel walls, resulting in blockage and energy-consuming treatments. Herein, we report the fabrication of low-adhesion graphene-oxide (GO)/gold-nanoparticle (GNP) hybrid films with a good underwater superaerophobicity, which have the ability to arbitrarily manipulate bubbles in water. Owing to the hydrophilic nature of GO nanosheets and hierarchical structures of aggregated GNPs, the GO/GNP hybrid films showed low adhesion towards bubbles in water. Thus, bubbles could be freely manipulated using home-made tools coated with these low-adhesion, underwater superaerophobic GO/GNP hybrid films. The controlled 1D and 2D movements of one bubble and merging/reaction of two bubbles can be achieved. This study provides a new avenue to design new strategies for bubble manipulations, and further extends the application of superwettable 2D materials in interface fields involving gas phases.

Axisymmetric and Nonaxisymmetric Oscillations of Sessile Compound Droplets in an Open Digital Microfluidic Platform

Manipulating droplets of biological fluids in an electrowetting on dielectric (EWOD)-based digital microfluidic platform is a significant challenge because of biofouling and surface contamination. This problem is often addressed by operating in an oil environment. We study an alternate configuration of sessile compound droplets having an aqueous core surrounded by a smaller oil shell. In contrast to the conventional EWOD platform, an open digital microfluidic platform enabled by the core-shell configuration will allow electrical, mechanical, or optical probes to get unrestricted access to the droplet, thus enabling highly flexible and dynamically reconfigurable lab-on-chip systems. Understanding droplet oscillations is essential as they are known to enhance mixing. To our knowledge, this is the first study of axisymmetric and nonaxisymmetric oscillations of compound droplets actuated using EWOD platforms. Mode shapes for both axisymmetric and nonaxisymmetric oscillations were studied and explained. Enhancement in the axisymmetric oscillation of the core by decreasing the shell volume was obtained experimentally and modeled theoretically. Smaller shell volumes reduce the damping losses, allowing the appearance of nonaxisymmetric modes over a larger range of operating parameters. The oscillation frequency regime for obtaining prominent nonaxisymmetric oscillations for different shell volumes was identified. Compound droplets provide a mechanism to reduce biofouling, sample contamination, and evaporation. We demonstrate axisymmetric and nonaxisymmetric oscillations of compound droplets with the biological core of red blood cells, providing crucial first steps for promoting applications such as rapid efficient assays, mixing of biological fluids, and fluidic photonics on hysteresis-free surfaces.

Parametric studies on droplet generation reproducibility for applications with biological relevant fluids

Although the great potential of droplet based microfluidic technologies for routine applications in industry and academia has been successfully demonstrated over the past years, its inherent potential is not fully exploited till now. Especially regarding to the droplet generation reproducibility and stability, two pivotally important parameters for successful applications, there is still a need for improvement. This is even more considerable when droplets are created to investigate tissue fragments or cell cultures (e.g. suspended cells or 3D cell cultures) over days or even weeks. In this study we present microfluidic chips composed of a plasma coated polymer, which allow surfactants-free, highly reproducible and stable droplet generation from fluids like cell culture media. We demonstrate how different microfluidic designs and different flow rates (and flow rate ratios) affect the reproducibility of the droplet generation process and display the applicability for a wide variety of bio(techno)logically relevant media.

Droplet-based electro-coalescence for probing threshold disjoining pressure

In this work, we investigate the coalescence of emulsion droplets in a controlled electric field. Two contacting droplets stabilized by surfactants can be forced to coalesce into a combined one when the applied voltage is above a critical value. The critical voltages change with the types, concentrations of surfactants and temperature. By exploring the drainage of a thin oil film trapped between emulsions, we interpret that the coalescence occurs as the electric compression overcomes the disjoining pressure barrier and squeezes the film to a critical thickness. Based on this, we have devised an approach to probe the threshold disjoining pressure which can help predict the emulsion stability and surfactant efficacy quantitatively. We have confirmed the validity of our approach for measuring the threshold disjoining pressure by comparing the result with other proven tests that involve centrifugation and thermal heating. Our approach is simple, reliable and robust in predicting emulsion stability and will facilitate the design of emulsion-based formulations by accelerating the testing of emulsion stability.