Electrohydrodynamic Flow Research Articles

Lattice Boltzmann analysis of conjugate heat transfer in the presence of electrohydrodynamic flow

The externally imposed direct current (DC) electric field mightily impacts on the conjugate heat transfer existing somewhere between the solid and liquid phases, which is numerically investigated via Lattice Boltzmann method (LBM) in this paper. The model is a square cavity filled with dielectric liquid, in which has a centred heat-generating solid body, and the spatially inhomogeneous physical properties such as thermal conductivity, heat capacity, permittivity and mobility are considered distinctively. The lattice Boltzmann method is used to solve the entirely coupled equations and the developed code is first verified by three cases for the hydrostatic solution, conjugate heat transfer with and without heat source. The LBM results show the highly agree well with both the derived analytical solution and the results obtained by other papers. Then, enhancement effect of the flow and heat transfer when electrohydrodynamic (EHD) forces in existence are studied under the different governing parameters and the internal heat source intensity. It is found that the solid-liquid two phases conjugate heat transfer can be significantly enhanced when applying a DC electric field across the domain.

Control of electrohydrodynamic (EHD) flow by secondary electric potential

Non-thermal plasma (NTP) is generated around the surface of electrode where the electrons of gas molecules are excited by high external energy. Electrohydrodynamic (EHD) flow is produced by collision between electrically neutral molecules and positive ions emitted by ionization. Because of the characteristics of electrically driven flow, the NTP has been focused on for next generation application, and widely applied to industrial processes. However, most studies are limited to symmetric configuration to control the momentum of EHD flow. Hence, to better control that flow, secondary electric potential should be considered at the multi-electrode configuration, so that local enhancement of heat transfer can be observed. To date, EHD flow by asymmetric formation of the electric field has not been well researched or analyzed. In the present work, the symmetric configuration was firstly considered to obtain the reliability of EHD flow. Numerical simulation and experiments were carried out to validate the current, velocity profile, and Nusselt number. The particle image velocimetry (PIV) method was used to identify the velocity distribution of EHD flow. Also, the convective heat transfer induced by EHD flow could be analyzed. Secondly, the asymmetric configuration was considered by applying various magnitudes of secondary electric potential, which affected the momentum and direction of the EHD flow, which could be observed by the results of numerical simulation and the PIV method. To consider the application of heat transfer, it was verified that vertically controlled EHD flow could enhance local cooling performance, and showed increase of thermal efficiency. Through the visualization, it was noted that the ratio between main and secondary electric potential determined the extent of bias. Finally, the analytic model to predict the velocity profile could be proposed, and the regime map for the occurrence of reverse flow was presented.

Electric field induced macroscopic cellular phase of nanoparticles.

A suspension of nanoparticles with very low volume fraction is found to assemble into a macroscopic cellular phase that is composed of particle-rich walls and particle-free voids under the collective influence of AC and DC voltages. Systematic study of this phase transition shows that it was the result of electrophoretic assembly into a two-dimensional configuration followed by spinodal decomposition into particle-rich walls and particle-poor cells mediated principally by electrohydrodynamic flow. This mechanistic understanding reveals two characteristics needed for a cellular phase to form, namely (1) a system that is considered two dimensional and (2) short-range attractive, long-range repulsive interparticle interactions. In addition to determining the mechanism underpinning the formation of the cellular phase, this work presents a method to reversibly assemble microscale continuous structures out of nanoscale particles in a manner that may enable the creation of materials that impact diverse fields including energy storage and filtration.

Underwater shock wave induced by pulsed discharge on water

Plasmas on liquids have provided significant applications in material, environmental, and biological sciences. The mechanisms of these chemical reactions in liquids have been primarily described by the plasma–liquid interactions and convection by an electrohydrodynamic flow. Although shock waves play a significant role in radical formation, agitation and cell destruction, not much information is available on underwater shock waves induced by the surface discharge on water. In this study, an underwater shock wave generated by the pulsed surface discharge on water using the laser shadowgraph method has been demonstrated. The results reveal that the shock wave generated by the discharge on water was transmitted into the water. The mean velocity of the shock wave reached 1.7 km s−1. The results indicate that the surface discharge accelerates the reaction in the water by the combined action of the underwater shock wave and the plasma reaction at the air–water interface. The results are expected to aid in the understanding the mechanisms of existing applications, such as decomposition, synthesis, and sterilization.

Characterization of electrohydrodynamic flow in a plate-plate electrostatic precipitator with a wire-cylinder pre-charger by data-driven vortex and residence time analysis

The two-stage electrostatic precipitator (ESP) is widely used for indoor air cleaning field, while both the Particle Image Velocimetry (PIV) observations and well-characterized quantitative descriptions are probably scarce. In this work, we propose to use vortex analysis, which is the main quantitative method for flow field, and the relative residence time method (RRT) to evaluate the electrohydrodynamic flow (EHD) in a plate-plate ESP. The effects of electrical energization, pre-charger polarity and primary gas flow velocity are studied. Two vortex indexes sort the particle flow as five patterns and increase almost linearly with applied voltage. Four final particle statuses, penetrated, collected, stagnant and back-mixed are clarified by RRT method. The data generated by RRT are utilized to derive the PIV index for further analysis of the ESP performance. The trend, which rises first and drops later, suggests PIV index is expected to pave a new way to clarify the relationship between the ESP flow patterns and particle collection efficiency.

Numerical Analysis of Electrohydrodynamic Flow in a Circular Cylindrical Conduit by Using Neuro Evolutionary Technique

This paper analyzes the mathematical model of electrohydrodynamic (EHD) fluid flow in a circular cylindrical conduit with an ion drag configuration. The phenomenon was modelled as a nonlinear differential equation. Furthermore, an application of artificial neural networks (ANNs) with a generalized normal distribution optimization algorithm (GNDO) and sequential quadratic programming (SQP) were utilized to suggest approximate solutions for the velocity, displacements, and acceleration profiles of the fluid by varying the Hartmann electric number (Ha2) and the strength of nonlinearity (α). ANNs were used to model the fitness function for the governing equation in terms of mean square error (MSE), which was further optimized initially by GNDO to exploit the global search. Then SQP was implemented to complement its local convergence. Numerical solutions obtained by the design scheme were compared with RK-4, the least square method (LSM), and the orthonormal Bernstein collocation method (OBCM). Stability, convergence, and robustness of the proposed algorithm were endorsed by the statistics and analysis on results of absolute errors, mean absolute deviation (MAD), Theil’s inequality coefficient (TIC), and error in Nash Sutcliffe efficiency (ENSE).

Lucas polynomials based spectral methods for solving the fractional order electrohydrodynamics flow model

The aim of this paper is to study the fractional order nonlinear singular boundary value problem arising in electrohydrodynamics flow, which describes the velocity of the ionized fluid in a circular cylindrical conduit. For this purpose, Lucas polynomials are employed via spectral collocation and Galerkin methods for reducing the nonlinear singular differential equation of fractional order to an algebraic nonlinear system, which is solved by Newton iterative method. The problem includes two important parameters, including the nonlinearity and Hartman numbers. The effect of these parameters and the influence of fractional order derivatives on the velocity of the fluid is discussed. Based on the reported results in tables and figures, for large values of Hartman number and the fractional derivatives, as well as weak nonlinearity parameter, the velocity profile was increased. The purposed schemes are simple and attractive, and their accuracy and efficiency, were confirmed by studying some cases of main problem and a comparison of the numerical results with the results obtained by some other methods.

Polyhedral Micromotors of Metal-Organic Frameworks: Symmetry Breaking and Propulsion.

Colloidal micromotors can autonomously propel due to their broken symmetry that leads to unbalanced local mechanical forces. Most commonly, micromotors are synthesized to possess a Janus structure or its variants, having two components distinct in shape, composition, or surface joined together on opposite sides. Here, we report on an alternative approach for creating micromotors, where microcrystals of metal-organic frameworks (MOFs) with various polyhedral shapes are propelled under an AC electric field. In these cases, symmetry breaking is realized by orienting the polyhedral particles in a unique direction to generate uneven electrohydrodynamic flow. The particle orientations are controlled by a delicate competition between the electric and gravitational forces exerted on the particle, which we rationalize using experiments and a theoretical model. Furthermore, by leveraging the MOF types and shapes, or surface properties, we show that the propulsion of MOF motors can be tuned or reversed. Because of the flexibility in designing MOFs and their one-step scalable synthesis, our strategy is simple yet versatile for making well-defined functional micromotors.

A Hybrid Metaheuristic Based on Neurocomputing for Analysis of Unipolar Electrohydrodynamic Pump Flow.

A unipolar electrohydrodynamic (UP-EHD) pump flow is studied with known electric potential at the emitter and zero electric potential at the collector. The model is designed for electric potential, charge density, and electric field. The dimensionless parameters, namely the electrical source number , the electrical Reynolds number , and electrical slip number , are considered with wide ranges of variation to analyze the UP-EHD pump flow. To interpret the pump flow of the UP-EHD model, a hybrid metaheuristic solver is designed, consisting of the recently developed technique sine–cosine algorithm (SCA) and sequential quadratic programming (SQP) under the influence of an artificial neural network. The method is abbreviated as ANN-SCA-SQP. The superiority of the technique is shown by comparing the solution with reference solutions. For a large data set, the technique is executed for one hundred independent experiments. The performance is evaluated through performance operators and convergence plots.

Enhancement of heat transfer at boiling in electrohydrodynamic flow

The influence of the electric field strength and interelectrode spacing on the heat transfer intensity at boiling in an electrohydrodynamic flow was studied. It was stated that the heat transfer coefficient increases with the increasing of the field strength. The influence of the interelectrode spacing is ambiguous. The efficiency of the action of a electrohydrodynamic flow on the heat transfer intensity at boiling was evaluated using the ratio of the heat transfer coefficient at boiling in the field to the heat transfer coefficient at boiling without the field. The relationships for calculation were obtained that satisfactorily agree with the experimental data.

Unsteady Electrohydrodynamic Stagnation Point Flow of Hybrid Nanofluid Past a Convective Heated Stretch/Shrink Sheet

Unsteady electrohydrodynamic hybrid nanofluid Al 2 O 3 ‐ Cu / H 2 O past a convective heat stretched/shrinked sheet is examined. A stagnation point fluid flow with velocity slip constrains and heat source or sink is deliberated. The combined set of PDEs is translated into ODEs by including approved similarity transformations. HAM is applied for the solution to the obtained nonlinear system. The magnetic input factor, Prandtl number, electric field factor, Eckert number, heat source factor, and unstable factor are the governing parameters. The impact of these factors on the temperature and velocity profiles features of the problem is considered with explanation. Intensification in values of electric and magnetic fields parameters enhanced the heat transfer rate. The greater Prandtl number lessens the temperature. Amplification in temperature is perceived for Eckert parameter. The heat transferred rate of hybrid nanofluid in the entire domain increases as the heat source increases, while the heat sink has the opposite effect. Skin friction and Nusselt number is increased for increasing values of magnetic field parameters. It is also noted that Nusselt number lessens for raising in Pr , E , and Ec . Furthermore, it is eminent that the hybrid nanofluid possesses better result compared to the nanofluid.

Electrically active and thermally passive liquid-crystal device toward smart glass

Cholesteric liquid crystals (CLCs) are an important soft material for display and sensing technologies due to their unique optical and thermal properties, which are susceptible to external perturbations such as the electric field and temperature variation. The typically periodic structure of a CLC with a twist between molecular layers permits its chiral photonic crystallinity and the resulting selective reflection of a bandwidth in the otherwise generally high transmission spectrum of incident light. Here we report on a novel dual-mode CLC device as smart glass that enables the transparency to be self-adaptive to temperature and is simultaneously characterized by a fully on-demand, electrically controlled function, allowing users to regulate the suitable or desired extent of transparency in accordance with their living climate zones or personal needs. The working principle is based on the controllable strength of voltage-induced electrohydrodynamic flow, generating temperature-dependent dynamic scattering for passive control. Moreover, the transmission can be reversibly modulated and switched by applied AC voltage for active control between the transparent and opaque states. As a proof of concept, the characteristic Bragg reflection can be designed to sit in the near-infrared region to partially block unwanted thermal radiation in the optically transparent state.

(Invited) Stabilizing Effect of Impinging Plasma Jet on the Water Surface

Year by year, plasma processing with atmospheric pressure plasma jets is increasingly gaining attention and diversifying into various scientific and industrial fields; plasma jet systems have the merit of being able to release plasma into target liquids or solids. Thus, naturally, part of plasma jet research has directed towards investigating interfacial phenomenon between plasma–liquid or –solid and understanding underlying principles thereof. Since impinging plasma jet on the liquid surface has been considered as a basis of plasma jet processing, for example, active efforts to assess the transport of electrons and other reactive species from plasma to liquid have been made. However, despite its scientific and practical significance, surprisingly little attention has been given to the stabilizing effect of plasma jets on plasma–liquid interfaces thus far. In this contribution, we demonstrate the stabilization of liquid instabilities by weakly ionized gas for the case of a helium gas jet impinging on water, which profoundly affects the water free surface. Special attention is given to the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, induced by the momentum transfer from accelerated charged particles to neutral gas under a strong electric field. A plasma jet consisting of periodic pulsed ionization waves, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional EHD gas flow and parallel electric field to the gas–water interface caused by plasma interacting ‘in the cavity’ render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering new insights into physical processes and revealing an interdependence between weakly ionized gases and deformable dielectric matter, including plasma–water systems, and their potential stabilization.

Numerical simulation of sorbent injection for mercury removal within an electrostatic precipitator: In-flight plus wall-bounded mechanism

Powder sorbent injection upstream of an electrostatic precipitator (ESP) is of great potential for mercury removal from coal-fired power plants. Nevertheless, the migration and transformation of mercury and sorbents within an ESP remains poorly understand ascribing to internal inaccessibility of an operating ESP for direct measurement. A multi-field model was established to simulate mercury removal by sorbent injection within an ESP. The particle diameter, sorbent concentration, equilibrium constant (K), and operation voltage were key factors for affecting mercury removal. When the particle diameter increased from 20 to 80 μm, the in-flight (EIn-flight) and wall-bounded (EWall-bounded) adsorption efficiency decreased from 48.1% and 5.0% to 3.3% and 2.2%, respectively, indicating that decreasing the particle diameter would benefit mercury removal. Besides, EIn-flight and EWall-bounded climbed to 91.9% and 5.3% when increasing the sorbent concentration by four folds (i.e., from 2 to 8 g/m3), suggesting a promotional effect of higher sorbent concentration. The K value exhibited a similar influential trend as sorbent concentration, i.e., changing the K value from 300 to 60000 m3/kg enhanced EIn-flight and EWall-bounded to 48.3% and 5.1%, respectively. The electrohydrodynamic (EHD) flow played dual roles under various voltages. When the voltage was lower than 20 kV, the EHD flow slightly promoted mercury removal. However, an obvious decline of EIn-flight and EWall-bounded was observed when the voltage climbed from 20 to 35 kV. Such results adequately clarified key factors for affecting mercury removal performance, which provides the fundamental to elucidate the behaviors of particulate matter precipitation and mercury capture within ESPs.

Towards efficient solar demulsification (I): A solar electrical role on interfacial film of emulsions

Oil/water separation has become a significant issue since emulsions are detected in many industrial wastewater and severely threaten the ecological environment. Developing separation technique driven by solar energy with high separation efficiency is quite meaningful and challenging. In this work, solar electrical role on an interfacial film of emulsions was developed for efficient oil/water separation. Emulsified oil/water droplets from the ASP flooding were chosen to characterize the influence of solar electrical role on demulsification. Droplet size distribution, zeta potential, interfacial tension, viscosity, microscope image, and rheological properties were systematically studied to explore the mechanism of electrical role on the instability of oil droplets. Experimental results indicate solar electrical role on the oil-water interface can change the properties of the interface, help the oil-water emulsion instability, and promote oil-water separation. The instability mechanism of the emulsion can be summarized as the following. Firstly, the charged oil/water droplets interfere with the original electric field, which generates a force along with the droplet's tangential direction. Secondly, the force drives charged groups to radially migrate away from the electrode, which causes the entangled polymer to loosen, expand, and rearrange interface charges, resulting in decreasing in the rigidity and zeta potential. Thirdly, the force also promotes the electrohydrodynamic flow of oil/water droplets. The adjacent oil/water droplets mutually entrained in electrohydrodynamic fluid flow, resulting in coalescence. Furthermore, the interface-active components can be electrochemically oxidized to small molecular. The in-depth research of solar electrical role on interfacial film provides theoretical support for coupling of solar photo effect and thermal effect, to obtain highly efficient solar demulsification.

Analysis and optimization of the electrohydrodynamic (EHD) flow with a wire-to-two-cylinder configuration for convective heat transfer

Electrohydrodynamic (EHD) flow is a promising field, especially for gas pumping applications. The EHD flow could be produced by the formation of non-thermal plasma (NTP) on a surface with high curvature. In this study, an analytical model was employed, and the experiment was carried out with the configuration of a wire and two cylinders in an open space. The Townsend relation between electric signals was checked for the stable generation of corona discharge, and the empirical equation was employed to verify the relation between parameters, such as current, voltage, and momentum. In this process, the momentum of EHD flow can be expressed as a function of the design variables, so that the characteristics of the NTP pump can be estimated. The efficiency of the system could be determined from the experimental results. The optimal point obtained from the electrical and hydraulic characteristics showed an efficiency of 2.6%, which was 2 times larger than that of the NTP pump reported in previous works. Heat transfer experiments were also carried out for the cooling effect of EHD flow as a pump using a heat plate with constant heat flux, which was placed parallel to the cylinder electrodes. For various cylinder electrode heights and voltages, the heat plate was cooled by the EHD flow, and the temperature was measured using thermocouples. The cooling performance of the EHD flow was evaluated by comparison with the heat transfer coefficient of natural convection. The heat transfer coefficient and Nusselt number were calculated, and their correlation could be obtained. Additionally, an infrared thermal camera was used for spatial reduction, and the global temperature of the overall heat plate could be obtained.

Nonlinear behavior of electrohydrodynamic flow in viscoelastic fluids

The nonlinear evolution of electrohydrodynamic flow subjected to unipolar injection in the dielectric liquid is extended from Newtonian fluids to viscoelastic fluids. The effect of viscoelasticity not only precipitates the onset of chaos but also leads to new transition sequences to chaos. Moreover, an asymmetric steady flow pattern is observed in a perfectly symmetric geometry. In viscoelastic fluids, the electric current transfer is reduced in most cases, but for weakly elastic fluid, it may be enhanced.

Controlled engineering of multifunctional porous structures using tri-needle co-axial electrohydrodynamic flow and sacrificial media

Three-dimensional porous architectures have gained popularity as advanced materials in emerging therapeutics, drug delivery systems and tissue regeneration. This is largely due to their porosity, inter-connectivity, and mechanical properties. However, current approaches to derive such three-dimensional structures are stochastic and therefore density uniformity varies, with innermost components exhibiting dense attributes. This also limits the encapsulation ability and quality of compounds. In this study, multifunctional structures comprising multi-layered particles are developed. Polycaprolactone (PCL)/ethyl cellulose (EC)/ganoderma lucidum polysaccharide (GLP) is the outer layer; silicone oil (SO) is the intermediate layer; PCL and magnetic Fe3O4 nanoparticles (MNP) are the inner-most (core) component. Particles are initially formed using single step tri-needle co-axial electrostatic jetting, followed by sacrificial template removal to enable multifunctional porous structures. Using this approach, deployment of layered bodies enables the formation of homogeneous particles (150 µm) with tunable pore sizes of 100 s of nanometers to 350 s of microns. The process is versatile, and results indicate multi-scale pore size and hybrid pore connectivity are regulated through manipulation of the sacrificial agent. These structures are therefore promising for several multi-faceted healthcare applications e.g. targeted drug delivery, 3D cell culture, and real-time MRI imaging.

AC electrohydrodynamic Landau–Squire flows around a conducting nanotip

Utilizing the joint singular natures of electric field and hydrodynamic flow around a sharp nanotip, we report new electrohydrodynamic Landau–Squire-type flows under the actions of alternating current (AC) electric fields, markedly different from the classical Landau–Squire flow generated by pump discharge using nanotubes or nanopores. Making use of the locally diverging electric field prevailing near the conical tip, we are able to generate a diversity of AC electrohydrodynamic flows with the signature of a 1/r point-force-like decay at distance r from the tip. Specifically, we find AC electrothermal jet and Faradaic streaming out of the tip at applied frequencies in the MHz and 102 Hz regimes, respectively. Yet at intermediate frequencies of 1–100 kHz, the jet flow can be reversed to an AC electro-osmotic impinging flow. The characteristics of these AC jet flows are very distinct from AC flows over planar electrodes. For the AC electrothermal jet, we observe experimentally that its speed varies with the driving voltage V as V3, in contrast to the common V4 dependence according to the classical theory reported by Ramos et al. (J. Phys. D: Appl. Phys, vol. 31, 1998, pp. 2338–2353). Additionally, the flow speed does not increase with the solution conductivity as commonly thought. These experimental findings can be rationalized by means of local Joule heating and double layer charging mechanisms in such a way that the nanotip actually becomes a local hotspot charged with heated tangential currents. The measured speed of the AC Faradaic streaming is found to vary as V3/2 logV, which can be interpreted by the local Faradaic leakage in balance with tangential conduction. These unusual flow characteristics signify that a conical electrode geometry may fundamentally alter the features of AC electrohydrodynamic flows. Such peculiar electrohydrodynamic flows may also provide new avenues for expediting molecular sensing or sample transport in prevalent electrochemical or microfluidic applications.

Reconfigurable artificial microswimmers with internal feedback

Self-propelling microparticles are often proposed as synthetic models for biological microswimmers, yet they lack the internally regulated adaptation of their biological counterparts. Conversely, adaptation can be encoded in larger-scale soft-robotic devices but remains elusive to transfer to the colloidal scale. Here, we create responsive microswimmers, powered by electro-hydrodynamic flows, which can adapt their motility via internal reconfiguration. Using sequential capillary assembly, we fabricate deterministic colloidal clusters comprising soft thermo-responsive microgels and light-absorbing particles. Light absorption induces preferential local heating and triggers the volume phase transition of the microgels, leading to an adaptation of the clusters’ motility, which is orthogonal to their propulsion scheme. We rationalize this response via the coupling between self-propulsion and variations of particle shape and dielectric properties upon heating. Harnessing such coupling allows for strategies to achieve local dynamical control with simple illumination patterns, revealing exciting opportunities for developing tactic active materials.