With the inspiration from electrowetting-controlled droplets, the potential advantages of electrowetting for bubble dynamics are investigated experimentally and numerically. In our experimental system, a 100 nanometer thin film gold metal was used as an electrode, and a 6.5 micrometer polydimethylsiloxane (PDMS) was spin-coated on the electrode acting both as an dielectric layer and hydrophobic surface. A two-phase model coupled with a electrostatics was used in our simulation work, where the body force due to the electric field acts as an external force. Our numerical results demonstrated that electrowetting can help the detachment of a small bubble by changing the apparent contact angle. Similar results were observed in our experiments that with electrowetting on dielectric, the contact angle of bubble on a hydrophobic surface will obviously decrease when a certain electrical field is applied either with a small size bubble (diameter around 1mm) or a relatively larger size bubble (diameter around 3 mm). When the applied voltage becomes high enough, both the experimental and numerical results demonstrate the characteristics of bubble detachment within a thin film liquid layer.

The photon transport and energy conversion of a near-field thermophotovoltaic (TPV) system with a selective emitter composed of alternate tungsten and alumina layers and a photovoltaic cell sandwiched by electrical contacts are theoretically investigated in this paper. Fluctuational electrodynamics along with the dyadic Green’s function for a multilayered structure is applied to calculate the spectral heat flux, and photocurrent generation and electrical power output are solved from the photon-coupled charge transport equations. The tungsten and alumina layer thicknesses are optimized to match the spectral heat flux with the bandgap of TPV cell. The spectral heat flux is much enhanced when plain tungsten emitter is replaced with the multilayer emitter due to the mechanism of surface plasmon polariton coupling in the tungsten thin film. In addition, the invalidity of effective medium theory to predict photon transport in the near field with multilayer emitters is discussed. Effects of a gold back reflector and indium tin oxide front coating with nanometer thickness, which could practically act as the electrodes to collect the photon-generated charges on the TPV cell, are explored. Conversion efficiency of 23.7% and electrical power output of 0.31 MW/m 2 are achieved at 100 nm vacuum gap when the emitter and receiver are respectively at temperatures of 2000 K and 300 K.

In this work, the reusable Lab-on-a-Chip or LOC was fabricated and developed to reduce its investment cost and to ease its operations. A sandwiched acrylic set was chosen to assemble the PDMS LOC which was aimed to investigate biological samples, especially the samples from animals. Chicken and sheep blood were chosen as the samples to implement two-phase flows with different red-blood-cell shapes. The chicken blood flow represented the viscous two-phase flow with the oval shape particles while the sheep blood flow represented the inadhesive samples with the round particles. The LOC electrodes from different materials as the sputtered nickel plate, the aluminium foil, the copper plate, and the gold foil were examined. Ratios between the anticoagulant solution and the biological samples were studied to find their effects on sample velocities and to find the best image to characterize the RBC flow behaviors. Since it was hard to characterize the particle flow in the inadhesive flows, the result pictures were analyzed and presented in terms of color intensities per unit area by using a computer program called “ImageJ”. The sheep-blood-flow results were validated with the hematology results which were the hematocrits to find their relationships between the hematocrits and the RBC flows. Among different LOC electrodes, the sputtered nickel electrodes were the most suitable electrodes in this current application. We found that the suitable anticoagulant-sample ratios for chicken and sheep samples were 5:1 and 1:1 by volume, respectively. The normal-health-condition sheep with the standard hematocrits higher than 28% showed the average-different-color intensities per unit area between cathode and anode at 39.485 pixels per unit area while the lower standard hematocrit samples, the hematocrits were lower than 28%, showed the average-different-color intensities at 14.641 pixels per unit area, the lower intensity the lower hematocrit. So the LOC coupled with “ImageJ” exhibited their capabilities to investigate the sheep blood conditions, especially, this coupled technique consumed less time than the traditional hematology process.

Microfluidics and its applications to Lab-on-a-Chip have attracted a lot of attention. Because of the small length scale, the flow is characterized by a low Re number. The governing equations become linear. Boundary element method (BEM) is a very good option for simulating the fluid flow with high accuracy. In this paper, we present a 2D numerical modeling of the electrothermal flow using BEM. In electrothermal flow the volumetric force is caused by electric field and temperature gradient. The physics is mathematically modeled by (i) Laplace equation for the electrical potential, (ii) Poisson equation for the heat conduction caused by Joule heating, (iii) continuity and Stokes equation for the low Reynolds number flow. We begin by solving the electrical potential and electric field. The heat conduction is caused by the Joule heating as the heat generation term. Superposition principle is used to solve for the temperature field. The Coulomb and dielectric forces are generated by the electrical field and temperature gradient of the system. We analyze the Stokes flow problem by superposition of fundamental solution for free-space velocity caused by body force and BEM for the corresponding homogeneous Stokes equation. It is well known that a singularity integral arises when the source point approaches the field point. To overcome this problem, we solve the free-space velocity analytically. For the BEM part, we also calculate all the integral terms analytically. With this effort, our solution is more accurate. In addition, we improve the robustness of the matrix system by combining the velocity integral equation with the traction integral equation. Our purpose is to design a pump for the microfluidics system. Since the system is a long channel, the flow is fully developed in the area far away from the electrodes. With this assumption, the velocity profile is parabolic at the inlet and outlet of the channel. So we can get appropriate boundary conditions for the BEM part of Stokes equation. Consequently, we can simulate the electrothermal flow in an open channel. In this paper, we will present the formulation and implementation of BEM to model electrothermal flow. Results of electrical potential, temperature field, Joule heating, electrothermal force, and velocity field will be presented.

Room temperature liquid metal has been widely used in many MEMS applications, such as integrated heaters, sensors, electrodes and stretchable wires. Injecting the liquid metal into microchannels provides a simple, rapid and low-cost way to fabricate micro heaters and sensors. The liquid-metal-filled microstructures can be designed in any shape and easily integrated into micro devices. In this study, a liquid-metal based thermal micro-system was proposed for on-chip cell culture purpose. The thermal micro-system consisted of two same microchannels filled with the liquid metal as electrical heaters. At the same time, the heater also worked as a resistance temperature sensor to control the heating process. The temperature sensor was calibrated from 20 °C to 70 °C to give an accurate temperature control for the microsystem. To justify whether this micro-system is capable of providing a uniform temperature distribution, Rhodamine B was filled into the micro cell culture chamber of interest to monitor the temperature distribution. Thermal analysis was numerically carried out to reveal the temperature field of the chip. This thermal micro-system has great potential use in many microfluidic applications, such as on-chip PCR, temperature gradient focusing, protein crystallization or chemical synthesis.

We report an electrowetting-controlled cooling system with site-specific treatments on the heat source (evaporator or hot spot) surfaces. Electrowetting-on-dielectric (EWOD) has great potential in thermal management because EWOD-driven droplet transport has unique characteristics of prompt response, low power consumption and programmable paths without the need for any mechanical moving parts. Prompt and fast droplet transport is necessary for adaptive and active cooling of high heat flux targets. Using a multi-channel DC/AC control system, we carried out sequenced activation of AC voltages on coplanar electrodes and transmitted a droplet to the spot target along a programmable path. With high positioning accuracy at the chip level, we have successfully transmitted a water droplet of 15 μL at speeds as high as ∼10 cm/s. We further improved electrowetting cooling performance by coating a fine copper screen on the cooling targets. The capillarity associated with the copper screen facilitates the delivered droplets automatically spreading and clinging to the target surfaces. As a result, heat transfer is in the more efficient form of filmwise evaporation at the evaporator sites. To maintain a thin film with proper thickness on the hot spots, we implemented EWOD-assisted droplet splitting and merging to precisely control the droplet volume to avoid fluid flooding (accumulation) on the hot spot surfaces. Our investigation indicates that thin-film evaporation is a high-efficiency heat transfer mechanism on a hydrophilized hot spot surface. Based on EWOD technique with surface treatments, the superheat on a hot spot of 4mm × 4mm was maintained well below 30°C even when the heat flux reached as high as 80W/cm 2 . The closed loop of this novel thermal management system can potentially function as a wickless vapor chamber or heat pipe with enhanced heat dissipation capabilities.

Although there are several reports demonstrating the use of aqueous two-phase systems (ATPS) for biological sample separation in miniaturized devices, they only used ATPS prepared in macro scale by following the conventional way. Formation of ATPS in a confined micro scale environment and their characteristics has not been studied yet. While ATPS formation in macro scale is supported by vigorous mixing and gravitational force to form two phases, both mixing capability and gravitational force aid are insignificant in micro scale. Hence, in this study, we investigated the formation of ATPS under these inferior conditions of micro scale. An electrowetting on a dielectric (EWOD) digital microfluidic chip was used as the confined miniaturized environment. In an EWOD device, micro/nano scale droplets are manipulated on an array of electrodes by sequential application of voltages to the electrodes. The common polyethylene glycol (PEG) and dextran (DEX) polymer/polymer ATPS was used as the model system. Nanoliter volume droplets of the two pure polymer solutions were brought into contact, mixed, and successfully formed ATPS on an EWOD chip. Furthermore, it was possible to separate the two phases into two droplets, demonstrating the promising capability of on-chip biological sample separations effectively by integrating formation of ATPS and separation process using ATPS in a single chip. This study also compares the characteristics of ATPS generated in microfluidic chip with that generated in conventional macro scale. The effect of mixing performance on the ATPS formation is studied experimentally.

Electrothermal transport and crystallization dynamics govern the speed and reliability of multibit phase change memory (PCM). This work develops a transient simulation code incorporating the nanoscale electrical, thermal, and crystalline growth models to investigate various cell structures and programming strategies. The simulation evaluates the multibit performances of two standard PCM structures, the “mushroom” cell and the “confined pillar” cell, with feature sizes smaller than 40nm. This paper implements the simulation code to explore a more compact and reliable architecture, the stacked vertical cell, for precise control of the Joule heating and the intermediate resistance states. For an electrode area of 10nm × 20nm, very low programming current of 60μA – 90μA generates sufficient heating power to amorphize the phase change elements sequentially, resulting four distinct resistance levels for multibit applications.

The present numerical work describes the simulation and analysis of the absorptance and absorption efficiency of a solar cell, where the effect of utilizing photonic crystals as an active material of the cell was studied. The study was performed by numerical simulations using a computational code based on the Finite Element Method [1]. The results were obtained for photonic crystals with periodicity in both one and two dimensions [2]. In the first one, periodicity, thickness of the active material, and distance with respect to the electrode for hole collection were varied, and two organic materials for the active zone were tested, P3HT:PCBM and TDPT:PCBM. In the case of crystals with periodicity in two dimensions, only the period in one of the two dimensions was varied, based on the cell with the highest efficiency of absorption proposed for cells with periodic photonic crystals in one dimension. All simulations were obtained for waves with TM polarization, zero angle of incidence and wavelengths between 400 and 700 nm.

The paper describes results from an experimental study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling at atmospheric pressure with polished smooth boiling surface. A micro scaled electrode with slits for bubbles to come out was designed in order to create non uniform high electric field strength and to produce electrohydrodynamics (EHD) convection with the application of dc voltage. The application of high electric field strongly enhanced the heat flux and the heat transfer coefficient. From observations of the behavior of bubbles over the electrode and the boiling surface condition, the instability between the liquid and the vapor increased the heat flux, the heat transfer coefficient and the CHF.

A continuously growing need for high energy density miniaturized power sources for portable electronic applications has spurred the development of a variety of microscale fuel cells. For portable applications, membrane-based fuel cells using small organic fuels (i.e., methanol, formic acid) are among the most promising configurations as they benefit from the high energy density and easy storage of the liquid fuels. Unfortunately, the performance of these fuel cells is often hindered by membrane-related issues such as water management (i.e., electrode dry-out / flooding) and fuel crossover. Furthermore, high costs of, for example, catalysts and membranes as well as durability concerns still hinder commercialization efforts. To address these challenges we have developed membraneless laminar flow-based fuel cells (LFFCs), which exploit microscale transport phenomena (laminar flow) to compartmentalize streams within a single microchannel. The properties of various fuel and media flexible LFFCs will be presented and novel strategies for improving fuel utilization and power density will be discussed. Furthermore, the performance of a scaled-out 14-channel LFFC prototype is presented. We have also developed a microfluidic fuel cell as a powerful analytical platform to investigate and optimize the complex processes that govern the performance of catalysts and electrodes in an operating fuel cell. This platform bridges the gap between a conventional 3-electrode electrochemical cell and a fuel cell, as it allows for standard electrochemical analysis (e.g., CV, CA, EIS) as well as fuel cell analysis (e.g., IV curves).

Atomistic modeling and/or experimental methods were conducted to study transport processes in the porous electrodes of polymer electrolyte fuel cells (PEFCs) and of mediator enhanced polymer electrolyte supercapacitors (MEPESCs). The simulations show that vibrations of the Pt nanocrystallines on rhe carbon supports significantly impacts diffusion in the Nafion electrolyte clusters between the carbon supports and enhances the diffusivity of hydrogen, oxygen, methanol, water, and hydronium up to 8 times the nominal value. Charging the carbon support alters the diffusivities. It was visualized that charging of Pt nanocrystallines resulted in disintegration of the Pt catalysts leading to a reduction in catalytic activity of the catalyst layer. Re-precipitation and re-crystallization of the Pt under a full discharging condition can also be visualized using the molecular dynamics (MD) modeling method. MD simulations also indicate that the mobility of the iodine mediators is at least 10 times greater than other small ions in the Nafion electrolyte. The degradation of the MEPESC is dominated by the crossover of I − ions but not I 2 in Nafion.

Based on CFD commercial software and the developed program which involved multi-component flow and diffusion, mass/heat transfer, electrochemical reaction and current field calculation, the performance of planar-electrode-support solid oxide fuel cell with different porosity anodes was investigated. The distributions of species concentration, temperature, electric potential, current and current density were obtained. The results show that this SOFC cell has good performance on species diffusion, current transfer and power output when anode porosity varies between 0.3 and 0.4. This work may be of significance to further research on the anode material.

The need for an infrastructure to provide hydrogen as a next generation energy carrier is ever increasing. High temperature solid oxide electrolysis cells (SOECs) have been proven to be a viable technology in the production of hydrogen [1]. With the increasing use of SOECs in various operating environments it is important to be able to specify the best SOEC for any given situation. We have developed a straightforward model to estimate cell performance in a timely and inexpensive manner. Composite electrode planer type SOEC models have been developed previously. It is a common assumption that all electrochemical reactions in these cells occur at the interface of the electrolyte and the electrode [2]. It has been shown by S. Gewies et al. [3] that the reactions occurring throughout a Ni/YSZ cermet electrode occur in a nonlinear fashion. Our one dimensional model has been developed to optimize SOECs with composite electrodes. This model takes into account ohmic, activation, and concentration polarizations. The electrochemical reaction that occurs within the electrode functional layers has been accounted for in the calculation of the concentration polarization. This is believed to give a more realistic view of the mass transfer that occurs in SOECs with composite electrodes via a simple and straightforward 1-D model.

Due to the high power density and local temperature increase on nanoscopic asperities of solid metal contacts, traditional MEMS contact switches suffer from contact welding, pitting, electromigration and oxidation. Particularly, when MEMS switches are used to handle high power, solid metal contacts pose serious limitation on the contact reliability. A self-healing RF MEMS switch, which utilizes liquid gallium contacts to take the place of the traditional solid metal-to-metal contacts, is proposed in this paper. Electrostatic actuation is used to drive a silicon nitride bridge with upper electrodes. When the bridge is pulled down, liquid gallium droplets work as an interface between the upper and lower contact electrodes. The loss of the gallium droplets can be avoided due to the unwettability of the material surrounding the contact electrodes. The switch is fabricated using a surface micromachining process. A coupled-field finite element analysis (FEA) is used to model the electric current, heating and thermal conduction of the contacts. The model includes deformable gallium droplets with 4 μm base diameter. The two sides of the droplets are connected to the upper and lower solid metal contact electrodes, respectively. By using the FEA models, the electric and thermal characteristics of the gallium droplets featuring a variety of geometric parameters have been studied. 1 A current handling capability of the liquid gallium contact is verified by the FEA models.

Recently, we have demonstrated an optically induced AC electrokinetic technique that rapidly, continuously and selectively concentrates colloids on an electrode surface [1–3]. This is demonstrated with a highly focused near-infrared (1,064 nm) laser beam applied to parallel plate electrodes separated by 50 μm without any additional surface modification or patterning of the electrodes. This dynamic optically-induced technique can be applied towards a variety of lab-on-a-chip applications. This paper will explain its physical mechanisms and showcase recent results regarding its particle sorting capabilities. This dynamic, optically induced fluid and particle manipulation technique could be used for a variety of lab-on-a-chip applications.

AC electrothermal effect generates temperature gradients in the fluids, consequently inducing free charges that move in electric fields, and produce microflows in microchannel. This paper reports dramatic improvements in flow rate over conventional ac electrokinetic micropumps by exploiting asymmetry in electric potentials over the electrodes. A micropump consisting of planar asymmetric electrode array was tested using AC signals with and without a DC bias. The pumping velocity is much faster with a DC voltage, in some cases by an order of magnitude, reaching 3.2 mm/sec linear velocities with only 9 Vrms. Our hypothesis attributes the increase of ACET force to the ion generation near electrode by reactions and the resulting high conductivity gradient. The discovery presents an exciting opportunity for microfluidics, and further improvement can be anticipated with some optimization.

Electrohydrodynamic (EHD) micropumps with three-dimensional 50μm × 50μm micropillar electrodes were fabricated and tested in this study. Two basic electrode configurations were investigated: (i) micropillar emitter and collector electrodes (symmetric) and (ii) micropillar emitter and planar collector electrodes (asymmetric). The micropumps were fabricated using chromium/gold planar electrodes with 3-D Nickel micro-pillars on a glass substrate that was integrated within a 100 μm high PDMS microchannel. The effect of the spanwise micropillar spacing spacing on the pump performance was determined. The pumps were tested using HFE-7100 as the working fluid for the maximum pressure generation under a no flow condition. The micropumps with the asymmetric electrode design generated a significantly higher pressure head and flow rate than the corresponding micropumps with symmetric electrode configuration for the same applied voltage, with lower power consumption. A decrease in the spanwise spacing of the micropillar electrodes increased the pump performance for the symmetric configuration, while the performance decreased for the asymmetric configuration.

The multiphysiochemical transport in electroosmosis of dilute electrolyte solutions (<1mM) through microporous media with granular random structures has been modeled in this work by our numerical framework consisting of three steps. First, the three-dimensional microstructures of porous media are reproduced by a random generation-growth method. Then the effects of chemical adsorption and electrical dissociation at the solid-liquid interfaces are considered to determine the electrical boundary conditions, which vary with the ionic concentration, the pH, and the temperature. Finally the nonlinear governing equations for the electrokinetic transport are solved by a highly efficient lattice Poisson-Boltzmann algorithm. The simulation results indicate that the electroosmotic permeability through the granular microporous media increases monotonically with the porosity, the ionic concentration, the pH and the environmental temperature. When the surface electric potential is higher than 50 mV, the permeability increases with the electric potential exponentially. The electroosmotic permeability increases with the pH exponentially, but with the temperature linearly. The present modeling results may improve our understanding of hydrodynamic and electrokinetic transport in geophysical systems, and help guide the design of porous electrodes in micro energy systems.

Atomization has been widely applied in pulmonary drug delivery as a promising technology to transport drug formulations directly to the respiratory tract in the form of inhaled particles or droplets. Because of the targeted treatment, the drug can be delivered directly to the site of inflammation, thus the need for systemic exposure and the possibility of side effects are both reduced. Therefore pulmonary drug delivery has significant advantages over other methods in the treatment of respiratory diseases such as asthma. The most common atomization methods employed in pulmonary drug delivery are jet atomization and ultrasonic atomization. However, the difficulty is in producing monodispersed particles/droplets in a size range of 1–5 micron meter in diameter, necessary for deposition in the targeted lung area or lower respiratory airways, within a controllable fashion. In this paper, we demonstrate surface acoustic wave (SAW) atomization as an efficient technique to generate monodispersed aerosol to produce the required size distribution. The SAW atomizer is made of a 127.86 Y-X rotated single-crystal lithium niobate piezoelectric substrate, which is patterned with chromium-aluminum interdigital transducer (IDT) electrodes via UV lithography. When an alternating electric field is applied onto lithium niobate substrate through the IDT, a SAW, propagating across substrate surface with ten nanometer order amplitudes, is generated. When the SAW meets the liquid which is placed upon substrate, the acoustic energy carried by the wave induces atomization of the working fluid, which contains salbutamol as a model drug. In order to measure the size distribution of the atomized droplets, two methods are used. One is the laser diffraction based Spraytec technique and the other is an in-vitro lung modelthe one stage glass twin impinger. The former revealed that the mean diameter of the aerosol atomized was around 3 um which were confirmed by the lung model that demonstrated that nearly 80% of atomized drug aerosol was deposited in the simulated lung area. Moreover, the SAW atomizer only requires 1–3 W driving power, suggesting that it can be miniaturized for portable consumer use.