Dielectrophoresis-driven spreading of immersed liquid droplets.

Liquid-liquid extraction techniques are one of the major tools in chemical engineering, analytical chemistry, and biology, especially in a system where two immiscible liquids have an interface solutes exchange between the two liquid phases along the interface up to a point where the concentration ratios in the two liquids reach their equilibrium values [1]. Solutes including nucleic acids and proteins of interests can be extracted from one liquid phase to the other immiscible liquid phase as a preparation step for many analytical processes. There are several advantages in miniaturizing the liquid-liquid extraction methods to on-chip level extraction. Usual advantages of miniaturization are the reduction in the sample size and portability. In addition, transport phenomena is faster in Micro-systems than in ordinary size systems, and therefore, one may expect that liquid-liquid extraction takes less time to achieve in miniaturized devices. It is due to shorter diffusion time in micro scale as well as high surface to volume ratio of Microsystems. Electrowetting on dielectric (EWOD) digital microfluidics is an efficient platform to process droplet based analytical processes [2]. Nanoliter (nL) or smaller volume of aqueous liquid droplets can be generated and transported on a chip by EWOD process. In addition to the high surface to volume ratio, high chemical potential can be expected in droplet based extraction when the droplets are in motion. In this paper, we propose to use room temperature ionic liquid (RTIL) as a second liquid phase for extraction, which forms immiscible interface with aqueous solutions. Properties of RTIL can be tailored by choice of cation, anion and substituents. RTIL has been investigated as replacements for the organic solvents and various “task-specific” ionic liquid are being developed which exhibit many attractive properties such as very low vapor pressure, high thermal stability [3]. We recently published EWOD properties of various RTILs toward microfluidic applications [4]. To demonstrate liquid-liquid micro extraction on chip, we fabricated and tested EWOD digital microfluidic devices. Fig. 1 shows (a) top and (b) cross sectional views of EWOD device. Two model extraction systems were tested. One is organic dye extracted from RTIL (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide or BMIMNTf2) to water and the other is iodine (I2) extracted from water to BMIMNTf2. The later model experiment is demonstrated in Fig. 2. Droplets of aqueous solution and BMIMNTf2 solution were generated on chip reservoir then transported for extraction and separated by EWOD actuation. When an aqueous solution and BMIMNTf2 solution join together, they created an interface, since water and BMIMNTf2 are immiscible. Extraction of I2 was done along the interface. After successful extraction, two immiscible liquid phases were separated by EWOD actuation and formed two separate droplets. From the result shown in Fig 2 (g), it is expected that extraction performance at the interface of moving droplet would be enhanced compared to the stationary droplet, because a moving interface prevent the chemical equilibrium, thus more chemical extraction potential can be provided with a moving interface than at a stationary interface. This demonstration is the first step toward total analysis system. The presented result opens the way to on-chip micro extraction, which will be readily integrated with other sample preparation microfluidic components and detection components. Currently, micro extraction systems for larger molecules such as nucleic acids, proteins and biological cells are being developed for further analytical applications.

Application of voltage across a liquid-dielectric interface resulting in the change of fluid contact angle is known as electrowetting on dielectric (EWOD) effect [1]. EWOD actuation is one of the most preferred techniques to move liquid at microscale due to relatively low voltages, low currents and power consumption requirements, and the absence of electrolysis. Our recent work has shown that nanofluids containing semiconductor nanoparticles exhibit a very strong EWOD effect [2, 3]. In particular, in the tested voltage range of 0 to 60 V, nanofluids loaded with molecularly-capped bismuth telluride nanoparticles show enhanced stability and increased actuation range of contact angle change. Developing a fundamental understanding of the role of nanostructure inclusions in controlling and potentially enhancing the EWOD effect would pave the way for the efficient use of EWOD with nanofluids in a wide range or microfluidic applications. Here we extend the studies on the EWOD behavior of nanofluids, to a system containing noble metal nano particles. In this abstract we report the preliminary results of the electro-wetting studies on nanofluids containing silver nano particles.

Microwater droplet splitting and merging in a parallel-plate electrowetting-on-dielectric (EWOD) device have been studied numerically. The transient governing equations for the microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The interface between liquid and gas is tracked by a coupled level set (LS) and volume-of-fluid (CLSVOF) method. A continuum surface force (CSF) model is employed to model the surface tension at the interface. Contact angle hysteresis which is an essential component in EWOD modeling is implemented together with a simplified model for the viscous stresses exerted by the two plates at the solid–liquid interface. The results of the numerical model have been validated with published experimental data and the physics of droplet motion within the EWOD device has been examined. A parametric study has been performed in which the effects of channel height and several other parameters on the fluid motion have been studied.

Electrowetting-on-dielectric (EWOD) technology has been considered as a promising candidate for digital microfluidic (DMF) applications due to its outstanding flexibility and integrability. The dielectric layer with a hydrophobic surface is the key element of an EWOD device, determining its driving voltage, reliability, and lifetime. Hereby, inspired by the ionic-liquid-filled structuring polymer with high capacitance independent on thickness, namely ion gel (IG), we develop a polymer (P)-ion gel-amorphous fluoropolymer, namely, PIGAF, composite film as a replaceable hydrophobic dielectric layer for fabrication of a high-efficiency and stable EWOD-DMF device at relatively low voltage. The results show that the proposed EWOD devices using the PIGAF-based dielectric layer can achieve a large contact angle (θ) change of ∼50° and excellent reversibility with a contact angle hysteresis of ≤5° at a relatively low voltage of 30 Vrms. More importantly, the EWOD actuation voltage did not change obviously with the PIGAF film thickness in the range of several to tens of microns, enabling the thickness of the film to be adjusted according to the demand within a certain range while keeping the actuation voltage low. An EWOD-DMF device can be prepared by simply stacking a PIGAF film onto a PCB board, demonstrating stable droplet actuation (motion) at 30 Vrms and 1 kHz as well as a maximum moving velocity of 69 mm/s at 140 Vrms and 1 kHz. The PIGAF film was highly stable and reliable, maintaining excellent EWOD performance after multiple droplet manipulations (≥50 cycles) or long-term storage of 1 year. The proposed EWOD-DMF device has been demonstrated for digital chemical reactions and biomedical sensing applications.

Electrowetting (EW) effect is defined as changing in the interfacial energy of solid–liquid interface which causes to change in solid–liquid contact angle. In EW on dielectric (EWOD) actuations, the microliter drops are usually placed between two substrates which have some advantages. By modifying the voltages applied to the electrodes, it is possible to manipulate the droplets by setting up surface tension gradients along the plate. In this paper, the free energy based lattice Boltzmann method which has been recently extended by the authors for modeling and simulation of EW phenomenon has been applied to simulate EWOD. Also here we report the results of simulation of EW on superhydrophobic surfaces and pinning problem. The results have been compared with experimental data and show good accuracy of the numerical simulation.

The electrowetting on dielectric (EWOD) technique has considerable potential for microfluidic and biomedical applications. The Lippmann-Young model based on the force balance concept has long been used to predict the contact angles of droplets under electrowetting. However, recent experimental evidence has indicated that this model fails to provide accurate predictions of the lower contact angles associated with saturation conditions at higher electric potentials. Hence, the study simulates the internal flow in an actuated droplet and treats it as stagnation-point flow. This kinetic energy is then taken into consideration while calculating the contact angles using an energy balance model. The energy of an actuated droplet is contributed by the combination of the side surface tension energy, the base tension energy, the dielectric energy, and the kinetic energy when deriving the energy balance model. Consequently, the new energy balance model modifies the Lippmann-Young equation, thereby providing enhanced reasonable predictions of the droplet contact angle across the higher electric potential where the contact angles are close to the saturated condition.

This paper reviews the principles, operations, and applications of bubble-based electrowetting-on-dielectric (EWOD). EWOD has proved to be an efficient tool in digital microfluidics that employs discrete droplets, and various applications that use the principles of EWOD have been developed from lab-on-a-chip to optical systems. Similar to its use with droplets, EWOD can also be applied to gaseous bubbles. This review begins with a discussion of the principles of EWOD for a bubble on an electrode covered with a hydrophobic dielectric layer. It then addresses EWOD actuation and the transportation of a bubble in an aqueous medium, along with a physical explanation of bubble motion. The operation of EWOD is then extended to the on-chip creation/elimination and splitting of bubbles. In particular, micro-mixers and pumps are discussed as potential applications of these operations. Unlike droplets, bubbles can be easily oscillated by external excitation, which provides additional functionalities. By integrating EWOD with external excitation, a number of new advanced applications are introduced, including the capture/separation of particles and the propulsion of objects. In these advanced operations, cavitational microstreaming flows and acoustic radiation forces are mainly responsible for the physical mechanisms. This paper also discusses these advanced operations along with their underlying physics. It is expected that in addition to bubble oscillation, other bubble actuation modes will create new functionalities and new potential applications.

We present a mixed finite element method for a model of the flow in a Hele-Shaw cell of 2-D fluid droplets surrounded by air driven by surface tension and actuated by an electric field. The application of interest regards a micro-fluidic device called ElectroWetting on Dielectric (EWOD). Our analysis first focuses on the time discrete (continuous in space) problem and is presented in a mixed variational framework, which incorporates curvature as a natural boundary condition. The model includes a viscous damping term for interface motion, as well as contact line pinning (sticking of the interface) and is captured in our formulation by a variational inequality. The semi-discrete problem uses a semiimplicit time discretization of curvature. We prove the well-posedness of the semi-discrete problem and fully discrete problem when discretized with iso-parametric finite elements. We derive a priori error estimates for the space discretization. We also prove the convergence of an Uzawa algorithm for solving the semi-discrete EWOD system with inequality constraint. We conclude with a discussion about experimental orders of convergence.

In this study, we introduce a novel Electrowetting-on-Dielectric (EWOD) sensor designed to quantify marine dispersants at the spill point. The sensor quantifies changes in the surface tension of liquid droplets at varying dispersant concentrations through the deformation response of the droplet under applied voltage. Analyzed responses include droplet length and contact angle (CA) on the device surface upon sensor activation. This sensor offers significant advantages over existing chemical methods, which are costly and complex. Moreover, compared to conventional methods based on the same principle, it demonstrates enhanced sensitivity at low concentrations. Additionally, the sensor’s portability enables instantaneous and in situ measurements of marine dispersant concentrations, thus providing a crucial tool for effective oil spill response by facilitating on-site decision-making and offering higher temporal resolution for studies on the marine dispersant’s environmental impact. The device’s potential extends beyond marine dispersants to detecting various contaminants affecting surface tension. Its adaptability underscores the EWOD device’s role as a versatile tool for environmental monitoring and on-site analysis, addressing the urgent need for efficient and sustainable solutions in environmental management.

Discussion notes on “Disjoining pressure of planar adsorbed films”, by J.R. Henderson

High-speed droplet actuation on single-plate electrode arrays

Active surface tension driven micropump using droplet/meniscus pressure gradient

Solid–liquid interface (SLI) properties of the Ni–Zr B33 phase were determined from molecular dynamics simulations. In order to perform these measurements, a new semi-empirical potential for Ni–Zr alloy was developed that well reproduces the material properties required to model SLIs in the Ni50.0Zr50.0 alloy. In particular, the developed potential is shown to provide that the solid phase emerging from the liquid Ni50.0Zr50.0 alloy is B33 (apart from a small fraction of point defects), in agreement with the experimental phase diagram. The SLI properties obtained using the developed potential exhibit an extraordinary degree of anisotropy. It is observed that anisotropies in both the interfacial free energy and mobility are an order of magnitude larger than those measured to date in any other metallic compound. Moreover, the [0 1 0] interface is shown to play a significant role in the observed anisotropy. Our data suggest that the [0 1 0] interface simultaneously corresponds to the lowest mobility, the lowest free energy and the highest stiffness of all inclinations in B33 Ni–Zr. This finding can be understood by taking into account a rather complicated crystal structure in this crystallographic direction.

The free energy involved in the formation of an interface between two phases (e.g., a solid-liquid interface) is referred to as the interfacial free energy. For the case of solidification, the interfacial free energy dictates the height of the energy barrier required to nucleate stable clusters of the newly forming solid phase and is essential for producing an accurate solidification kinetics model using classical nucleation theory (CNT)-based methods. While various methods have been proposed for modeling the interfacial free energy for solid-liquid interfaces in prior literature, many of these formulations involve making restrictive assumptions or approximations, such as the system being at or near equilibrium (i.e., the system temperature is approximately equal to the melt temperature) or that the system is at pressures close to atmospheric. However, these approximations and assumptions may break down in highly non-equilibrium situations, such as in dynamic-compression experiments where metastable liquids that are undercooled by hundreds of kelvin or overpressurized by several gigapascals or more are formed before eventually solidifying. We derive a solid-liquid interfacial free-energy model for such high-pressure conditions by considering the enthalpies of interactions between pairs of atoms or molecules. We also consider the contribution of interface roughness (disordering) by incorporating a multilayer interface model known as the Temkin n-layer model. Our formulation is applicable to a diverse variety of materials, and we demonstrate it by developing models specifically for two different materials: water and gallium. We apply our interfacial free-energy formulation to CNT-based kinetics simulations of several suites of dynamic-compression experiments that cause liquid water to solidify to the high-pressure solid polymorph ice VII and have found good agreement to the observed kinetics with only minor empirical fitting.

Solid–liquid interface properties play a crucial role in the adsorption and adhesion of different microorganisms to the solid. There are some methods to inhibit microorganisms’ adsorption at the solid–liquid interface and their adhesion to the solid. These methods can be divided into bulk phase and surface modification. They are often based on the surfactants’ effect on the wettability of the solid in a given system, due to the fact that adsorption and wetting properties of the food additive antimicrobial surfactants (sucrose monolaurate and sucrose monodecanoate as well as some other sugar-based ones (n-octyl-β-d-glucopyranoside, n-dodecyl-β-d-glucopyranoside, n-dodecyl-β-d-maltoside)) in the solid-aqueous solution of surfactant-air system were considered. Quantitative description of adsorption of the studied compounds at the solid–liquid interface was made based on the contact angle of the aqueous solutions of studied surfactants on polytetrafluoroethylene, polyethylene, poly(methyl methacrylate), polyamide and quartz surface and their surface tension. From the above-mentioned considerations, it can be seen that during the wettability process of the studied solids, surfactants are oriented in a specific direction depending on the type of the solid and surfactant. This specific orientation and adsorption of surfactant molecules at the solid–water interface cause changes of the solid surface properties and its wettability, which was successfully predicted in the studied systems.