Liquid Dielectrophoresis Research Articles

Deflection of sliding droplets by dielectrophoresis force on a superhydrophobic surface

In this study, we experimentally identify the effect of liquid dielectrophoresis (LDEP) force on a superhydrophobic surface in directing the trajectory of moving water droplets across designed interdigitated electrodes and show that this method is capable of rapidly selecting droplets at a high speed (200 mm/s). As the droplets traverse down the surface by the electric field, their deflection on the edge of these electrodes is achieved successively, allowing for the selective manipulation of discrete droplets. A series of experiments were conducted to validate the relationships among droplet deflections, applied electric fields, and dynamic contact angles. Our findings reveal that the principal driving force behind the droplet deflections is the LDEP force, which can provide instant manipulation of moving droplets rather than a variation in contact angles brought about by electrowetting. This study presents a proof-of-concept experiment utilizing LDEP for high-throughput droplet selection and also highlights the potential applications of this mechanism in high-speed digital microfluidics (DMF) and biological separation methodologies.

A tumor on a chip for studying immune-cell infiltration into tumor under chemo/immunotherapy treatments

The combo treatment of chemo/immunotherapy is a promising method in treating cancer patients. The results of Keynote-189 phase III clinical trial showed that chemotherapy drug plus immunotherapy drug reduced mortality compared with chemotherapy drug alone, in which the underlying mechanism has not been well demonstrated. The treatment combination also needs to be tuned for each patient, i.e., personalized medicine. Tumor-on-a-chip aims to mimic the tumor microenvironment (TME) with an in-vitro lung cancer-immune model to provide a potential solution for this application. Gelatin methacryloyl (GelMA)-cancer cells were driven on the ITO electrode track using liquid dielectrophoresis (LDEP) to form the tumor-like hydrogel. Jurkat T cells surrounded the solidified GelMA-A549 cells, resembling the cancer-immune microenvironment. The immune-cell infiltration phenomena of Jurkat T cells into the tumor were activated and enhanced via either IL-1β-treated A549 cells or atezolizumab (immune drugs) on the tumor lab chip. The death of cancer cells significantly increased ∼35% after chemo/immunotherapy combination treatment compared with the single treatment. The distribution of dead cancer cells treated with chemo/immunotherapy revealed their distinct theoretical mechanisms, indicating the promising application of this tumor lab chip for cancer patients in personalized medicine treatments.

Droplet Motion Driven by Liquid Dielectrophoresis in the Low-Frequency Range.

Electrohydrodynamic wetting manipulation plays a major role in modern microfluidic technologies such as lab-on-a-chip applications and digital microfluidics. Liquid dielectrophoresis (LDEP) is a common driving mechanism, which induces hydrodynamic motion in liquids by the application of nonhomogeneous electrical fields. Among strategies to analyze droplet movement, systematic research on the influence of different frequencies under AC voltage is missing. In this paper, we therefore present a first study covering the motion characteristics of LDEP-driven droplets of the dielectric liquids ethylene glycol and glycerol carbonate in the driving voltage frequency range from 50 Hz to 1600 Hz. A correlation between the switching speed of LDEP-actuated droplets in a planar electrode configuration and the frequency of the applied voltage is shown. Hereby, motion times of different-sized droplets could be reduced by up to a factor of 5.3. A possible excitation of the droplets within their range of eigenfrequencies is investigated using numerical calculations. The featured fluidic device is designed using larger-sized electrodes rather than typical finger or strip electrodes, which are commonly employed in LDEP devices. The influence of the electrode shape is considered simulatively by studying the electric field gradients.

Electric-Field-Induced Selective Directed Transport of Diverse Droplets.

Droplet directional transport is one of the central topics in microfluidics and lab-on-a-chip applications. Selective transport of diverse droplets, particularly in another liquid phase environment with controlled directions, is still challenging. In this work, we propose an electric-field gradient-driven droplet directional transport platform facilitated by a robust lubricant surface. On the platform, we clearly demonstrated a liquid-inherent critical frequency-dominated selective transport of diverse droplets and a driving mechanism transition from electrowetting to liquid dielectrophoresis. Enlightened by the Kelvin-Helmholtz theory, we first realize the directional droplet transport in another liquid phase whenever a permittivity difference exists. Co-transport of multiple droplets and various combinations of droplet types, as well as multifunctional droplet transport modes, are realized based on the presented powerful electric-field gradient-driven platform, overcoming the limitations of the surrounding environment, liquid conductivity, and intrinsic solid-liquid wetting property existing in traditional droplet transport strategies. This work may inspire new applications in liquid separation, multiphase microfluidic manipulation, chemical reagent selection, and so on.

Study on liquid dielectrophoresis based on double flexible electrodes simulating interdigitated pattern electrodes

Dielectrophoresis affects the surface wettability by applying a non-uniform electric field to dipoles inside dielectric liquid, achieving adjustable droplet contact angle and overcoming the saturation limitation of contact angle caused by the electrowettability effect. However, it is difficult to realize useful three-dimensional tunable optical devices because most of the driving electrodes need to be patterned. In this work, a model of double flexible electrodes simulating planar interdigitated pattern electrodes is proposed based on the dielectrophoresis. Double flexible electrodes, which are wrapped with an insulating dielectric layer and are not conductive to each other are arranged at close intervals and wound along the plane substrate to form a two-dimensional planar line wall. A hydrophobic layer is used to fill the gap and increase the initial contact angle. Ultimately, the “droplet-interdigitated planar line wall” dielectrophoresis driven-droplet model is formed after the dielectric droplets have been deposited on the line wall surface. Firstly, considering the influence of penetration depth and electrode gap area, Young’s equation is theoretically modified to adapt to this model. Then, the finite element algorithm simulation is used to used to comparatively analyze the potential distribution of this model and the planar interdigitated pattern electrode model. The field strength distributions of the electrodes with different wire diameters and insulating layer thickness values are analyzed. It can be found that with the increase of the diameter of the electrode wire and the thickness of the insulating layer, the morphology of the model changes from the tip electrode into the planar electrode, the surface field strength attenuates exponentially and the peak value decreases. This shows that the structure of this electrode in this model is superior to that of the planar electrode. After that, the contact angle of the model is measured experimentally in a range of 58°－90° under 0–250 <i>V</i><sub>rms</sub> voltage, which is in line with the theoretical expectation. At the same time, neither obvious contact angle lag nor saturation is observed in the experiment. Finally, the new electrophoretic driving droplet model constructed in this paper transforms the dielectric electrophoretic driving mode from a two-dimensional planar electrode to a one-dimensional flexible linear electrode. Because of its flexibility and plasticity, it is convenient to form a three-dimensional cavity and can be applied to more complex device structures.

Experimental and theoretical investigation of frequency-dependent and conductivity-dependent threshold actuation voltage of electrowetting-on-dielectric and liquid dielectrophoresis on a one-plate device

Actuation voltage is critical in microfluidic manipulation using electrowetting-on-dielectric (EWOD) and liquid dielectrophoresis (LDEP). Compared to parallel electrode devices, the liquid manipulation of coplanar electrodes requires a higher actuation voltage due to its structure. This study derives an equation related to the threshold actuation voltage and all important parameters (droplet volume, fluid permittivity, fluid conductivity, applied frequency, surface tension, thickness and permittivity of dielectric materials, and electrode width). The equation is also used to analyze and explain frequency-dependent, and conductivity-dependent threshold actuation voltage on a coplanar electrode structure in the experiment. Three water-based solutions with different electrical conductivities (3 *10−4-10−2 S/m) were tested for the threshold voltage of liquid actuation using a wide range of signal frequencies (0.1–100 kHz). The theoretical and experimental data reveal that our model can successfully explain the effects of conductivity and frequency on liquid actuation. In addition, the effects of EWOD and LDEP on the threshold actuation voltage of microfluidics on a one-plate device are also discussed with the model and experiment. This study covers a common range for important parameters (applied frequency, and fluid conductivity) related to the actuation voltage in microfluidic actuation on a coplanar electrode structure. The theory and experimental data can be used to predict the applied voltage of EWOD and LDEP for a broad range of buffers and organic solvents.

Introducing electric field fabrication: A method of additive manufacturing via liquid dielectrophoresis

Biomedical devices with millimeter and micron-scaled features have been a promising approach to single-cell analysis, diagnostics, and fundamental biological and chemical studies. These devices, however, have not been able to fully embrace the advantages of additive manufacturing (AM) that offers quick prototypes and complexities not achievable via traditional 2D fabrication techniques (e.g., soft lithography). This slow adoption of AM can be attributed in part to limited material selection, resolution, and inability to easily integrate components mid-print. Here, we present the feasibility of using liquid dielectrophoresis to manipulate and shape a droplet of build material, paired with subsequent curing and stacking, to generate 3D parts. This Electric Field Fabrication (EFF) technique is an additive manufacturing method that offers advantages such as new printable materials and mixed-media parts without post-assembly for biomedical applications.

Dielectrowetting on curved surfaces

Programmable fluidic systems on curved and flexible substrates are of increasing interest. One approach to achieving programmability is the controlled sequential wetting and dewetting on a surface using voltage actuation. In particular, liquid dielectrophoresis techniques have recently been shown to provide the ability to form a spread liquid film on a normally liquid repellent, but rigid, substrate via applying a spatially periodic electrical potential underneath an initial sessile droplet. In this work, we demonstrate the creation of thin, rectangular shaped, films of electrically insulating liquid on the side of a curved and flexible liquid repellant substrate using dielectrophoresis forces. We find that the experimental threshold voltage VT(κs) for film formation has a monotonic dependence on the value of the substrate curvature κs in the range −0.4 mm−1 < κs < 0.26 mm−1. By considering the balance of stresses acting on the films, including the Laplace pressure and the Maxwell stress, we develop an analytical theoretical expression that is in excellent quantitative agreement with our curvature dependent experimental threshold voltage measurements. The resulting physical insights and the demonstration of programmable wettability on curved and flexible substrates with both positive and negative curvature provide the foundations for applications in imaging, displays, and biochemical analysis.

Active heat transfer enhancement by interface-localized liquid dielectrophoresis using interdigitated electrodes

We introduced an active heat transfer control between graphene and water using interdigitated electrodes (IDEs). Oppositely charged co-planer electrodes embedded on a graphene surface created a non-uniform electric field. Resulted interface localized liquid dielectrophoresis (LDEP) perpendicular to surface enhanced the water/graphene coupling and decreased interfacial thermal resistance (ITR) substantially. We correlated the theoretical calculations of average electric field strength near surface with Kapitza values measured at corresponding electrode configurations. We obtained a unified linear variation of Kapitza as a function of average electric strength independent of electrode size and charge. By increasing the electric field strength, we measured up to 96% decrease of Kapitza near electrodes. Since the IDEs generated electric field was only interface localized, it required lower electrode charges than any parallel-plate capacitor systems. We showed that ITR remains effective in heat transfer behavior for systems as big as 100nm such that interface localized electric field can at least increase the heat removal 50% by eliminating the ITR from both graphene/water interfaces of a channel system. By converting hydrophobic few-layer graphene to super-hydrophilic condition with ultra-low Kapitza, current results are important for graphene-based materials considered for the solution of the thermal management problem of current and next generation micro/nano-electronics.

Tilting micromirror platform based on liquid dielectrophoresis

This study presents an electrically actuated tilting micro platform based on liquid dielectrophoresis with three axes movement using three droplets situated 120° apart from each other. The interdigitated electrodes produce a non-uniform electric field that generates a body force. The dielectrophoretic mechanism is responsive within at least 30 ms, and it eliminates the solid-solid contact. The tilting platform enabled an angular coverage up to 0.9° (±0.02°), with a maximum displacement of 120 µm. The tilting micromirror platform has beam steering characteristics suitable for various optical applications. The actuating platform sensor is a cost-effective and simple alternative method to study liquid dielectrophoresis without measuring the droplet contact angle. Furthermore, the unique configuration without any solid-solid contact offers a potential improvement for applications in optics, actuators, and other conventional microelectromechanical systems.

Motion of generated dumbbell-shaped satellite droplets during liquid dielectrophoresis

Actuation of sub-millimeter sized droplets has important implications in heat transfer, microfluidics, and self-cleaning surfaces. Here, we report the motion of satellite droplets generated around a primary droplet actuated using liquid dielectrophoresis (L-DEP). Satellite droplets spanning the electrode gap grow due to the electric field induced enhanced merging of droplets. These satellite droplets attain a dumbbell shape due to the applied field. Interestingly, satellite droplets follow the primary drop interface maintaining a constant gap between themselves and the primary drop. The motion of satellite droplets was captured using high-speed imaging, and the role of temperature gradient was verified by measuring temperature distribution using an IR camera. Through observations and simulations, we qualitatively believe that this behavior arises from (a) repulsive electrostatic and temperature gradient forces; and (b) attractive vapor concentration gradient forces. Simulations were used to estimate the direction of individual forces. Interfacial charges from the applied electric field give rise to the electrostatic repulsion. During L-DEP, the primary droplet heats due to induced current. Thermal conduction through the substrate sets up a temperate gradient, which adds to the repulsion forces between the droplets. Evaporation of the primary droplet leads to the vapor concentration gradient, responsible for the attraction force between the droplets.

Electrical actuation of dielectric droplets by negative liquid dielectrophoresis.

This paper presents an electrical actuation scheme of dielectric droplet by negative liquid dielectrophoresis. A general model of lumped parameter electromechanics for evaluating the electromechanical force acting on the droplets is established. The model reveals the influence of actuation voltage, device geometry, and dielectric parameter on the actuation force for both conductive and dielectric medium. Using this model, we compare the actuation forces for four liquid combinations in the parallel-plate geometry and predict the low voltage actuation of dielectric droplets by negative dielectrophoresis. Parallel experimental results demonstrate such electric actuation of dielectric droplets, including droplet transport, splitting, merging, and dispending. All these dielectric droplet manipulations are achieved at voltages <100 Vrms . The frequency dependence of droplet actuation velocity in aqueous solution is discussed and the existence of surfactant molecules is believed to play an important role by realigning with the AC electric field. Finally, we present coplanar manipulation of oil and water droplets and formation of oil-in-water emulsion droplet by applying the same low voltage.

Lattice Boltzmann Simulations of Multiphase Dielectric Fluids.

The dynamic effect of an electric field on dielectric liquids is called liquid dielectrophoresis. It is widely used in several industrial and scientific applications, including inkjet printing, microfabrication, and optical devices. Numerical simulations of liquid-dielectrophoresis are necessary to understand the fundamental physics of the phenomenon, but also to explore situations that might be difficult or expensive to implement experimentally. However, such modeling is challenging, as one needs to solve the electrostatic and fluid dynamics equations simultaneously. Here, we formulate a new lattice-Boltzmann method capable of modeling the dynamics of immiscible dielectric fluids coupled with electric fields within a single framework, thus eliminating the need of using separate algorithms to solve the electrostatic and fluid dynamics equations. We validate the numerical method by comparing it with analytical solutions and previously reported experimental results. Beyond the benchmarking of the method, we study the spreading of a droplet using a dielectrowetting setup and quantify the mechanism driving the variation of the apparent contact angle of the droplet with the applied voltage. Our method provides a useful tool to study liquid-dielectrophoresis and can be used to model dielectric fluids in general, such as liquid-liquid and liquid-gas systems.

Continuous Droplet-Actuating Platforms via an Electric Field Gradient: Electrowetting and Liquid Dielectrophoresis.

This work develops a technology for actuating droplets of any size without the requirement for high voltages or active control systems, which are typically found in competitive systems. The droplet actuation relies on two microelectrodes separated by a variable gap distance to generate an electrostatic gradient. The physical mechanism for the droplet motion is a combination of liquid dielectrophoresis and electrowetting. Investigating the system behavior as a function of the driving frequency identified the relative contribution of these two mechanisms and the optimum operating conditions. A fixed signal frequency of 0.5 kHz actuated various liquids and contaminants. Droplet actuation was demonstrated on several platforms, including linear, radial-symmetric, and bilateral-symmetric droplet motion. The electrode designs are scalable and can be fabricated on a flexible and optically transparent substrate: these key advancements will enable consumer applications that were previously inaccessible. A self-cleaning platform was also tested under laboratory conditions and on the road. This technology has significant potential in microfluidics and self-cleaning platforms, for example, in the automotive sector to clean body parts, camera covers, and sensors.

Programmable droplet actuating platform using liquid dielectrophoresis

Droplet motion has been a long-standing interest in microfluidics as it is often limited by the high operating voltages, which hampers the development of consumer applications. Forces generated by liquid dielectrophoresis (L-DEP) can enhance surface wetting, without requiring chemical modification or surface texturing. This work presents a droplet actuating platform to control the wetting behaviour of water droplets using L-DEP. The exploitation of high electric fields at the droplet solid–liquid interface reduced the operating voltages. The operating voltage can be further reduced, to as low as 30 V, by introducing a lubricant layer to minimise the droplet contact angle hysteresis, thus requiring a smaller L-DEP bulk force. The outcomes of this study will provide a new pathway for developing energy-efficient and reliable droplet actuating platforms to clean a variety of surfaces. A particular interest will be the application of this system in the automobile sector, for instance, to clean headlamps, sensors, cameras, door mirrors, front side glass, and rear screen.

Field-effect pump: liquid dielectrophoresis along a virtual microchannel with source-gate-drain electric fields.

We investigate liquid dielectrophoresis (LDEP) to implement field-effect pumps (FEPs) that drive liquids from source, via gate, toward drain electric fields between parallel plates without external pumps or the problem of dead volume. The appropriate gate electric field establishes a wall-less virtual microchannel to transfer the liquid from source to drain with an adjustable flow rate (Q) controlled by the difference of the square of the electric field strength (ΔE2DS). Analogous to field-effect transistors (FETs), the FEPs can operate in a "linear", "transition" or "saturation" region depending on ΔE2GD and ΔE2DS. With a sufficient ΔE2GD and a small ΔE2DS, the FEPs operated in the linear region where Q was linearly proportional to ΔE2DS and inversely proportional to the flow resistance R that was mainly determined by the length (L), width (W) and height (H) of a stable and fully-occupied virtual microchannel. With an insufficient ΔE2GD and a moderate to large ΔE2DS, narrowing, tapering and even pinch-off of virtual microchannels were observed, which increased R and changed the operation into the transition or saturation region. A field-effect stream merger regulating two streams was built based on two FEPs with shared gate and drain electrodes. The versatility of FEPs was demonstrated with preliminary studies on whole blood and particle solutions.

Active Heat Transfer Enhancement by Interface-Localized Liquid Dielectrophoresis Using Interdigitated Electrodes

Active Heat Transfer Enhancement by Interface-Localized Liquid Dielectrophoresis Using Interdigitated Electrodes

Local Heat Transfer Control using Liquid Dielectrophoresis at Graphene/Water Interfaces

Graphene-based materials are considered for the solution of the thermal management problem of current and next generation micro/nano-electronics with high heat generation densities. However, the hydrophobic nature of few-layer graphene makes passing heat to a fluid very challenging. We introduced an active and local manipulation of heat transfer between graphene and water using an applied, non-uniform electric field. When water undergoes electric field induced orientation polarization and liquid dielectrophoresis, a substantial increase in heat transfer develops due to a decrease in interfacial thermal resistance and increase in thermal conductivity. By using two locally embedded pin and plate electrodes of different sizes, we demonstrated a two-dimensional heat transfer control between two parallel few-layer graphene slabs. We obtained local heat transfer increase up to nine times at pin electrode region with an ultra-low Kapitza resistance through the studied non-uniform electric field strength range creating highly-ordered compressed water in the experimentally measured density limits. With this technique, heat can be (i) distributed from a smaller location to a larger section and/or (ii) collected to a smaller section from a larger region. Current results are important for hot spot cooling and/or heat focusing applications.

Microfluidic patterning using a digital microfluidic system

There are some issues in printing technology, such as stencils having a brief life span and limitations in patterning; therefore, we propose microfluidic patterning using an electrowetting-on-dielectric (EWOD) and liquid-dielectrophoresis (L-DEP) system. Propylene carbonate liquid was generated, moved, separated, and positioned, and was patterned into various English alphabetic characters. Unlike traditional printing techniques, patterns become limited and fixed, so the shape of the desired pattern cannot be modified arbitrarily. Each single English alphabetic pattern can be formed with a volume of droplet of about 100 nl; the width of the pattern can be decreased to about 0.15 mm or less. Using an EWOD system, we have implemented an open-plate L-DEP system and a two-plate L-DEP system, for the patterning of all English alphabetic characters with the same volume of liquid at a fixed voltage.

Bubble Control, Levitation, and Manipulation Using Dielectrophoresis

AbstractBubbles attached to surfaces are ubiquitous in nature and in industry. However, bubbles are problematic in important technologies, including causing damage to the operation of microfluidic devices and being parasitic during heat transfer processes, so considerable efforts have been made to develop mechanical and electrical methods to remove bubbles from surfaces. In this work, liquid dielectrophoresis is used to force a captive air bubble to detach away from an inverted solid surface and, crucially, the detached bubble is then held stationary in place below the surface at a distance controlled by the voltage. In this “levitated” state, the bubble is separated from the surface by liquid layer with a voltage‐selected thickness at which the dielectrophoresis force exactly counterbalances the gravitational buoyancy force. The techniques described here provide exceptional command over repeatable cycles of bubble detachment, levitation, and reattachment. A theoretical analysis is presented that explains the observed detachment–reattachment hysteresis in which bubble levitation is maintained with voltages in an order of magnitude lower than those used to create detachment. The precision surface bubble removal and control concepts are relevant to situations such as nucleate boiling and microgravity environments and offer an approach toward “wall‐less” bubble microfluidic devices.