Search

The effect of electrode orientation in an electrohydrodynamic (EHD) gas pump inside a square channel is experimentally examined for its effective use in thermal management. The two-stage gas pump w...

An electrohydrodynamic (EHD) gas pump inside a square channel is experimentally evaluated for its effectiveness in flow delivery. The gas pump with three sets of electrode assemblies (separately wi...

In this work, a numerical model for the simulation of two-phase electrohydrodynamic (EHD) problems is proposed. It is characterized by a physically consistent treatment of surface tension as well as a jump in the electric material properties. The formulation is based on a finite element method enriched with special shape functions, capable of accurate capturing discontinuities both in the fluid pressure and the gradient of the electric potential. Phase interface is, thus, represented as a zero-thickness boundary. The proposed methodology allows modeling the electric force as an interfacial one, strictly abiding with the physics. The approach is tested using the droplet deformation benchmarks. Moreover, application of the method to study a three-dimensional (3D) case, not characterized by symmetry of revolution, is shown. The proposed methodology defines a basis for an enriched finite element method for a wide range of EHD problems. The authors acknowledge the financial support of the Ministerio de Ciencia, Innovaci on e Universidades of Spain via the “Severo Ochoa Programme” for Centres of Excellence in R&D (Referece No. CEX2018-000797-S) given to the International Centre for Numerical Methods in Engineering (CIMNE). The work of C. Narvaez-Mu~noz was supported by the “Severo Ochoa Ph.D. Scholarship” Reference No. PRE2020-096632. Parts of this work were done in the framework of DIDRO project (Toward establishing a Digital twin for manufacturing via drop-on-demand inkjet printing. Proyectos Estrat egicos Orientados a la Transici on Ecol ogica y a la Transici on Digital. Reference No. TED2921-130471B-I00) supported by the Ministerio de Ciencia, Innovaci on e Universidades of Spain. M. Hashemi acknowledges the funding received from European Union’s Horizon 2020 Research and Innovation Programme (European High-Performance Computing Joint Undertaking Grant Agreement No. 955558) as part of EFLOWS4HPC project. P. Ryzhakov and J. Pons-Prats are Serra Hunter fellows.

Printed flexible electrical heaters with excellent heating performance and mechanical durability are highly desirable for deicing and wearable thermotherapy devices. However, the performance of the conventional heaters is stilled limited by low-resolution fabrication methods when applied in high-precision heating in desirable regions. Moreover, the poor conductivity and mechanical stability of the ink also increase the power consumption. Herein, a high-resolution (45 μm) heater with low power consumption was fabricated by a facile electrohydrodynamic (EHD) printing method. A highly printable and stable hybrid conductive ink was obtained by doping PANI nanoparticles into silver flake/thermoplastic polyurethane (TPU) composite. After adding 0.5 wt% PANI nanoparticles into 40 wt% silver flake/TPU composite and low temperature sintering (80 °C), the bulk resistivity decreased from 96.03 × 10−5 Ω·m to 1.26 × 10−5 Ω·m. Thanks to the ultrahigh conductivity of the ink, the EHD printed flexible heater shows high saturation temperature (127.0 °C) under low applied voltage (2 V), wide heating range (33.9 °C~127.0 °C) under a small range of driving voltages (0.5 V ~ 2.0 V), the rapid response time (20 s) and excellent repeatability during 10-time cyclic heating-cooling possess. Furthermore, the printed flexible heaters exhibit great flexibility and durability. The resistance of the heater remains stable after 3000 outer bending cycles with a radius of 0.5 mm, indicating outstanding mechanical stability. Moreover, the heater can be attached to the human body, showing the potential for emerging wearable electronic applications.

Surface dielectric barrier discharges (SDBDs) at atmospheric pressure can be achieved by direct air discharge. It can produce reactive species without expensive vacuum chambers as well as generate the ionic wind to control the airflow or accelerate actuators. For the purpose of extending the discharge zone to satisfy the larger area treatment of materiel or enhance the electrohydrodynamic (EHD) force, an efficient way is to arrange several single modules of SDBDs in series. In this article, several kinds of SDBDs with different geometries and voltage polarities of multi-electrode are studied to investigate the discharge dynamical characteristics, such as the discharge uniformity, the streamer length, the number of streamers, and the EHD force during the discharge. The spatial distribution of the electric field (or potential gradient) established by the electrode array is crucial to determine the discharge behaviors.

The paper demonstrates a dual-cell smart window based on dichroic-dyed-doped liquid crystal that is switchable among transparent, dark and scattering states. The dual-cell forms the crossed-polaris...

Electrohydrodynamic (EHD) jet printing enables rapid prototyping high-resolution and low-cost lines with width of micrometer or even nanometer. However, EHD printing always suffers from nozzle clogging when the nozzle inner-diameter decrease to micro-scale. Thus fabrication of low cost nozzles becomes significantly important. In this work, 50 μm wide and 12.5 μm deep PMMA (Polymethyl Methacrylate) nozzles were fabricated without using traditional expensive glass capillary pulling approach. To replicate PMMA nozzle with high precision, the embossing condition was optimized according to replication precision, the deformation rate, and maximum stress. To nearly fully bond PMMA nozzle with intact PMMA microchannel, the bonding condition was optimized according the bonding rate and dimension loss of PMMA microchannel. The availability of the fabricated PMMA nozzle was finally verified by EHD printing experiments.

I would like to express my respect to Dzyaloshinski—co-author Method of Quantum Field Theory in Statistical Physics (Englewood Cliffs: Prentice Hall, 1963) written by Abrikosov, Gorkov, and Dzyaloshinski. This book had become the table book for young theoreticians. He is co-author of scientific discovery of magnetoelectric effect—classical result of the modern physics. Today the terms vector of Dzyaloshinskii and Dzyaloshinskii–Moriya interaction have become generally accepted. He always supports new ideas and search of new materials, for example, liquid crystals. The last ones have unique properties, for example they have unusual changes in specific phase diagrams and in the structures of boundaries of ferroelectric domains, which are bound to defects and impurities in liquid-crystalline lattices. It is shown here that besides the “true” electrohydrodynamic (EHD) and flexoelectric (FE) instabilities with endless y-stripes on xy-plane (EHD), at observance of necessary conditions, and x-stripes (FE), there are short EHD-formations and in the plane system which lead to a peculiar phase diagram in the oscillating electric field.

In this study, the heat transfer by forced convection over a circular cylinder was studied numerically in the presence of an electric field and the effects of the installation angle of discharge el...

Microfluidic chips made by traditional materials (glass and silicon) are still important for fluorescence tests, biocompatible experiments, and high temperature applications. However, the majority of the present bonding methods suffer from ultra-clean requirement, complicated fabrication process, and low production efficiency. In the present work, an Electrohydrodynamic printing assist bonding method was proposed. By this method, the ultraviolet-cured-glue dots were printed onto the silicon substrate, and then the patterned glass and silicon substrate can be bonded together at room temperature. The influence of printing condition (nozzle inner-diameter, applied voltage, printing height, and flow rate) on the diameter of printed dot was analyzed by experiments. By the optimized printing condition, the glass-silicon microfluidic chip can be well bonded. The bonding strength and leakage test demonstrated the high bonding quality of the microfluidic chip (bonding strength of 28 MPa and leakage pressure of 3.5 MPa).

This paper is focused on the research of airflow generating through the use of high-voltage electrohydrodynamic devices. For this purpose, the authors built several electrohydrodynamic airflow generators with one point electrode and one tube electrode of varying dimensions and compared their efficiency in generating the airflow in order to find an optimal design. The character of the flow was also analyzed with the help of particle image velocimetry, and velocity vector maps and velocity profile were acquired. In addition, a possible practical cooling application was proposed and realized with positive results. Lastly, the products present in the generated airflow were tested for ozone and nitrogen oxides, which could have detrimental effects on human health and material integrity. In both cases, the concentration has been found to be below permissible limits.

Electrohydrodynamic (EHD) printing has great potential to be used in the field of electronic industry, precision optics and biological engineering. However, it is still a challenge to achieve a high scaling ratio for line patterns production by EHD jet printing technique, which can effectively prevent the needles to be blocked. In this paper, the influence of the key process parameters on the line width of EHD jet printing was investigated. The printing needle with the inner diameter of 890 μm was used, and the line with width of 3.02 μm was printed by optimizing the parameters. The scaling ratio between the needle inner diameter and the printed line width was up to 295 times. At the same time, the function model between the flow rate and printed line width of PVP/PEO composite was deduced. The error was found to be less than 1% by the verification of experiments. By utilizing this model, the printed line width can be predicted by controlling the flow rate, which will greatly improve the work efficiency. By controlling the flow rate, the goal of printing on demand can be realized. The proposed method and model can provide support for the fabrication of micro‐nano scale structure.

In recent years, there has been an increasing interest in the research in soft actuators that exhibit complex programmable deformations. Soft electrothermal actuators use electricity as the stimulus to generate heat, and the mismatch between the thermal expansions of the two structural layers causes the actuator to bend. Complex programmable deformations of soft electrothermal actuators are difficult due to the limitations of the conventional fabrication methods. In this article, we report a new approach to fabricate soft electrothermal actuators, in which the resistive heater of the electrothermal actuator is directly printed using electrohydrodynamic (EHD) printing. The direct patterning capabilities of EHD printing allow the free-form design of the heater. By changing the design of the heating pattern on the actuator, different heat distributions can be achieved and utilized to realize complex programmable deformations, including uniform bending, customized bending, folding, and twisting. Finite element analysis (FEA) was used to validate the thermal distribution and deformation for different actuator designs. Lastly, several integrated demonstrations are presented, including complex structures obtained from folding, a two-degree-of-freedom soft robotic arm, and soft walkers.

In this work, systems with serial and alternating electrical connection of three electrohydrodynamic cells based on corona discharge in atmospheric pressure air were studied. Numerical simulations of the systems was conducted. The volume force acting in the drift regions of the multi-cascade system was calculated and a good agreement was obtained between experimental and theoretical data.

In this work, we demonstrated a smart window application that could control white light diffraction based on electrohydrodynamic instability. The device could switch between the transparent and dif...

This paper aims to present the simulation work and obtain optimized parameters for the development of drop-on-demand electrohydrodynamic jet (DoD E-Jet) to print control and stable micro-structures on a flexible insulating substrate. In this work, the novel comparison of three types of combination needle structures was developed based on the multiphase flow (liquid–air) technique, in order to achieve optimal printing conditions for a flexible PET substrate. According to simulation results, steel-quartz needle combination provides very unique compensations in the controllability and stability of E-Jet. Printing on a flexible substrate was challenging, but parameters used in simulation validate the possibilities for DoD E-Jet printing method. Optimize working parameters were achieved by the numerical simulation executed to generate developed and stable E-Jet morphology. In addition, various stable and uniform microscale droplets and structures were directly printed on a flexible polymer substrate with the help of collective impact of electrical force, viscous force, and internal pressure force throughout DoD E-Jet printing process. The results of numerical simulation and experimental work exhibited an excellent and promising E-Jet printing tool for flexible electronic systems.

Electrohydrodynamic (EHD) jet printing is a promising direct writing method to produce micro- and nano-scale dots due to its easy manipulation, high resolution, and low cost. The effect of printing conditions on the diameter of printed dots was widely studied by both experiments and simulations. However, positional precision is also important for EHD printing. There is no published work on numerical simulation for the trajectory analysis of the ejected droplet. In the present work, a finite element model was established to investigate the droplet trajectory. The influencing factors, such as nozzle size, nozzle angle, applied voltage, ink density, and charge number in one droplet, were considered during numerical simulation. The influence of influencing factors on the electric intensity, droplet speed, and deposition direction was analyzed. The proposed simulation model provides a useful tool to analyze the droplet formation process and optimize the printing parameters to improve the positional precision of EHD printing.

EHD (Electrohydrodynamic) printing is a photomask-free and direct writing technique for alternative fabrication of high resolution micro- and nano-structures with low cost and simple equipment. It is critical to understand the motion trajectory of the ejected droplet under electric filed so as to precisely control the printing process. However, there is no work which analyzed this issue previously. Thus in this paper, a finite element model was established to study the electric field distribution near the nozzle and analyze the motion trajectory of the ejected droplet during EHD printing. By using established finite element method, the electric field distribution in the nozzle and near the nozzle tip was investigated. The influence of printing distance and applied potential on the maximum electric field intensity was analyzed. Based on the electric field distribution study, the droplet trajectory analysis was carried out to confirm the motion velocity and direction of the ejected droplets.

The exploration and application of electrokinetic techniques in micro total analysis systems have become ubiquitous in recent years, and scientists are expanding the use of such techniques in areas where comparable active or passive methods are not as successful. In this work, for the first time, we utilize the concept of AC electroosmosis to design a phase-controlled field-effect micromixer that benefits from a three-finger sinusoidally shaped electrodes. Analogous to field-effect transistor devices, the principle of operation for the proposed micromixer is governed by the source-gate and source-drain voltage potentials that are modulated by introducing a phase lag between the driving electrodes. At an optimized flow rate and biasing scheme, we demonstrate that the source, gate, and drain voltage phase relations can be configured such that the micromixer switches from an unmixed state (phase shift of 0°) to a mixed state (phase shift of 180°). High mixing efficiencies beyond 90% was achieved at a volumetric flow rate of 4 µL/min corresponding to ~13.9 mm/s at optimized voltage excitation conditions. Finally, we employed the proposed micromixer for the synthesis of nanoscale lipid-based drug delivery vesicles through the process of electrohydrodynamic-mediated nanoprecipitation. The phase-controlled electrohydrodynamic mixing utilized for the nanoprecipitation technique proved that nanoparticles of improved monodispersity and concentration can be produced when mixing efficiency is enhanced by tuning the phase shifts between electrodes. Researchers in Canada have developed a device which can efficiently mix water in microfluidic devices. The dominance of viscosity in the microscale dynamics means that mixing only occurs via diffusion, making micromixing devices a key tool for working at these scales. Paresa Modarres and Maryam Tabrizian at McGill University engineered an active mixing device consisting of three sinusoidal electrodes running through the water channel. Applying a voltage across these electrodes generates an electric field which causes the water streams to mix. By varying the phase of the electrodes, they could control the strength of mixing, switching the micromixer between a mixing and non-mixing state. Finally, the duo tests the new device by using it to synthesize lipid nano-vesicles from a mixed solution of water and lipid-containing ethanol.

The generation of spatially localized, soliton-like hydrodynamic disturbances in microscale fluidic systems is an intriguing challenge. Herein, we introduce nonequilibrium solitons in nematic liquid crystals stimulated by an electric field. These dynamic solitons are robust as long as the electric field is maintained. Interestingly, their kinetic behaviours depend on the field condition—Tuning of the amplitude and frequency of the applied electric field alters the solitons to self-assemble into lattice ordering like physical particles or to command them to various dynamic states. Our key property to the realisation is the electrohydrodynamic instability due to the coupling between the fluid elasticity and the background convection. This paper describes a new mechanism for realising dynamic solitons in fluid systems on the basis of the electrohydrodynamic phenomena., Electric field induced convective instabilities in liquid crystal slabs can assume a localized shape. The authors show how to generate, manipulate and shepard these dissipative solitary excitations that do not require topological constraints for stabilization.

In this study, the visualization of the flow inside a Taylor cone formed during an electrohydrodynamic (EHD) spraying is conducted to analyze its stability among five liquid candidates. A micro-PIV with a micro-nozzle is used for the visualization, and the physical properties as well as measured values are utilized in the analysis. First, in forming the Taylor cone, the electrohydrodynamic force is required to be sufficiently large in order to overcome the surface tension of the liquid. Thus, among the five liquids tested here, three, in this case IPA, EtOH, and MeOH, form a Taylor cone due to the relatively low surface tension levels as compared to the others. Once electrohydrodynamic jetting occurs, the average and maximum velocities become monotonically proportional to the average current. As the velocities are the smallest in using IPA, the circulation flow becomes superior to the extrusive flow, which yields the stable formation of a Taylor cone. Also, low fluctuation of the instantaneous currents supports the stable formation of IPA. Consequently, IPA shows the most stable formation of the Taylor cone in our condition due to the lowest average current and low-level surface tension. Eventually, micro-PIV would be a good tool in choosing an optimal fluid for stable EHD spraying.

There is a compelling need for delicate nanomaterial design with various intricate functions and applications. Electrohydrodynamics applies electrostatic force to overcome the surface tension of a liquid jet, shrinking the jet through intrinsic jetting instability into submicron fibers or spheres, with versatility from a huge selection of materials, feasibility of extracellular matrix structure mimicry and multi-compartmentalization for tissue engineering and drug delivery. The process typically involves the collection and drying of fibers at a solid substrate, but the introduction of a liquid phase collection by replacing the solid collector with a coagulation bath can introduce a variety of new opportunities for both chemical and physical functionalizations in one single step. The so-called wet electrohydrodynamics is an emerging technique that enables a facile, homogeneous functionalization of the intrinsic large surface area of the submicron fibers/spheres. With a thorough literature sweep, we herein highlight the three main engineering features integrated through the single step wet electrospinning process in terms of creating functional biomaterials: (i) The fabrication of 3D macrostructures, (ii) in situ chemical functionalization, and (iii) tunable nano-topography. Through an emerging technique, wet electrohydrodynamics has demonstrated a great potential in interdisciplinary research for the development of functional 3D interfaces and materials with pertinent applications in all fields where secondary structured, functional surface is desired. Among these, engineered biomaterials bridging materials science with biology have already shown particular potential. [Figure not available: see fulltext.].

Electrohydrodynamic instabilities (EHDI) in liquid crystals form uniform and continuously variable diffractive structures when subject to certain material and geometry determined conditions. A one-dimensional grating is one such diffractive structure, where the refractive index changes periodically in a direction parallel to the initial liquid crystal director. The period of this structure has been shown previously to vary continuously between the values of the cell gap and half-cell gap approximately, allowing continuous angular modulation of optical beams but with a limited angular range. In this work, the lower pitch limit is shown to also be governed in part by the ratio of the splay and bend elastic constants (k11/k33) of the liquid crystal. A host nematic liquid crystal with standard elastic constant ratios (k11/k335) than for those previously used in literature EHDI studies. The EHDI gratings formed in this new mixture exhibit pitch lengths significantly below half-cell gap, allowing up to 50% wider angle continuous steering of light. This improves the potential for application in beamsteering and diffractive optical devices.

This study deals with the use of the electrohydrodynamic effects in flat miniature heat pipes (FMHP). A prototype is conceived and modeled. The equations of the model are developed on the basis of ...

Electrohydrodynamics is among the most promising techniques for manipulating liquids in microsystems. The electric stress actuates, generates, and coalesces droplets of small sizes; it also accelerates, focuses, and controls the motion of fine jets. In this review, the current understanding of dynamic regimes of electrically driven drops and jets in multiphase microsystems is summarized. The experimental description and underlying mechanism of force interplay and instabilities are discussed. Conditions for controlled transitions among different regimes are also provided. Emerging new phenomena either due to special interfacial properties or geometric confinement are emphasized, and simple scaling arguments proposed in the literature are introduced. The review provides useful perspectives for investigations involving electrically driven droplets and jets.

Electrohydrodynamic instability in liquid crystals is investigated for smart window applications. By comparing different species of ions doped in liquid crystals, the positive charge of the organic part of the salt is the origin of the generation of vortexes and leads to the activation of electrohydrodynamic instability. By replacing conventional electrolytes with zwitterions, compared to electrolyte-doped liquid crystals, zwitterion-doped liquid crystals have a broader range of optimal frequency from 10 Hz to 5 kHz. The devices can be switched between the transparent state and the light scattering state for thousands of times without showing any fatigue. A scattering colour is obtained by incorporating a dichroic dye in zwitterion-doped liquid crystal. A patterned device is designed and shows a localised light scattering and colour effect in the presence of an electric field. This strategy will make electrohydrodynamic instability possible to enrich the applications across smart windows, projection screens and information displays.

For flow control, plasma actuators have the advantages of no moving parts. An experimental study was carried out to generate vortexes using a microplasma actuator. Also, a 3-D numerical simulation code was developed to calculate the flow generated by the microplasma actuator. The numerical simulation used the Suzen–Huang model coupled with Navier–Stokes equations. Our microplasma actuator has a thin dielectric layer with a thickness of $25~\mu \text{m}$ between the grounded and high-voltage energized electrodes, which enables to drive our device at less than 1 kV. The high-voltage and grounded electrodes have both holes. In the series of experiments, an ac voltage with an amplitude 1 kV and a frequency of 20 kHz was applied to the electrode. The induced flow was visualized using an Nd: YVO4 532-nm laser, and the flow velocity was measured using the particle tracking velocimetry (PTV) method. Incense smoke was utilized as a tracer particle. The electrohydrodynamic (EHD) flow was induced around the holes of high-voltage electrode, thus vortexes appeared above these holes. In order to study the basic phenomena of the flow, one and four holes were isolated from the electrode; thus, the phenomena could be observed in a simplified version of the electrode. The 3-D numerical simulation code showed similar results both in values and flow configuration compared with the experimental results.

In this study, the pulse phenomenon is found in the electrohydrodynamic (EHD) process using an ionic liquid, which is similar to other liquids, but there are significant differences in pulse characteristics. The frequency, waveform, charge, and images of the pulse in the EHD process with the ionic liquid are studied experimentally. The results show that the pulse process is divided into two stages: a low-frequency pulse stage and a high-frequency pulse stage. We propose that the pulse is caused by the air corona discharge and the frequency is influenced by the anchoring radius of liquid oscillation in the electric field. The different anchoring radii of the two stages are the main reason for the obvious difference in the frequency. The main difference between the ionic liquid and other solutions is the conductivity and the surface tension coefficient, which is the cause of corona discharge rather than electrospray.

The interface between two immiscible fluids can become unstable under the effect of an imposed tangential electric field along with a stagnation point flow. This canonical situation, which arises in a wide range of electrohydrodynamic systems including at the equator of electrified droplets, can result in unstable interface deflections where the perturbed interface gets drawn along the extensional axis of the flow while experiencing strong charge build-up. Here, we present analytical and numerical analyses of the stability of a planar interface separating two immiscible fluid layers subject to a tangential electric field and a stagnation point flow. The interfacial charge dynamics is captured by a conservation equation accounting for Ohmic conduction, advection by the flow and finite charge relaxation. Using this model, we perform a local linear stability analysis in the vicinity of the stagnation point to study the behaviour of the system in terms of the relevant dimensionless groups of the problem. The local theory is complemented with a numerical normal-mode linear stability analysis based on the full system of equations and boundary conditions using the boundary element method. Our analysis demonstrates the subtle interplay of charge convection and conduction in the dynamics of the system, which oppose one another in the dominant unstable eigenmode. Finally, numerical simulations of the full nonlinear problem demonstrate how the coupling of flow and interfacial charge dynamics can give rise to nonlinear phenomena such as tip formation and the growth of charge density shocks.

Numerical and experimental papers on the enhancement of evaporation from liquid surfaces due to the electrohydrodynamic flow are reviewed in this work. Impressive advances have been obtained during the last two decades in the augmentation of evaporation from liquid surfaces due to electrohydrodynamic flow, especially through numerical simulations. This flow is generated by the electric corona discharge in ambient gas, which is produced by the application of a high dc voltage to a sharp electrode. In all studies, sharp electrodes were placed over the liquid surface and enhancement in evaporation rate was observed. Different water models were proposed in the numerical simulations, but only the conducting models are physically correct.

Various enhancement techniques are proposed in the literature to alleviate heat transfer issues arising from the low thermal conductivity of the phase change materials (PCM) in latent heat thermal energy storage systems (LHTESS). The identified techniques include employment of fins, insertion of metal structures, addition of high conductivity micro/nanoparticles, micro-encapsulation, macro-encapsulation and cascaded PCMs arrangement. However, these conventional techniques tend to reduce the storage capacity as they generally add additional components/materials into the storage medium. On the other hand, if techniques such as direct contact heat exchange, ultrasonic vibration, electrohydrodynamics and movable PCM are employed, the storage volume would remain unaffected. Hence, the said techniques seem to have gained importance in PCM research in recent times. Although several review papers elaborating conventional techniques are available, none can be found on the aforementioned alternative class. Driven by the current scenario, this review paper intends to summarize past research on alternative heat transfer enhancement techniques employed for LHTESS. The critical analysis of the potential of each technique in enhancing the phase change heat transfer rate and their practical applicability are presented. Further, the present review evaluates relative merits/demerits and challenges/issues/limitations of these techniques to provide guidelines for future research.

Drops subjected to electric fields can deform into singular shapes exhibiting apparent sharp tips. At high field strengths, a perfectly conducting drop surrounded by a perfectly insulating exterior fluid deforms into a prolate-shaped drop with conical ends and can exist in hydrostatic equilibrium. On the conical ends, capillary stress, which is due to the out-of-plane curvature and is singular, balances electric normal stress which is also singular. If the two phases are not perfect conductors/insulators but are both leaky dielectrics and the drop is much more conducting and viscous than the exterior, electric tangential stress disrupts the hydrostatic force balance and leads to jet emission from the cone's apex. If, however, the physical situation is inverted so that a weakly conducting, slightly viscous drop is immersed in a highly conducting, more viscous exterior, the drop deforms into an oblate lens-like profile before eventually becoming unstable. In experiments, the equator of a lenticular drop superficially resembles a wedge prior to instability. Such a drop disintegrates by equatorial streaming by ejecting a thin liquid sheet from its equator. We show theoretically by performing a local analysis that a lenticular drop's equatorial profile can be a wedge only if an approximate form of the surface charge transport equation – continuity of normal current condition – is used. Moreover, we demonstrate via numerical simulation that such wedge-shaped drops do not become unstable and therefore cannot emit equatorial sheets. We then show by transient simulations how equatorial streaming can occur when charge transport along the interface is analysed without approximation.

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.

The breakup process of a charged, leaky-dielectric jet subjected to an axial perturbation is computationally analysed from the perspectives of linear and nonlinear dynamics using the arbitrary Lagrangian–Eulerian technique. The linear dynamics of the leaky-dielectric jet is quantitatively predicted by the dispersion relation from the linear stability analysis. Regarding the nonlinear dynamics, it is found that the charge relaxation is responsible for the radial compression of satellite droplets, which is validated by experiments. Two types of charge relaxations, namely, ohmic conduction and surface charge convection, define the pinching process into three breakup modes, i.e. ligament pinching, end pinching and transition pinching. In the ligament-pinching mode, the ohmic conduction dominates the jet breakup since the charge relaxes to the jet ligament instantaneously. In contrast, the surface charge convection takes effect in the end-pinching mode since the surface charge is convected to the jet end via fluid flow. When the ohmic conduction is comparable to the surface charge convection, the breakup occurs simultaneously at the end and the ligament. Finally, the influences of the perturbed wavenumber, the electric field intensity and the viscosity on the breakup mode and the local dynamics at pinch-off are comprehensively discussed.

Colloidal particles like polystyrene beads and metallic micro and nanoparticles are known to assemble in crystal-like structures near an electrode surface under both DC and AC electric fields. Various studies have shown that this self-assembly is governed by a balance between an attractive electrohydrodynamic (EHD) force and an induced dipole-dipole repulsion (Trau et al., 1997). The EHD force originates from electrolyte flow caused by interaction between the electric field and the polarized double layers of both the particles and the electrode surface. The particles are found to either aggregate or repel from each other on application of electric field depending on the mobility of the ions in the electrolyte (Woehl et al., 2014). The particle motion in the electrode plane is studied well under various conditions however, not as many references are available in the literature that discuss the effects of the AC electric field on their out-of-plane motion, especially at high frequencies (>10 kHz). Haughey and Earnshaw (1998), and Fagan et al. (2005) have studied the particle motion perpendicular to the electrode plane and their average height from the electrode mostly in presence of DC or low frequency AC (10 kHz) electric field on the particles’ motion perpendicular to the electrode plane.

Colloids in low-frequency (

Anisotropic particles have been widely used to make micro/nanomotors that convert chemical, ultrasonic, electrical, or magnetic energy into mechanical energy. The moving directions of most colloidal motors are, however, difficult to control. For example, asymmetric dimers with two lobes of different sizes, ζ-potential, or chemical composition have shown rich propulsion behaviors under alternating current (AC) electric fields due to unbalanced electrohydrodynamic flow. While they always propel in a direction perpendicular to the applied electric field, their moving directions along the substrate are hard to control, limiting their applications for cargo delivery. Inspired by two separate engine and steering wheel systems in automobiles, we use orthogonally applied AC electric field and direct current (DC) magnetic field to control the dimer's speed and direction independently. To this end, we first synthesize magnetic dimers by coating dopamine-functionalized nanoparticles on geometrically asymmetric polystyrene dimers. We further characterize their static and dynamic susceptibilities by measuring the hysteresis diagram and rotation speed experimentally and comparing them with theoretical predictions. The synthesized dimers align their long axes quickly with a planar DC magnetic field, allowing us to control the particles' orientation accurately. The propulsion speed of the dimers, on the other hand, is tunable by an AC electric field applied perpendicularly to the substrate. As a result, we can direct the particle's motion with predesigned trajectories of complex shapes. Our bulk-synthesis approach has the potential to make other types of magnetically anisotropic particles. And the combination of electric and magnetic fields will help pave the way for the assembly of magnetically anisotropic particles into complex structures.

Low-power trapping of nanoscale objects can be achieved by using the enhanced fields near plasmonic nanoantennas. Unfortunately, in this approach the trap site is limited to the position of the plasmonic hotspots and continuous dynamic manipulation is not feasible. Here, we report a low-frequency electrothermoplasmonic tweezer (LFET) that provides low-power, high-stability and continuous dynamic manipulation of a single nanodiamond. LFET harnesses the combined action of the laser illumination of a plasmonic nanopillar antenna array and low-frequency alternating current (ac) electric field to establish an electrohydrodynamic potential capable of the stable trapping and dynamic manipulation of single nanodiamonds. We experimentally demonstrate the fast transport, trapping, and dynamic manipulation of a single nanodiamond using a low-frequency ac field below 5 kHz and low-laser power of 1 mW. This nanotweezer platform for nanodiamond manipulation holds promise for the scalable assembly of single photon sources for quantum information processing and low noise quantum sensors.

The threshold structural instability arising in the thin layer of a nematic liquid crystal (nematic) along the surface of an electrode during the flow of a weak direct injection current is described. Local, limited in length Lу, nuclei (precursors) of electrohydrodynamic and flexoelectric instabilities are assumed to be in this thin layer. In the case of electrohydrodynamic instability, such precursors have been called “bullets” (solitons) because of their specific appearance, and their length Lу is a measure of local perturbation of the orientational structure of a nematic. In the case of flexoelectric instability, pieces Lу are formed by an irregular system of short polarized flexoelectric domains. Such an instability corresponds to a system consisting of groups of stripes, which are characterized by the opposite motion of “bullets” along these stripes and the average velocity of this movement.

The electrohydrodynamics of a weakly conducting buoyant drop under the combined influence of gravity and a uniform electric field is studied computationally, focusing on the inertia-dominated regime. Numerical simulations are performed for both perfectly dielectric and leaky dielectric drops over a wide range of dimensionless parameters to explore the interplay of fluid inertia and electrical stress to govern the drop shape and charge convection. For perfectly dielectric drops, the fluid inertia alters the drop shape and the deformation behaviour of the drop follows a non-monotonic path. The drop shape at steady state exhibits the transition from oblate to prolate shape on increasing the electric field strength, in sharp contrast to the cases concerning the Stokes flow regime. Similar behaviour is also obtained for leaky dielectric drops for certain fluid properties. For leaky dielectric drops, the fluid inertia also affects the convective transport of charges at the drop surface and thereby alters the drop dynamics. Unlike the Stokes flow regime, where surface charge convection has little effect on the settling speed, the same modifies the drop settling speed quite significantly in the finite inertial regime depending on the combination of electrical conductivity ratio and permittivity ratio. For oblate drops at low capillary number, charge convection alters drop shape, while keeping the nature of deformation unaltered. However, for relatively large capillary number, the oblate drop transforms into a dimpled shape due to charge convection. For all cases, an interesting fact is noticed that under the combined action of electric and inertial forces, the resultant deformation is less than the summation of the deformations caused by individual effects, when inertial effects are strong. These results are likely to provide deep insights into the interplay of various nonlinearities towards altering electrohydrodynamic settling of drops and bubbles.

A study was made of the electrohydrodynamic (EHD) effect on the water evaporation rate of a channel flow with a needle-arrayed electrode system using both numerical and experimental methods. The lo...

This paper extends the analysis of solute dispersion in electrohydrodynamic flows to the case of band broadening in polyelectrolyte-grafted (soft) capillaries by accounting for the effects of ion partitioning, irreversible catalytic reaction and pulsatile flow actuation. In the Debye–Hückel limit, we present the benchmark solutions of electric potential and velocity distribution pertinent to steady and oscillatory mixed electroosmotic–pressure-driven flows in soft capillaries. Afterwards, the mathematical models of band broadening based on the Taylor–Aris theory and generalized dispersion method are presented to investigate the late-time asymptotic state and all-time evolution of hydrodynamic dispersion, respectively. Also, to determine the heterogeneous dispersion behaviour of solute through all spatiotemporal stages and to relax the constraint of small zeta potentials, a full-scale numerical simulation of time-dependent solute transport in soft capillaries is presented by employing the second-order-accurate finite difference method. Then, by inspecting the dispersion of passive tracer particles in Poiseuille flows, we examine the accuracy of two analytical approaches against the simulation results of a custom-built numerical algorithm. Our findings from hydrodynamic dispersion in Poiseuille flows reveal that, compared to rigid capillaries, more time is required to approach the longitudinal normality and transverse uniformity of injected solute in soft capillaries. For the case of dispersion in mixed electrohydrodynamic flows, it is found that the characteristics of the soft interface, including the thickness, permittivity, fixed charge density and friction coefficient of the polymer coating layer, play a significant role in determining the Taylor diffusion coefficient, advection speed and dispersion rate of solutes in soft capillaries.

In this study, carbon-black (CB) conductive electrodes were successfully printed using the high-resolution electrohydrodynamic (EHD) jet printing technique. The wrapping of CB bundles with a polymeric surfactant, Triton X-100 (TX-100), enabled the CB/TX-100 composites to well disperse in ethanol/deionized water for use in the preparation of conductive inks for EHD jet printing. By adjusting the voltage and operation distance, the applied electrostatic and gravity forces to the loaded CB/TX-100 inks overcame the fluid forces (viscosity and surface tension) to elongate the droplet and provide continuous jet lines, where the ink widths were smaller than the diameter of the nozzle. The EHD-printed CB/TX-100 in the stable cone-jet mode formed conductive lines and various pattern shapes. These conductive lines were utilized as source and drain electrodes of organic field-effect transistors (OFETs) with solution-processed organic semiconductors. The OFET with printed CB/TX-100 electrodes exhibited better electrical performances, including a higher saturation mobility and smaller hysteresis, than those of the reference OFET with Au electrodes.

Inorganic materials face enormous challenges in designing and processing flexible devices that can be stretchable, crimped and folded. Organic materials have attracted wide attention because of their flexible properties and advantages in manufacturing flexible electronic devices. In this study, the flexible organic microcircuit pattern of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) material was fabricated by using the near-field electrohydrodynamic direct -writing method, the experimental results show that the bending deformation of the flexible substrate with different curvatures has no effect on the conductivity of the microcircuit pattern deposited on the flexible substrate, which has no effect on the performance of the flexible microcircuit pattern. In order to improve the conductivity of the flexible organic electronic microcircuit, the multi-layer microcircuit pattern was fabricated by using the near-field electrohydrodynamic direct-writing method. With the increase of the number of the direct-writing micro-pattern layer, the conductivity of the microcircuit patterns sintered at 105 °C for 10 min increase from 168.32 to 313.05 S/cm. Atomic force microscope was used to observe the morphology of the direct-writing microcircuit patterns. With the increase of the layer number of the microcircuit pattern, the internal density of the microcircuit pattern increases, and the microcircuit-pattern morphology becomes smoother. The experimental results show that the multi-layer direct- writing method can effectively improve the conductivity of the flexible organic electronic microcircuit pattern. This study provides a new method to fabricate the flexible organic microcircuit pattern with high conductivity by a non-contact and low-cost mode.

The mathematical theory of stability requires the analysis of the time evolution of arbitrary perturbations of the initial conditions in the system. However, in real systems arbitrary perturbations are possible only in thermodynamically non-equilibrium states. This paper deals with the surface stability of the liquid metal in electric field. The paper proposes the relevant theory, which differs from the Larmor–Tonks–Frenkel instability and experimentally confirmed by Serkov, et al. It is shown that the introduction of the dependence between the surface tension and curvature in the Larmor–Tonks–Frenkel instability changes the critical electric intensity by not over 5%. It is found that as a result of the Larmor–Tonks–Frenkel instability, the dependence between the critical electric intensity and the liquid metal temperature is (1 – T/T0)1/3, whereas in the proposed theory, this dependence is not observed.

This work reports a theoretical investigation of the effect of Joule heating and temperature-dependent zeta potential on the electroosmotic flow measurements in calorimetric flow sensing. Joule heating resulting from the applied electric field in electroosmotic flow increases the temperature of the liquid inside the sensor and, consequently, modifies the sensor performance. The model presented in this paper considers temperature dependence of the wall zeta potential on the sensor characteristics. Additionally, all liquid properties such as density, viscosity, relative permittivity, specific heat, thermal conductivity, and electrical conductivity are taken as temperature-dependent properties. A comparison between the characteristics of the modelled sensor in the presence and absence of Joule heating is presented. The effect of heater power on sensor characteristics is also discussed. Simulation results reveal that Joule heating and temperature dependence of zeta potential have a significant effect on the behaviour of calorimetric flow sensors, which must be considered when this type of sensor is used to measure electroosmotic flow. Temperature dependence of zeta potential, in particular, affected the velocity distribution inside the sensor considerably.

The electrohydrodynamic (EHD) effects on the flow field and forced convection heat transfer in a rectangular corrugated channel is numerically investigated in the present study. The numerical results are validated against experimental data for the smooth channel. The simulation results qualitatively agree with the experimental measurements. It is observed that in the presence of EHD in the rectangular corrugated channel, the thermal enhancement index at higher aspect ratios becomes better rather than the lower ones. Also, the application of the corona voltage, yields a stronger recirculation zone and causes a greater increment in the heat transfer rate.