Electrohydrodynamic Effects Research Articles

A numerical investigation of electrohydrodynamic (EHD) effects on bubble deformation under pseudo-nucleate boiling conditions

In this article, the electrohydrodynamic (EHD) effects on nucleate boiling are studied by developing a numerical modelling of EHD effect on bubble deformation in pseudo-nucleate boiling conditions. The volume of fluid (VOF) method is employed to track the interface between the gas–liquid two phases; the user-defined code is written and added to the commercial software FLUENT to solve the electric field and the corresponding electric body force. On this basis, the model is applied to study the EHD effects on heat transfer and fluid flows. An initial air bubble surrounded by liquid CCl 4 and attached to a horizontal superheated wall under the action of electric field is studied. The results of the EHD effect on bubble shape evolution are compared with those of available experiments showing good agreement. The mechanism of EHD enhancement of heat transfer and the EHD induced phenomena including bubble elongation and detachment are analyzed in detail.

Heat transfer enhancement by needle-arrayed electrodes – An EHD integrated cooling system

Natural convection heat transfer enhanced by electrohydrodynamics (EHD) is experimentally investigated in this work. Four kinds of needle-arrayed electrodes combined with a plate fin heat sink are made and tested under a controlled environment. Previous and present test results show that convection heat transfer is enhanced with the presence of EHD due to corona wind generation by the electron avalanche mechanism. To identify the effect of EHD, the enhancement ratio ( ξ), defined as the heat transfer coefficient with EHD ( h ec ) relative to that without EHD ( h nc ), is proposed to examine the performance. It is found that the negative corona has a lower threshold voltage and a better performance compared with the positive one. The enhancement ratio increases with electric power input and linearly with the 1/4 power of total corona current. Heat transfer enhancement is evident when a corona discharge above 1 μA is established. For moderate test condition, the enhancement ratio is around 3–5 with a preferable normalized electrode height of around 0.25–0.35 compared to the edge of the heat sink. For a given electrode height, there is an optimal electrode density below which the required area exposed to the corona wind is apparently insufficient, and beyond which the flow interference caused by adjacent electrodes offsets the effective performance. In particular, the enhancement ratio decreases with increasing the heat dissipation as a result of augmented buoyancy convection.

Influence of pH and Compression on Electrohydrodynamic Effects in Nanoporous Packed Beds

Fluid flow and charge transport in fine structures can be driven both by pressure gradients and by electric fields if electrochemical double layers are present on the surfaces. The interrelated electrohydrodynamic effects may be used to drive liquids without moving parts, for example, in dewatering or in electroosmotic chromatography, or to generate small electric currents. While the electrohydrodynamic transport is well understood for simple geometries, models for porous structures are complex. Furthermore, the interconnected porous structure of a packed bed itself strongly depends on the electrochemical double layers. In this study, the electrohydrodynamic transport in packed beds consisting of boehmite particles with an average diameter of 38 nm is investigated. We describe a new approach to the electrokinetic effects by treating the packed beds as theoretical sets of cylindrical capillaries. The charge transport and the electrically driven fluid flow predicted with this model agree well with experimental results. Furthermore, the hydraulic permeability was found to be a nonlinear function of the porosity, independent of whether the porosity change is caused by changing the compression or the electrochemical double layer.

Microscale Thermal Transport and Electromechanical Microfluidic Actuation

This paper summarizes recent advances in fundamental research on microscale thermal transport in phase-change processes and developments in novel microfluidic actuation methods using electromechanical effects. The research topics covered include thin-film transport, pool boiling heat transfer enhancement, convective flow boiling in microchannels, microscale ionic winds, micropumping using electrohydrodynamic effects including ion induction and dielectrophoresis, and electrowetting-based control of liquid droplet motion. Progress in microscale flow characterization and non-intrusive temperature measurement is also discussed.

Electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles

We have compared optical tracking and electric current measurements to study the electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles. Particle trajectories are measured in response to a voltage step, revealing spatial and temporal variations of the electric field when space-charge layers are created by charged inverse micelles. The particle trajectories and current measurements are in good agreement with simulations and analytical approximations based on drift and diffusion of charges. Electrohydrodynamic effects observed at high concentrations of charged inverse micelles are explained by injection of charged inverse micelles from the bulk into the space-charge layers.

Bacterial handling under the influence of non-uniform electric fields: dielectrophoretic and electrohydrodynamic effects

This paper describes the modeling and experimental verification of a castellated microelectrode array intended to handle biocells, based on common dielectrophoresis. The proposed microsystem was developed employing platinum electrodes deposited by lift-off, silicon micromachining, and photoresin patterning techniques. Having fabricated the microdevice it was tested employing Escherichia coli as bioparticle model. Positive dielectrophoresis could be verified with the selected cells for frequencies above 100 kHz, and electrohydrodynamic effects were observed as the dominant phenomena when working at lower frequencies. As a result, negative dielectrophoresis could not be observed because its occurrence overlaps with electrohydrodynamic effects; i.e. the viscous drag force acting on the particles is greater than the dielectrophoretic force at frequencies where negative dielectrophoresis should occur. The experiments illustrate the convenience of this kind of microdevices to micro handling biological objects, opening the possibility for using these microarrays with other bioparticles. Additionally, liquid motion as a result of electrohydrodynamic effects must be taken into account when designing bioparticle micromanipulators, and could be used as mechanism to clean the electrode surfaces, that is one of the most important problems related to this kind of devices.

Non-uniform electric field-induced yeast cell electrokinetic behavior

Common dielectrophoresis (c-DEP, i.e. neutral matter motion induced by non-uniform electric fields) has become a basic phenomenon of biochips intended for medical, biological and chemical assays, especially when they imply bioparticle handling. This paper deals with modelling and experimental verification of a castellated, c-DEP-based, microelectrode array intended to handle biological objects. The proposed microsystem was developed employing platinum electrodes patterned by lift-off, silicon micro machining and photoresin patterning techniques. Saccharomyces cerevisiae were used as test bioparticles for experimental verification. Yeast cells were repelled toward electrode bays and toward interelectrodic gaps tor frequencies around 20 MHz where there is minimum electric field strength, corresponding to a negative dielectrophoretic phenomenon. Yeast cell agglomerations were observed around electrode edges for frequencies of around 2 MHz where there is maximum electric field strength, thereby verifying the positive dielectrophoretic phenomenon. Bioparticles were separated from the electrode edges when the working frequency was reduced and they were dragged towards the electrode centre, remaining there while the frequency was low enough. Such atypical pattern may be explained due to the occurrence of positive dielectrophoresis overlap with electrohydrodynamic effects (i.e. the viscous drag force acting on the particles was greater than the dielectrophoretic force at frequencies where positive dielectrophoresis should occur). The experiments illustrated microsystem convenience in microhandling biological objects, the reby providing these microarrays’ possible use with other cells. Liquid motion resulting from electrohydrodynamic effects must also be taken into account when designing bioparticle micromanipulators, and could be used as a mechanism for cleaning electrode surfaces.

Simulating (electro)hydrodynamic effects in colloidal dispersions: Smoothed profile method

Previously, we have proposed a direct simulation scheme for colloidal dispersions in a Newtonian solvent (Phys. Rev. E 71, 036707 (2005)). An improved formulation called the "Smoothed Profile (SP) method" is presented here in which simultaneous time-marching is used for the host fluid and colloids. The SP method is a direct numerical simulation of particulate flows and provides a coupling scheme between the continuum fluid dynamics and rigid-body dynamics through utilization of a smoothed profile for the colloidal particles. Moreover, the improved formulation includes an extension to incorporate multicomponent fluids, allowing systems such as charged colloids in electrolyte solutions to be studied. The dynamics of the colloidal dispersions are solved with the same computational cost as required for solving non-particulate flows. Numerical results which assess the hydrodynamic interactions of colloidal dispersions are presented to validate the SP method. The SP method is not restricted to particular constitutive models of the host fluids and can hence be applied to colloidal dispersions in complex fluids.

Electrohydrodynamic interaction induced by a dielectric barrier discharge

In the present work an experimental investigation on the electrohydrodynamic (EHD) interaction effect, induced by means of a dielectric barrier discharge on a 1atm subsonic air flow, is described. The air flow is obtained in an open circuit blowing wind tunnel at different air speeds. A plane plasma panel equipped with a dielectric discharge barrier system is immersed in the flow. A single phase sinusoidal power supply system, a superposition of a single phase sinusoidal system with a dc voltage supply, and a three phase symmetrical power supply configuration are considered. Electric, fluid dynamic, and spectroscopic diagnostic techniques are utilized. Pitot probe measurements are performed in the boundary layer on the plasma panel surface. Schlieren imaging is done to visualize the plasma sheath. Vibrational and rotational temperatures are evaluated by means of spectroscopic techniques. The force induced on the flow by the dielectric barrier discharge acts on the ions of the non-neutral sheath ahead of the plasma filament when travelling on the panel surface only. This causes a reduction of the time interval during which the force is active. The EHD effect and the momentum transfer to the gas flow in the boundary layer are reduced when the flow speed grows. An increase of the supply frequency and of the supply voltage causes an increase of the momentum transfer. In these experiments no significant increase of the efficiency of EHD effect is observed when utilizing the superposition of a dc voltage to a single phase power supply or a three phase power supply.

Hydrodynamic Flows in Electrowetting

Hydrodynamic flows are generated inside a droplet in electrowetting when an ac voltage is applied. To discover the characteristics and origin of the flows, we investigated the flow pattern for a sessile droplet for various needle-electrode positions, electrolyte concentrations, and applied electrical frequencies. Two distinct types of flows were observed under current experimental conditions. In the typical experimental condition, a quite fast flow appears in the low-frequency range of about 10 Hz to 15 kHz. A different type of flow is observed in the high-frequency range of about 35 to 256 kHz, but this frequency range depends significantly on the electrolyte concentration. Most typically, the flow directions are different for the two flows. A shape oscillation of a droplet was observed in the low-frequency range by a high-speed camera. The flow in the low-frequency range is insensitive to the conductivity of the solution and may be caused by the interfacial oscillation of the droplet. The flow at high frequency is very sensitive to the conductivity of the solution and electrode position, so the high-frequency flow is believed to be caused by some electrohydrodynamic effect.

Humidity Characteristics of Plasma Synthetic Jet Actuator

This paper describes humidity characteristics of Plasma Synthetic Jet Actuator (PSJA). PSJA is flow control device using the Electro Hydro Dynamic (EHD) effect of the glow-discharge. PSJA can be downsized, controlled easily, and has quick control response. Though the flow control around an airfoil with PSJA is mainly studied, its characteristics have not been clarified. So, it is necessary to investigate humidity characteristics of induced flow by PSJA. In this paper, the induced flow from PSJA was measured in the different relative humidity. The experimental results show that the effect of induce flow by PSJA had decreased with increase relative humidity. Therefore, when PSJA was used, relative humidity becomes an important parameter.

Numerical analysis of thermal and electrohydrodynamic effects in travelling-wave dielectrophoretic devices

Travelling-wave dielectrophoretic phenomena involving cell detection and manipulation are very interesting for biomedical and biotechnological applications. However, electrohydrodynamic phenomena may interfere with the action of dielectrophoresis. The main goal of our study is to calculate the interplay of these effects. In so doing, we have been able to go beyond the order-of-magnitude estimations, which were available in the literature. In this framework we have taken into account the effects of using square signals, which calls for a calculation of the combined polarisability effects due to the superposition of different frequencies. Thermal gradients are also a triggering agent of electrohydrodynamic effects. We have undertaken the pertinent thermal simulations with a concern for the influence of different boundary conditions. The dynamics of the thermal fields were also assessed in order to estimate the characteristic times of electrohydrodynamic phenomena. All these results have been used in a study of the fluid flow pattern motion at different frequencies and its influence on the particle behaviour.

Microextraction in a tetrabutylammonium bromide/ammonium sulfate aqueous two-phase system and electrohydrodynamic generation of a micro-droplet

Microextraction of methyl orange in the aqueous two-phase system (ATPS) formed by dissolving tetrabutylammonium bromide (TBAB) and ammonium sulfate (AS) is reported. Methyl orange was transported from the AS-rich phase to TBAB-rich phase across the interface of the two immiscible phases. The electrohydrodynamic effect on the shape of the interface of two immiscible flows was also observed by applying dc voltage at the T-junction of the microchannel and the generation of a droplet of AS-rich phase was observed when the potential difference between positive and negative electrodes exceeds a threshold voltage. The minimum voltage necessary for the droplet generation depends on pH due to the degree of dissociation and charge accumulation.

Numerical Investigation of the Flow Profiles in the Electrically Enhanced Cyclone

A numerical model for simulation of the electrohydrodynamic flow in an electrically enhanced cyclone is presented. A finite element approach was applied to solve the coupled equations for the positive corona-induced electric field. Three-dimensional simulations of gas flow were carried using Reynolds-Averaged Navier-Stokes equations including the Reynolds stress model and the electrohydrodynamic effect. Numerical results show that the change in the flow profile because of the influence of the corona-induced electric field is apparent when the inlet flow rate is low but is negligible at higher flow rates.

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Special Issue on the Magnetic and Electric Field Effects at the Micro- and Nano-Scale Systems and Flows in Material Processing

Numerical modeling of the electrohydrodynamic effect to natural convection in vertical channels

The electrohydrodynamic effect to natural convection inside the vertical channels is numerically investigated by computational fluid dynamics technique. The range of parameters considered are 10 4 = Ra = 10 7, 7.5 = V 0 = 17.5 kV, and 2 = aspect ratio = 10. Flow and temperature distributions are affected with supplied voltage at the wire electrodes, and the heat transfer enhancement is significantly influenced at low Rayleigh number. The augmented volume flow rate of fluid is indicated in relation with the number of electrodes. Moreover, heat transfer enhancement also depended on the electrode arrangement while the number of electrodes is initially fixed. The relation between channel aspect ratio and number of electrodes that performs the maximum heat transfer is expressed incorporating with the optimum concerning parameters.

Insulator-based dielectrophoresis in viscous media—Simulation of particle and droplet velocity

The velocity of micro-particles in a nonuniform electric field was examined as a function of electrical potential and particle size to illustrate the possible application of dielectrophoresis (DEP) as a new separation technique in viscous media. A new comprehensive model is presented that combines the effects of DEP and electrohydrodynamic forces on particle motion. The current model simulation takes into account the possible significant influence of electrohydrodynamic effects depending on the particle size, electrode distance, and voltage applied during DEP particle separation. The heat generated as a consequence of high electric field strength leads to density gradients in the liquid, thus inducing buoyancy forces that cause fluid convective motion. Experimental velocity measurements using two materials having extreme properties, i.e. polyethylene (PE) particles (diameter range 100–2000 μm) and water droplets (diameter range 25–275 μm), both suspended in a viscous medium (silicone oil), correspond with the proposed theoretical predictions. The comprehensive model presented was applied to insulator-based DEP in a direct current (dc) electric field, but it is expected to allow predictions of various similar systems.

Numerical modeling of the effect of number of electrodes on natural convection in an EHD fluid

Numerical modeling of the electrohydrodynamic effect on natural convection in enclosures is investigated. The interactions between electric field, flow field, and temperature field are analyzed by using the computational fluid dynamics technique. Flow pattern and temperature distribution are substantially affected by the voltage supplied at the wire electrodes, especially at low Rayleigh number. It can be concluded that the fluid velocity and heat transfer coefficient in the presence of an electric field are significantly increased with when a large number of electrodes is used. Surprisingly, a minimum value of augmented heat transfer occurs with an intermediate number of electrodes. The optimized condition between the aspect ratio of an enclosure and number of electrodes which leads to maximum heat transfer enhancement is expressed in terms of the parameters concerned.

Electrode arrangement effect on natural convection

Numerical and experimental studies of the electrohydrodynamic effect on natural convection in an enclosure are investigated. The governing equations of electric field, flow field and temperature field are also represented. It is found that the flow pattern is affected by the supplied voltage at the wire electrodes. Augmented heat transfer increases at low Rayleigh number and high number of electrodes. The uniform electrode arrangement along an enclosure provides maximum heat transfer when the number of electrodes is fixed.

A Planar Array of Micro-Fabricated Electrospray Emitters for Thruster Applications

This paper reports the design, fabrication, and experimental characterization of a planar array of micro-fabricated electrospray emitters intended for space propulsion applications in micro–satellites. The engine uses the ionic liquid EMI-BF <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$_4$</tex> as propellant. Electrospray engines take advantage of the electrohydrodynamic effect known as Taylor cone to produce thrust. The array is designed with an open architecture and it is composed of a set of spikes, i.e., emitters, coming out from a propellant pool. There are two configurations for the emitters: fully sharpened slender emitters, i.e., pencils, and truncated pyramidal emitters, i.e., volcanoes. The arrays have between 4 and 1024 emitters in an active area of 0.64 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ cm^2$</tex> . The surface of the engine (tank and emitters) is covered with “black silicon” that acts as wicking material. The micro-fabrication of the engine is described. The paper reports experimental characterization of the hydraulics system including wettability tests, current/emitter–voltage characteristics, and imprints of the exit stream on a collector. Preliminary results demonstrating the feasibility of obtaining substantially larger emission currents at the same extraction voltage by controlling the temperature are also reported. The paper compares the experimental current/emitter–voltage characteristics with relevant theories of field emission of electrons.1433