Electrical Actuation of Dielectric Droplets

Electrical actuation of liquid droplets at the microscale offers promising applications in the fields of microfluidics and lab-on-a-chip devices. Much prior research has targeted the electrical actuation of electrically conducting liquid droplets; however, the actuation of dielectric droplets has remained relatively unexplored, despite the advantages associated with the use of a dielectric droplet. This paper presents modeling and experimental results on the electrical actuation of dielectric droplets between two flat plates. A first-order analytical model, based on the energy-minimization principle, is developed to estimate the electrical actuation force on a dielectric droplet as it moves between two flat plates. Two versions of this analytical model are benchmarked for their suitability and accuracy against a detailed numerical model. The actuation force prediction is then combined with available semi-analytical expressions for predicting the forces opposing droplet motion to develop a model that predicts transient droplet motion under electrical actuation. Electrical actuation of dielectric droplets is experimentally demonstrated by moving transformer oil droplets between two flat plates under the influence of an actuation voltage. Droplet velocities and their dependence on the plate spacing and the applied voltage are experimentally measured and showed reasonable agreement with predictions from the models developed

Characterizing Microfluidic Operations Underlying an Electrowetting Heat Pipe on the International Space Station

Electrowetting heat pipes (EHPs) are a newly conceptualized class of heat pipes, wherein the adiabatic wick section is replaced by electrowetting-based pumping of the condensate (as droplets) to the evaporator. Specific advantages include the ability to transport high heat loads over long distances, low thermal resistance and power consumption, and the absence of moving mechanical parts. In this work, we describe characterization of key microfluidic operations (droplet motion and splitting) underlying the EHP on the International Space Station (ISS). The testing was performed under the Advanced Passive Thermal eXperiment (APTx) project, a project to test a suite of passive thermal control devices funded by the ISS Technology Demonstration Office at NASA JSC (Johnson Space Center). A rapid manufacturing method was used to fabricate the electrowetting device on a printed circuit board. Key device-related considerations were to ensure reliability and package the experimental hardware within a confined space. Onboard the ISS, experiments were conducted to study electrowetting-based droplet motion and droplet splitting, by imaging droplet manipulation operations via pre-programmed electrical actuation sequences. An applied electric field of 36 Volts per micron resulted in droplet speeds approaching 10 millimeters per second. Droplet splitting dynamics were observed and the time required to split droplets was quantified. Droplet motion data was analyzed to estimate the contact line friction coefficient. Overall, this demonstration is the first-ever electrowetting experiment in space. The obtained results are useful for future design of the EHP and other electrowetting-based systems for microgravity applications

Temperature Dependent Droplet Impact Dynamics on Flat and Textured Surfaces

Droplet impact dynamics determines the performance of surfaces used in many applications such as anti-icing, condensation, boiling, and heat transfer. We study impact dynamics of water droplets on surfaces with chemistry/texture ranging from hydrophilic to superhydrophobic and across a temperature range spanning below freezing to near boiling conditions. Droplet retraction shows very strong temperature dependence especially on hydrophilic surfaces; it is seen that lower substrate temperatures lead to lesser retraction. Physics-based analyses show that the increased viscosity associated with lower temperatures combined with an increased work of adhesion can explain the decreased retraction. The present findings serve as a starting point to guide further studies of dynamic fluid-surface interaction at various temperatures. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692598

Pool boiling of nanofluids on biphilic surfaces: An experimental and numerical study

This study addresses the combination of customized surface modification with the use of nanofluids, to infer on its potential to enhance pool-boiling heat transfer. Hydrophilic surfaces patterned with superhydrophobic regions were developed and used as surface interfaces with different nanofluids (water with gold, silver, aluminum and alumina nanoparticles), in order to evaluate the effect of the nature and concentration of the nanoparticles in bubble dynamics and consequently in heat transfer processes. The main qualitative and quantitative analysis was based on extensive post-processing of synchronized high-speed and thermographic images. To study the nucleation of a single bubble in pool boiling condition, a numerical model was also implemented. The results show an evident benefit of using biphilic patterns with well-established distances between the superhydrophobic regions. This can be observed in the resulting plot of the dissipated heat flux for a biphilic pattern with seven superhydrophobic spots, δ = 1/d and an imposed heat flux of 2132 w/m2. In this case, the dissipated heat flux is almost constant (except in the instant t* ≈ 0.9 when it reaches a peak of 2400 W/m2), whilst when using only a single superhydrophobic spot, where the heat flux dissipation reaches the maximum shortly after the detachment of the bubble, dropping continuously until a new necking phase starts. The biphilic patterns also allow a controlled bubble coalescence, which promotes fluid convection at the hydrophilic spacing between the superhydrophobic regions, which clearly contributes to cool down the surface. This effect is noticeable in the case of employing the Ag 1 wt% nanofluid, with an imposed heat flux of 2132 W/m2, where the coalescence of the drops promotes a surface cooling, identified by a temperature drop of 0.7 °C in the hydrophilic areas. Those areas have an average temperature of 101.8 °C, whilst the average temperature of the superhydrophobic spots at coalescence time is of 102.9 °C. For low concentrations as the ones used in this work, the effect of the nanofluids was observed to play a minor role. This can be observed on the slight discrepancy of the heat dissipation decay that occurred in the necking stage of the bubbles for nanofluids with the same kind of nanoparticles and different concentration. For the Au 0.1 wt% nanofluid, a heat dissipation decay of 350 W/m2 was reported, whilst for the Au 0.5 wt% nanofluid, the same decay was only of 280 W/m2. The results of the numerical model concerning velocity fields indicated a sudden acceleration at the bubble detachment, as can be qualitatively analyzed in the thermographic images obtained in this work. Additionally, the temperature fields of the analyzed region present the same tendency as the experimental results.This work was funded by Portuguese national funds of FCT/MCTES (PIDDAC) through the base funding from the following research units: UIDB/00532/2020 (Transport Phenomena Research Center, CEFT), UIDB/04077/2020 (MEtRICs) and UIDP/04436/2020. The authors are also grateful for the funding of FCT through the projects LISBOA-01-0145-FEDER-030171/NORTE-01-0145-FEDER-030171 (PTDC/EME-SIS/30171/2017), funded by COMPETE2020, NORTE2020, PORTUGAL2020 and FEDER. The authors also acknowledge FCT for partially financing the research under the framework of the project UTAP-EXPL/CTE/0064/2017, financiado no ambito do Projeto 5665-Parcerias Internacionais de Ciencia e Tecnologia, UT Austin Programme. Mr Pedro Pontes also acknowledgesFCT for his fellowship ref. SFRH/BD/149286/2019

Energy Minimization-Based Analysis of Electrowetting for Microelectronics Cooling Applications

Electrowetting (EW)-induced droplet motion has been studied over the last decade in view of its promising applications in the field of microfluidics. The objective of the present work is to analyze the physics underlying two specific EW-based applications for microelectronics thermal management. The first of these involves heat absorption by liquid droplets moving on the surface of a chip under EW actuation. Droplet motion between two flat plates under the influence of an electrowetting voltage is analyzed. An energy minimization framework is employed to predict the actuation force on a droplet. This framework, in combination with semi-analytical models for the forces opposing droplet motion, is used to develop a model that predicts transient EW-induced droplet motion. The second application is targeted at hot-spot thermal management and relies on the control of droplet states on artificially structured surfaces through an applied EW voltage. The influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface is analyzed. An energy minimization-based modeling approach reveals the influence of interfacial energies, surface roughness parameters and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The model is used to establish preliminary criteria to design rough surfaces for use in the hot-spot mitigation application. The concept of an electrically tunable thermal resistance switch for hot-spot cooling applications is introduced and analyzed

Electrical actuation of liquid droplets on smooth and artificially microstructured surfaces

Electrically induced actuation of liquid droplet motion has significant applications in the field of MEMS and lab-on-a-chip devices. This dissertation presents results on the fundamentals governing the electrical actuation of liquid droplets on smooth and artificially microstructured surfaces. A physics-based analytical model is formulated to predict transient EW-induced motion of an electrically conducting droplet between flat plates. A generic modeling framework for predicting the actuation force on a generic liquid droplet (electrically conducting or insulating) under DC or AC actuation is also developed. It is seen that the electrical actuation force on a droplet depends on the electrical properties of the liquid and the dielectric layer, the dielectric layer thickness, plate spacing and the frequency of the AC actuation voltage. Detailed experiments which involve the measurement of droplet velocities are conducted to benchmark the predictions of the electromechanical model. The present work also studies droplet morphology and state transitions on artificially microstructured electrowetted surfaces. An analytical model which predicts the EW-induced Cassie-Wenzel transition on such surfaces is developed. Extensive experiments to study the EW-induced transition and quantify the reversibility of the transition are presented. It is seen that the presence of an energy barrier and dissipative forces hinder the reversibility of the Cassie-Wenzel transition. The dissertation also presents two novel concepts for enhanced control of droplet morphology on artificially microstructured surfaces

Electrical Actuation of Droplets for Microelectronics Cooling

Develop technologies enabling electrical actuation and control of droplets for providing chip-integrated thermal management solution

Heat pipes with electrical pumping of condensate

A heat pipe with electrical pumping. The heat pipe includes a condenser to condense vapor into liquid droplets and an evaporator for the liquid-vapor conversion. Furthermore, the heat pipe includes liquid conduits for carrying the liquid droplets, where every liquid conduit includes electrodes for moving the liquid droplets along the liquid conduits. Additionally, the heat pipe includes vapor conduits for carrying the vapor. After the liquid is condensed and droplets are formed, they are electrically pumped towards the evaporator by sequentially actuating a series of electrodes in the liquid conduits. By implementing electrical pumping instead of wick-based pumping, the heat transport capacity over long distances is greatly increased. Additionally, since the electrical force is greater than gravity, it is possible to develop orientation independent long heat pipes. Other benefits include planar form factors, noiselessness, high reliability due to the absence of moving mechanical parts and ultralow power consumption.Board of Regents, University of Texas Syste

Electrical actuation-induced droplet transport on smooth and superhydrophobic surfaces

Electrical control of liquid droplet motion and wettability has wide-ranging applications in the field of MEMS, lab-on-a-chip devices and surface engineering, in view of the resulting enhanced flow control opportunities, low power consumption and the absence of mechanical moving parts. This article summarizes recent progress towards understanding of the fundamentals underlying electrical actuation of droplets on smooth and superhydrophobic surfaces. Electrical actuation of liquid droplets with widely differing electrical properties on smooth surfaces is first discussed. Electromechanical considerations are employed to study the actuation force on a generic liquid droplet across the entire spectrum of electrical actuation regimes. The challenges in understanding the fluid flow and dissipation mechanisms associated with a discrete moving droplet are discussed. The role of electrical voltages, interfacial energies and surface morphology in determining droplet states (nonwetting Cassie state and wetting Wenzel state) and triggering state transitions on superhydrophobic surfaces is then mapped out. Critical phenomena associated with droplet transitions on superhydrophobic surfaces (energy barrier for the Cassie-Wenzel transition, lack of spontaneous reversibility of the Cassie-Wenzel transition, robustness of the Cassie state, and the role of the roughness elements) are analyzed. The article also highlights key avenues for future research in the fields of electrical actuation-based microfluidics and superhydrophobic surfaces

Electrowetting-based control of droplet transition and morphology on artificially microstructured surfaces

Electrowetting (EW) has recently been demonstrated as a powerful tool for controlling droplet morphology on smooth and artificially structured surfaces. The present work involves a systematic experimental investigation of the influence of electrowetting in determining and altering the state of astatic droplet resting on an artificially microstructured surface. Extensive experimentation is carried out to benchmark a previously developed energy-minimization-based model that analyzed the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The EW voltage required to trigger a transition from the Cassie state to the Wenzel state is experimentally determined for surfaces having a wide range of surface parameters (surface roughness and fraction of surface area covered with pillars). The reversibility of the Cassie-Wenzel transition upon the removal of the EW voltage is also quantified and analyzed. The experimental results from the present work form the basis for the design of surfaces that enable dynamic control of droplet morphology. A significant finding from the present work is that nonconservative dissipative forces have a significant influence in opposing fluid flow inside the microstructured surface that inhibits reversibility of the Cassie-Wenzel transition. The artificially structured surfaces considered in this work have microscale roughness feature sizes that permits direct visual observation of EW-induced Cassie-Wenzel droplet transition; this is the first reported visual confirmation of EW-induced droplet state transition