Continuous Electrowetting Research Articles

Application of liquid metal driven-abrasive flow to material removal for the inner surface of channel

Gallium-based eutectic liquid metal alloys possess unique properties such as deformability, high electrical conductivity, and low vapor pressure. These characteristics have generated significant interest in their application for stretchable electronics and microelectromechanical systems (MEMS). Precise manipulation of liquid metal within electrolytes is essential to meet specific functional requirements. This study investigates the electrostatic manipulation of liquid metal in an alkaline solution with abrasives for material removal from the inner surfaces of flow channels. The polarization of the double layer at the gallium‑indium alloy and electrolyte interface is analyzed, elucidating the principle of electrolyte propulsion via continuous electrowetting. A theoretical model is developed, and two- and three-dimensional transient and steady-state simulations of the liquid metal-driven abrasive flow are conducted. Results demonstrate that this method effectively removes material from the inner walls of straight channels during cyclic motion. Utilizing continuous electrowetting, an experimental apparatus was designed, where gallium‑indium liquid metal propelled silicon carbide abrasives against PMMA channel walls. Experimental results showed effective material removal, consistent with finite element simulations, confirming the feasibility of this innovative approach.

Study on the electrowetting and beam current characteristics of externally wetted ionic liquid electrospray thruster

It is the theoretical basis to analyze the hydrodynamics mechanism of ionic liquid in an electrostatic field to ensure that the ionic liquid electrospray thruster possesses good wettability. Stable and continuous electrowetting emitters are closely related to thrust noise, resolution, and stability. Therefore, it is urgent to explore the law of ionic liquid electrowetting emitters in an electrostatic field to improve the performance of thrusters. The article proposes a new hybrid emitter structure, which consists of a V-blade emitter and a square capillary. It also carries out numerical simulation of emitter electrowetting under normal gravity and atmospheric conditions and emission experiments to explore the process of ionic liquid layer spreading on the external surface of the emitter. The dimensions of the emitter d1, d2, and θ and the physical properties of ionic liquids jointly determine the electrowetting velocity, the threshold of wetting voltage, and the liquid layer thickness. By analyzing the influence of various variables on electrowetting, a complete emitter electrowetting law of ionic liquid electrospray thrusters is summarized, which provides an important basis for the optimization of the emitter structure. Considering the scale of the emitter and bond ≈1, the meniscus in the electrostatic field is mainly affected by gravity, surface tension, viscous force, and electrophoretic force. Accordingly, it is worth noting that electrophoretic force is calculated by the Maxwell stress tensor method that treats ionic liquids as a dielectric to ensure the continuity of the meniscus. Although it can satisfy the electrowetting study at the macroscale, the interaction between anions and cations at the micro-scale is ignored. The numerical simulation results and the experimental results are correlated. It is proved that the method used in this article is accurate enough to simulate electrowetting before the liquid layer breaks up to form ion emission.

Small universal mechanical module driven by a liquid metal droplet.

Gallium-based liquid metal droplets (LMDs) from micro-electromechanical systems (MEMS) have gained much attention due to their precise and sensitive controllability under an electric field. Considerable research progress has been made in the field of actuators by taking advantage of the continuous electrowetting (CEW) present within the solution. However, the motion generated is confined within the specific liquid environment and is lacking a way to transmit its motion outwardly, which undoubtedly serves as the greatest obstacle restricting any further development. Therefore, a driving module is proposed to generate rotational motion outside the solution for universality. Its performance can be easily tuned by adjusting the applied voltage. As an example of further application, the module is designed in the form of a pump that realizes the continuous/intermittent propulsion to mimic the veins/arteries of the human body without the problem in the previous LMD-based pumps. The feasibility of this pump in the on-chip in vitro analysis is proved by preparing a dynamic cell culture to simulate the movement of biofluids within human bodies. This study proposes an optional solution with an LMD-based motor for generating rotational motion and to expand current research on soft materials in actuators.

Liquid metal motor.

SummaryLiquid metal has demonstrated an enormous potential for developing soft functional devices and machines. However, current liquid metal enabled machines suffer from several issues, such as the requirement of a liquid environment, generation of weak actuating forces, and insufficient maneuverability. To overcome these restrictions, here, a motor is developed based on the electrical actuation of liquid metal droplets without the need for conventional electromagnets. The approach is distinguished by (1) the encapsulation of electrolyte and multiple liquid metal droplets within an enclosed system, and (2) the creation of stable and continuous torque outside a liquid environment. In addition, a liquid metal electrical brush is introduced to operate the motor with low friction and low wear. The unique driving mechanism endows the motor with several advantages, including low friction, no sparking, low noise, versatile working environment, and being built from soft materials that could offer new opportunities for developing soft robotics.

Pumping of electrolyte with mobile liquid metal droplets driven by continuous electrowetting: A full-scaled simulation study considering surface-coupled electrocapillary two-phase flow.

With the excellent merits of both solid conductors and rheological fluids, liquid metal (LM) provides new opportunities to serve as flexible building blocks of miniaturized electronic and fluidic devices. The phenomenon of continuous electrowetting (CEW) has been long utilized for actuating LM contents in buffer medium, wherein an externally imposed voltage difference is responsible of manipulating the interfacial tension of deformable LM droplets. CEW effectively lowers the surface tension at the LM/electrolyte interface by driving bipolar counterions to the surface of conducting droplet. Since surface tension coefficient relies sensitively on the local voltage drop across the induced double layer, an electric-analogy Marangoni effect occurs even under a rather weak electric field in the presence of a surface gradient of the interfacial tension. CEW of LM routinely induces unidirectional pumping of electrolyte in the direction of applied electric field, with LM droplet translating oppositely within the device channel. Although this subject has received great attention from the microfluidic society in the past decade, previous reports concerned either the individual delivery of the suspension medium or the transport of LM droplet. Starting from this point, we offer herein a fully coupled physical description of two-phase flow dynamics occurring in CEW. The proposed simulation model successfully incorporates the synergy of the interfacial electrokinetic momentum transfer, surface tension on a curved surface, contact angle at the three-phase contact line as well as the gravity force density. The spatial-temporal motion of the contact interface is traced instantly with a moving mesh approach. By direct numerical simulation, the importance of the direct-current bias, additional alternating-current forcing, droplet size, initial ion adsorption in the process of CEW is addressed. Additionally, it is discovered that increasing the number of LM droplet is more cost-effective than enhancing the volume of a single drop in terms of achieving an improvement of the resulted electrocapillary pump performance, while the translational speed of the discrete droplet carrier does not make an observable change in response to a variation in the drop number. These results prove invaluable in terms of an elaborate design of smart on-chip electrokinetic frameworks embedding flexible LM contents in modern micro-total-analytical systems.

Liquid metal droplet-enabled electrocapillary flow in biased alternating electric fields: a theoretical analysis from the perspective of induced-charge electrokinetics

The phenomenon of Marangoni chaotic electroconvection of electrolytes induced by liquid metal droplets has been serving as a method of choice for pumping of microfluidics in the past ten years. Although an external additional alternating voltage, usually being sufficiently small, was invariably employed in practical experiments for stabilizing the unidirectional liquid motion caused by a preexisted direct-current electric field, its subtle effect on the superimposed non-uniform surface tension and the final fluid transport state is still theoretically unclear. From the physical perspective of induced-charge electrokinetics, we present herein an analytical criteria for the conditions under which a sessile metal drop can actuate unidirectional liquid motion passing across it affected by a biased alternating voltage signal, wherein a mechanical balance between the linear pump pressure and nonlinear bipolar stress components exerted on the conducting drop surface has been taken into account. For accuracy validation, a numerical model was developed, in which multiple frequency electrochemical polarization was calculated under the Debye–Huckel limit, and coupled to the mechanical problem via inserting the resultant surface tension gradient as a source term of electrocapillary stress at the droplet–medium interface. The analytical solution accords well with the simulation results, in terms of the threshold voltage amplitude for effective liquid transport along the static potential gradient. At last, we demonstrate that the effect of continuous electrowetting can be fully exploited for simultaneous pumping and extraction of microscale impurities in closed fluid loops embedding a network of three-dimensional floating electrode plates for invoking auxiliary dielectrophoretic attraction. These results prove invaluable for achieving a high degree of freedom manipulation of particle and liquid contents in modern microfluidic systems with flexible conducting microdroplets.

Droplet-Based Microfluidic Thermal Management Methods for High Performance Electronic Devices

Advanced thermal management methods have been the key issues for the rapid development of the electronic industry following Moore’s law. Droplet-based microfluidic cooling technologies are considered as promising solutions to conquer the major challenges of high heat flux removal and nonuniform temperature distribution in confined spaces for high performance electronic devices. In this paper, we review the state-of-the-art droplet-based microfluidic cooling methods in the literature, including the basic theory of electrocapillarity, cooling applications of continuous electrowetting (CEW), electrowetting (EW) and electrowetting-on-dielectric (EWOD), and jumping droplet microfluidic liquid handling methods. The droplet-based microfluidic cooling methods have shown an attractive capability of microscale liquid manipulation and a relatively high heat flux removal for hot spots. Recommendations are made for further research to develop advanced liquid coolant materials and the optimization of system operation parameters.

In brief

PAGE 192 Researchers in the USA propose a two-dimensional pixelating technique for actuating liquid metal. Continuous electrowetting is used to move the liquid metal between reservoirs, which resulted in ON and OFF switching of adjacent pixels. This method removes the need for a physical switch to be employed in pixelated antennas. PAGE 227 An indoor positioning method based on a single LED and camera is given in work from China. By simultaneously measuring both the strength and the angle of arrival of the LED light, ‘fingerprint’ signals of the receiver position are identified. This work is proposed as an optimum solution for indoor positioning systems using optical communication. Liquid metal actuation between pixels using continuous electrowetting. PAGE 229 Work from Iran presents a low power, all-optical switch consisting of a microfibre loop resonator in one arm of a microfibre Mach–Zehnder interferometer. The design is numerically predicted to offer easy configuration, small dimensions, low threshold power and fast switching times in optical signal processing and communications applications. A single camera and LED are used to determine receiver positions indoors. PAGE 194 Researchers in Egypt are developing an accumulated grey level image (AGLI) method for lung cancer detection. The method works by classifying genetic mutations in DNA sequences of effective genes in lung cancer, using a combination of principal component analysis and AGLI. The reported computational approach could be used as a diagnostic aid to classify genetic mutations associated with lung cancer. Numerical predictions propose an efficient all-optical switch design for optical communication applications. PAGE 187 A simple polarisation reconfigurable printed monopole antenna is presented by researchers in China. The antenna structure consists of a printed monopole, two switching circuits, and a reflector, and is shown to have four operational states that are accessed by controlling the ON/OFF states of the switching circuits. The antenna is a candidate for use in multi-frequency and multi-polarisation systems. A computational diagnostic approach for identifying genetic mutations associated with lung cancer. Four operational states reported for a simple polarisation reconfigurable printed monopole antenna.

An Electrically Actuated Liquid-Metal Gain-Reconfigurable Antenna

A tunable liquid-metal antenna demonstrating gain reconfigurability is presented. This antenna uses a reconfigurable stub made with the liquid metal Galinstan. Using continuous electrowetting, a 60 Hz signal with an amplitude of 1 Vpp and 75% duty cycle can actuate the Galinstan continuously along a channel, tuning the stub length and antenna gain. Zero external power is required to maintain the position of the slug. The 5 GHz antenna offers more than 10 dB of analog gain tuning, from −5.90 to 4.43 dB.

Out‐of‐plane continuous electrowetting actuation of liquid metal

Liquid metal is actuated against the force of gravity using low-voltage, low-power continuous electrowetting. The liquid metal moves from a lower reservoir to an upper reservoir, a distance of 12 mm, in 1.66 s. This demonstrates that the electrokinetic actuation of liquid metal is not limited to two-dimensional movement.

Elucidating Electro-Hydrodynamics at a Liquid Metal Oxide-Electrolyte Interface Via Electrochemical Impedance Spectroscopy

This work characterizes the behavior of a eutectic alloy of gallium and indium (75% Ga, 25% In, by weight, ‘EGaIn’) in response to electric fields. The metal is a liquid at room temperature (M.P., 15.5 °C) and exhibits low toxicity. These fluidic metals may be injected into microfluidic systems, fibers, and capillary networks to form soft electronic devices that are soft and compliant. Once injected, the metal remains in its place because of the adhesive nature of its thin native oxide. Preventing the oxide adhesion within microchannels enables reversible actuation of EGaIn. Actuating liquid metals may be useful for soft actuators, reconfigurable optical displays, frequency tunable antennas, and other opto-fluidic technologies. In this work, we utilize low voltages (<2 V) to reversibly move droplets of EGaIn through microchannels. Pre-wetting the channels with an aqueous solution prior to injecting the metal prevents oxide adhesion; the water forms an interfacial ‘slip-layer’ the metal and channel wall. Thereafter, an applied electric field (~10-20 V/m) actuates the liquid metal by establishing a gradient of surface tension; this effect is known as continuous electrowetting (CEW). Although CEW has been utilized before with mercury, which is toxic, the adhesive nature of the Ga oxide complicates CEW behavior. This work utilizes optical microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) to characterize electro-hydrodynamics of CEW with EGaIn under a variety of conditions. Specifically, we compare electro-hydrodynamic behavior of EGaIn with and without the presence of an oxide ‘skin’. In addition, we elucidate the influence of material properties of the electrolyte on the metal-electrolyte interface. An equivalent circuit model from EIS experiments is proposed to understand the metal-electrolyte interface and to control the movement of the liquid metal within microfluidic systems.

Fluid Flow and Mixing Induced by AC Continuous Electrowetting of Liquid Metal Droplet

In this work, we proposed a novel design of a microfluidic mixer utilizing the amplified Marangoni chaotic advection induced by alternating current (AC) continuous electrowetting of a metal droplet situated in electrolyte solution, due to the linear and quadratic voltage-dependence of flow velocity at small or large voltages, respectively. Unlike previous researchers exploiting the unidirectional surface stress with direct current (DC) bias at droplet/medium interface for pumping of electrolytes where the resulting flow rate is linearly proportional to the field intensity, dominance of another kind of dipolar flow pattern caused by local Marangoni stress at the drop surface in a sufficiently intense AC electric field is demonstrated by both theoretical analysis and experimental observation, which exhibits a quadratic growth trend as a function of the applied voltage. The dipolar shear stress merely appears at larger voltages and greatly enhances the mixing performance by inducing chaotic advection between the neighboring laminar flow. The mixer design developed herein, on the basis of amplified Marangoni chaotic advection around a liquid metal droplet at larger AC voltages, has great potential for chemical reaction and microelectromechanical systems (MEMS) actuator applications because of generating high-throughput and excellent mixing performance at the same time.

Liquid metal actuation by electrical control of interfacial tension

By combining metallic electrical conductivity with low viscosity, liquid metals and liquid metal alloys offer new and exciting opportunities to serve as reconfigurable components of electronic, microfluidic, and electromagnetic devices. Here, we review the physics and applications of techniques that utilize voltage to manipulate the interfacial tension of liquid metals; such techniques include electrocapillarity, continuous electrowetting, electrowetting-on-dielectric, and electrochemistry. These techniques lower the interfacial tension between liquid metals and a surrounding electrolyte by driving charged species (or in the case of electrochemistry, chemical species) to the interface. The techniques are useful for manipulating and actuating liquid metals at sub-mm length scales where interfacial forces dominate. We focus on metals and alloys that are liquid near or below room temperature (mercury, gallium, and gallium-based alloys). The review includes discussion of mercury—despite its toxicity—because it has been utilized in numerous applications and it offers a way of introducing several phenomena without the complications associated with the oxide layer that forms on gallium and its alloys. The review focuses on the advantages, applications, opportunities, challenges, and limitations of utilizing voltage to control interfacial tension as a method to manipulate liquid metals.

Robust bidirectional continuous electrowetting based on metal–semiconductor (M–S) diodes

We demonstrate bidirectional continuous electrowetting by embedding metal–semiconductor diodes in the electrowetting substrate. Unlike conventional electrowetting on dielectric, bidirectional continuous electrowetting uses a single electrode pair to actuate a droplet through long distances. As long as the voltage potential is maintained between two end electrodes, the droplet moves toward the electrode with the higher potential. However, previously reported material systems had limited success in repeated actuation. In this work, diodes based on Schottky barriers were fabricated by forming metal–semiconductor junctions between titanium and high-resistivity n-type silicon. The performance enhancements were evaluated using current–voltage measurements of interface pairs. When the titanium is coated with gold to limit electrochemical reactions, the Schottky diodes achieved superior performance compared to electrochemical diodes previously studied. Droplet speed range from 8 to 240 mm/s is reported. Under repeated actuation, the speed of the droplet showed no degradation for up to 2000 cycles (experiment duration).

연속적 전기습윤 효과를 이용한 액체금속 액적의 채널 내 거동

In this paper, the movement of a liquid metal droplet in a channel by continuous electrowetting effect is analyzed. The channel is fabricated using two glass substrates and silicone rubber as spacers, and a mercury droplet and dilute sulfuric acid are added into the channel. The droplet is moved according to voltage applied at both ends of the channel through an electrolyte. According to the shape of the droplet and the applied voltage, the velocity of the droplet is changed. The velocity is proportional to the applied voltage and inversely proportional to the length of the droplet, both theoretically and experimentally. Contact angle hysteresis and a meniscus change were also found in the moving state. This implies the existence of a threshold in movement by Laplace pressure difference. The experiment indicated that the sliding angle was inversely proportional to the width of the droplet but that the voltage threshold was proportional to the width.

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension. Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries. Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( ̴500 mN/m to near zero). Furthermore, this method requires only a very modest potential (< 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

Continuous electrowetting at the low concentration electrolyte-insulator-semiconductor junction

Electrowetting (EW) has applications including displays, microactuation, miniaturized chemistry, adaptive optics, and energy harvesting—understanding the physics of EW junctions is of key importance. Here, the roles of semiconductor space-charge and electric double layer in continuous EW at an electrolyte-insulator-semiconductor junction are considered. A model is formulated in terms of experimental parameters—applied voltage, zero-bias wetting contact angle, semiconductor type and doping, insulator thickness and dielectric constant, and electrolyte concentration and dielectric constant. The model predicts, and experiments indicate, that the EW behavior is diminished for low concentration solutions (∼1–10 nM) and lowly doped silicon (1014–1015 cm−3).

Liquid Metal Actuator for Inducing Chaotic Advection

Chaotic advection plays an important role in microplatforms for a variety of applications. Currently used mechanisms for inducing chaotic advection in small scale, however, are limited by their complicated fabrication processes and relatively high power consumption. Here, a soft actuator is reported which utilizes a droplet of Galinstan liquid metal to induce harmonic Marangoni flow at the surface of liquid metal when activated by a sinusoidal signal. This liquid metal actuator has no rigid parts and employs continuous electrowetting effect to induce chaotic advection with exceptionally low power consumption. The theory behind the operation of this actuator is developed and validated via a series of experiments. The presented actuator can be readily integrated into other microfluidic components for a wide range of applications.

Precise droplet volume measurement and electrode-based volume metering in digital microfluidics

In this work, for the first time, we demonstrate nanoscale droplet generation from a continuous electrowetting microchannel using a simple and precise image-based droplet volume metering technique. One of the most popular ways of droplet generation in electrowetting devices is to split a droplet from a preloaded volume as a fluid reservoir. This method is effective, but lowers volume consistency after multiple droplets are generated. Impedance- and capacitance-based methods of volume metering have been successfully used in digital microfluidics, but require complex circuitry and feedback signal processing. In this work, we demonstrate nanoliter droplet generation from a continuous electrowetting channel used as a replenishable fluid reservoir which compensates for the loss of reservoir volume as droplets are sequentially split. This improves volume consistency especially for applications requiring multi-droplet generation. Based on the area of the electrode, the volume of each droplet split from the electrowetting channel can be obtained by a simple and precise image processing technique with no need for additional hardware and measurement errors of ±0.05 %. This simple technique can be used in a wide range of applications that require precise volume metering, such as immunoassay.

Continuous Electrowetting of Non-toxic Liquid Metal for RF Applications

Continuous electrowetting (CEW) is demonstrated to be an effective actuation mechanism for reconfigurable radio frequency (RF) devices that use non-toxic liquid-metal tuning elements. Previous research has shown CEW is an efficient means of electrically inducing motion in a liquid-metal slug, but precise control of the slug's position within fluidic channels has not been demonstrated. Here, the precise positioning of liquid-metal slugs is achieved using CEW actuation in conjunction with channels designed to minimize the liquid-metal surface energy at discrete locations. This approach leverages the high surface tension of liquid metal to control its resting position with submillimeter accuracy. The CEW actuation and fluidic channel design were optimized to create reconfigurable RF devices. In addition, solutions for the reliable actuation of a gallium-based, non-toxic liquid-metal alloy (Galinstan) are presented that mitigate the tendency of the alloy to form a surface oxide layer capable of wetting to the channel walls, inhibiting motion. A reconfigurable slot antenna utilizing these techniques to achieve a 15.2% tunable frequency bandwidth is demonstrated.