Droplets In Electric Field Research Articles

Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

Electric Field-Induced Water Condensation Visualized by Vapor-Phase Transmission Electron Microscopy.

Understanding the nanoscale water condensation dynamics in strong electric fields is important for improving the atmospheric modeling of cloud dynamics and emerging technologies utilizing electric fields for direct air moisture capture. Here, we use vapor-phase transmission electron microscopy (VPTEM) to directly image nanoscale condensation dynamics of sessile water droplets in electric fields. VPTEM imaging of saturated water vapor stimulated condensation of sessile water nanodroplets that grew to a size of ∼500 nm before evaporating over a time scale of a minute. Simulations showed that electron beam charging of the silicon nitride microfluidic channel windows generated electric fields of ∼108 V/m, which depressed the water vapor pressure and effected rapid nucleation of nanosized liquid water droplets. A mass balance model showed that droplet growth was consistent with electric field-induced condensation, while droplet evaporation was consistent with radiolysis-induced evaporation via conversion of water to hydrogen gas. The model quantified several electron beam-sample interactions and vapor transport properties, showed that electron beam heating was insignificant, and demonstrated that literature values significantly underestimated radiolytic hydrogen production and overestimated water vapor diffusivity. This work demonstrates a method for investigating water condensation in strong electric fields and under supersaturated conditions, which is relevant to vapor-liquid equilibrium in the troposphere. While this work identifies several electron beam-sample interactions that impact condensation dynamics, quantification of these phenomena here is expected to enable delineating these artifacts from the physics of interest and accounting for them when imaging more complex vapor-liquid equilibrium phenomena with VPTEM.

The first micro thrust measurement of the ultrasonic electric propulsion system

The Ultrasonic Electric Propulsion system is mainly used in space missions within the limits of 50 kg, and employs capillary standing waves as emitters to extract charged droplets in electric field. Due to multi-physics field such as ultrasonic vibration, liquid mechanics and electrostatic field in the Ultrasonic Electric Propulsion system, its thrust prediction always faces a big challenge. In this work, the thrust of Ultrasonic Electric Propulsion system is first measured by a measurement platform, and the test results are 565.23 μN, 715.65 μN and 842.73 μN with an accuracy in the order of 10% at 5 kV, 7 kV and 9 kV, and a flow rate of 5 ml/h and ultrasonic vibration frequency of 120 kHz. The test results also indicate that the thrust value increases with extraction voltage in the same flow rate with propellant and vibration frequency. Besides, the vacuum emission plume of Ultrasonic Electric Propulsion system is first investigated by a high-speed imaging technique, which reveals different emission conditions at different time such as parabolic emission trajectory and varied plume angles.

Dielectrophoresis Response of Water-in-Oil-in-Water Double Emulsion Droplets with Singular or Dual Cores.

Microfluidic technologies have enabled generation of exquisite multiple emulsion droplets, which have been used in many fields, including single-cell assays, micro-sized chemical reactions, and material syntheses. Electrical controlling is an important technique for droplet manipulation in microfluidic systems, but the dielectrophoretic behaviors of multiple emulsion droplets in electrical fields are rarely studied. Here, we report on the dielectrophoresis response of double emulsion droplets in AC electric fields in microfluidic channel. A core-shell model is utilized for analyzing the polarization of droplet interfaces and the overall dielectrophoresis (DEP) force. The water-in-oil-in-water droplets, generated by glass capillary devices, experience negative DEP at low field frequency. At high frequency, however, the polarity of DEP is tunable by adjusting droplet shell thickness or core conductivity. Then, the behavior of droplets with two inner cores is investigated, where the droplets undergo rotation before being repelled or attracted by the strong field area. This work should benefit a wide range of applications that require manipulation of double emulsion droplets by electric fields.

Investigation of the influence of low frequency electric fields on model water-in-oil emulsions

The article describes the mechanisms of coagulation and coalescence of emulsion droplets in electric fields, and also defines the frequency ranges at which coagulation and coalescence of emulsion droplets occurs, and the establishment times of these processes. The experiments revealed the tendency to increase the intensity of the studied processes with increasing field strength and salinity of the aqueous phase.

Evaporating hydrophilic and superhydrophobic droplets in electric fields

Abstracts Evaporating water droplets on a heated substrate are investigated in this work. Specifically, the influences of electric fields are studied in the context of the heat flux distribution beneath the droplets as well as the droplet mechanics and resulting shapes and forces. To facilitate a deeper understanding of the problem, both hydrophilic and superhydrophobic droplets are considered for an entire evaporation period with and without electric field effects. Both wetting scenarios show that the net radial directed electric force is directed inward, resulting in a compressive force which influences the droplet shape in such a way that it appears elongated. Conversely, the net vertically directed electric force is determined to be downwardly directed for hydrophilic droplets, pressing the droplet to the surface, whereas it is upwardly directed for the superhydrophobic droplets, representing a lifting force. With regard to the heat transfer to the droplets, only a pronounced electric field effect was observed for the superhydrophobic droplet. For all droplets, the contact line density, representing the ratio of the contact line perimeter to the total base area of the droplet, is determined to be a parameter that unifies the average heat flux from the heater to the droplets. This suggests that the heat transfer to the base of the droplet in the presence of an electric field is dominated by the electric fields influence, or lack thereof, on the contact line density.

Migration behaviors of leaky dielectric droplets with electric and hydrodynamic forces.

The external electric field enables separation and transport of droplets effectively in microfluidic devices. Herein, a volume-of-fluid (VOF) + level-set (LS) + smoothed physical parameters (SPP) method associated with the dynamically adaptive grid technique is extended to simulate three-dimensional leaky dielectric droplets in the electric field. The effects of electric and hydrodynamic forces on droplet behaviors are investigated. It is demonstrated that the electric force could act toward the outside or inside of a droplet and produce different droplet deformations. For the dielectrophoretic migration of droplets in the nonuniform electric field, the electric force has a dominant effect. It is found that when the electric conductivity ratio is greater than 1, an unbalanced electric force toward a stronger electric field is generated, bringing about the migration toward a stronger electric field. In contrast, when the electric conductivity ratio is smaller than 1, the unbalanced electric force direction is reversed and the droplet migrates toward a weaker electric field. The hydrodynamic force produces little promotion or hindrance to droplet migration. A greater permittivity ratio usually produces greater electric force and migration velocity. The droplet migrates along one direction in a symmetric nonuniform electric field but tends to migrate along the normal direction of electric potential profiles in an asymmetric nonuniform electric field.

Sharp numerical simulation of incompressible two-phase flows

We present a numerical method for simulating incompressible immiscible fluids, in two and three spatial dimensions. It is constructed as a modified pressure correction projection method on adaptive non-graded Oc/Quadtree Cartesian grids, using the level-set framework to capture the moving interface between the two fluids. The sharp treatment of the interface position, of the fluid parameter discontinuities, and of the interfacial jump conditions ensures convergence in the L∞-norm. Using a novel construction for the pressure guess, we are able to alleviate the standard time step restriction incurred by capillary forces. The solver is validated numerically and employed to simulate the dynamics of physically relevant problems such as rising bubbles and viscous droplets in electric fields.

High-throughput multiplexed fluorescence-activated droplet sorting

Fluorescence-activated droplet sorting (FADS) is one of the most important features provided by droplet-based microfluidics. However, to date, it does not allow to compete with the high-throughput multiplexed sorting capabilities offered by flow cytometery. Here, we demonstrate the use of a dielectrophoretic-based FADS, allowing to sort up to five different droplet populations simultaneously. Our system provides means to select droplets of different phenotypes in a single experimental run to separate initially heterogeneous populations. Our experimental results are rationalized with the help of a numerical model of the actuation of droplets in electric fields providing guidelines for the prediction of sorting designs for upscaled or downscaled microsystems.

Electrostatic-induced coalescing-jumping droplets on nanostructured superhydrophobic surfaces

Coalescing-jumping droplets from the condensation of water vapor on a non-wetting surface return to the substrate due to resistance forces. While some can coalesce with neighboring droplets and jump again, some adhere to the surface and become larger, leading to progressive flooding, limiting heat transfer performance. To address these issues, an electric field is utilized. This study investigates the jumping height, the droplet charge, the jumping angle, the gravitational force, the drag force, the inertia force and the electrostatic force of coalescing-jumping droplets in electric fields through experiment and mathematical models. The results show that an electric field can enhance the jumping height due to a significant increase in the electrostatic force. With the applied electric field, the maximum jumping height is over three times higher than those without. Additionally, the study reports the intersection point at the jumping droplet radius of 35 μm separating jumping droplet motion into two regimes; the drag-force-dominated regime where the small-sized droplets can jump and reach the top plate, and the gravitational-force-dominated regime where the larger droplets can jump, but return to the substrate. The other intersection point is between the gravitational force and the inertia force showing a decrease in the influence of the inertia force with a greater applied electric field. Moreover, it is also found that the average charge of the droplets is relatively constant in all pressure conditions and applied electric fields. The results of these findings can further advance knowledge on the enhancement of heat transfer and can be applied to several applications including self-cleaning, smart windows, thermal diodes and condensation heat transfer enhancement.

Numerical study of non-Newtonian droplets electrocoalescence

In the present study, characteristics of leaky dielectric droplets in electric fields are investigated numerically. In order to validate the numerical method, deformation of a leaky dielectric droplet in an electric field is studied and the numerical results are compared to Taylor theoretical model. After that, the effects of electrical parameters such as electric permittivity, conductivity, electric field strength, and physical parameters such as initial distance and size of droplets on the electrocoalescence are examined. Moreover, behaviors of non-Newtonian droplets in the electric field are surveyed to elucidate the underlying physics. According to the results, pseudoplastic droplets have more deformations in an electric field than Newtonian and dilatant ones, and they also have faster electrocoalescence.

Transient deformation dynamics of particle laden droplets in electric field

We study the transient deformation dynamics after application of uniform DC electric fields to particle laden droplets with different polyethylene particle coverage. Presence of interfacial particles result in reduced electrohydrodynamic circulation flows and charge convection, which in turn result in slower transient droplet deformation compared to pure uncovered droplets, and as well to increased steady-state droplet deformation. A prolate-oblate deformation transition is observed immediately after the application of the electric field. This anomaly is not visible or greatly reduced for droplets fully covered by particles.

3D model of a droplet in an electric field

Outdoor high-voltage equipment is exposed to rain and moisture. Water droplets on the surface of insulators influence negatively the insulating and hydrophobic material properties. The shape of the droplets signifies the state of the aging material. The present paper develops a numerical procedure to determine the three-dimensional shape of water droplets in strong electric fields. That includes the solution of a feed-back problem between the droplet shape and the electric field. Here we concentrate on the particular features in three dimensions and compare the resulting droplet shapes with a 2D droplet model.

A Hybrid Boundary/Finite Element Method for Simulating Viscous Flows and Shapes of Droplets in Electric Fields

A mixed boundary element and finite element numerical algorithm for the simultaneous prediction of the electric fields, viscous flow fields, thermal fields and surface deformation of electrically conducting droplets in an electrostatic field is described in this paper. The boundary element method is used for the computation of the electric potential distribution. This allows the boundary conditions at infinity to be directly incorporated into the boundary integral formulation, thereby obviating the need for discretization at infinity. The surface deformation is determined by solving the normal stress balance equation using the weighted residuals method. The fluid flow and thermal fields are calculated using the mixed finite element method. The computational algorithm for the simultaneous prediction of surface deformation and fluid flow involves two iterative loops, one for the electric field and surface deformation and the other for the surface tension driven viscous flows. The two loops are coupled through the droplet surface shapes for viscous fluid flow calculations and viscous stresses for updating the droplet shapes. Computing the surface deformation in a separate loop permits the freedom of applying different types of elements without complicating procedures for the internal flow and thermal calculations. Tests indicate that the quadratic, cubic spline and spectral boundary elements all give approximately the same accuracy for free surface calculations; however, the quadratic elements are preferred as they are easier to implement and also require less computing time. Linear elements, however, are less accurate. Numerical simulations are carried out for the simultaneous solution of free surface shapes and internal fluid flow and temperature distributions in droplets in electric fields under both microgravity and earthbound conditions. Results show that laser heating may induce a non-uniform temperature distribution in the droplets. This non-uniform thermal field results in a variation of surface tension along the surface of the droplet, which in turn produces a recirculating fluid flow in the droplet. The viscous stresses cause additional surface deformation by squeezing the surface areas above and below the equator plane.

Electrohydrodynamic behaviour of a drop subjected to a steady uniform electric field at finite electric Reynolds number

The electrohydrodynamic flow associated with a fluid drop in an electric field is a consequence of the tangential electric stress at the fluid interface. The tangential viscous stress due to the electrohydrodynamic flow arises to just balance the tangential electric stress at the fluid interface so that the traction boundary condition is satisfied. Influenced by both the local electric stress and viscous stress, the drop interface may exhibit various shapes. The presence of fluid flow also leads to charge convection phenomena. The relative significance of the charge convection effect is usually measured in terms of the electric Reynolds number, Re E , defined as the ratio of the timescales of charge convection by flow and that for charge relaxation by ohmic conduction. This work presents a quantitative analysis of the charge convection effects in a framework of the leaky dielectric model at finite Re E , which has not been considered in previous investigations. Axisymmetric steady flows driven by an applied uniform electric field about a deformable fluid drop suspended in an immiscible fluid are studied by computational means of the Galerkin finite–element method with supplementary asymptotic analysis. The results of finite–element computations are in general agreement with the prediction by the asymptotic analysis for spherical drops at vanishingly small Re E . A common effect of charge convection is found to reduce the intensity of electrohydrodynamic flow. As a consequence, oblate drops are predicted to be less deformed in an electric field when charge convection is taken into account. The prolate drops are often associated with an equator–to–pole flow, which convects charges toward the poles to form a charge distribution resembling that in a highly conducting drop immersed in an insulating medium. Therefore, charge convection tends to enhance the prolate drop deformation. In many cases, charge convection effects are found to be significant even at apparently small Re E , corresponding to the charge relaxation time–scale about 10 −3 s, suggesting that many experimental results reported in the previous publications could have been influenced by charge convection effects.

Electric field effects on the droplet structure in polymer dispersed cholesteric liquid crystals

Abstract The behaviour of polymer dispersed cholesteric droplets in electric fields has been investigated for materials with both negative and positive dielectric anisotropy. Microscopic observations parallel and perpendicular to the electric field were performed. In materials with εa < 0, the cholesteric pitch remains constant but a disclination ring occurs due to reorientation of the local helix directions. In systems with εa > 0, however, helical unwinding was observed.

Dynamics of droplets in uniform electric fields

The evolution of a liquid droplet in an electric field has been analyzed. Microgravity environment has been used to isolate droplet from all other effects. The droplet is deformed by induced polarization effect only. The deformation process consists of rapid stretching along the field lines, formation of Taylor cones, ejection of fine spray from the pointed ends and break-up of the main body of the droplet due to capillary instability. The experimental and theoretical results agree until formation of Taylor cones occurs.

Collision efficiency of charged cloud droplets in electric fields

Collision efficiencies are shown for 5- and 10-micron droplets colliding with 30-, 40-, and 50-micron drops. The charge on the drop is always positive, but the sign of the charge on the droplet is alternately positive and negative. The magnitude of the charge on the droplet extends from zero to 10−14 coulomb. The calculations are also extended to include electric field intensities of 0, 900, 2100, and 3600 v/cm oriented at angles of 0°, 90°, and 180° with respect to the direction of fall of the drop. The electrical effects were found to influence collision efficiencies only when the droplet charges were greater than about 10−16 coulomb or when the electric field intensity exceeded 900 v/cm. The pertinence of the calculations to the physics of electrified clouds is briefly discussed.

Comments on paper by N. R. Lindblad and R. G. Semonin, ‘Collision efficiency of cloud droplets in electric fields’

Journal of Geophysical Research (1896-1977)Volume 68, Issue 15 p. 4632-4632 Letters Comments on paper by N. R. Lindblad and R. G. Semonin, ‘Collision efficiency of cloud droplets in electric fields’ J. Doyne Sartor, J. Doyne SartorSearch for more papers by this author J. Doyne Sartor, J. Doyne SartorSearch for more papers by this author First published: 1 August 1963 https://doi.org/10.1029/JZ068i015p04632AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume68, Issue151 August 1963Pages 4632-4632 RelatedInformation

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