Dielectric Liquid Films Research Articles

Heat flux from the surface in the process of the rupture of a thin liquid film by an electric field

In this article, the cooling of a solid surface by an evaporating film of a dielectric liquid and the influence of an electric field on this process are studied. The mesoscopic lattice Boltzmann method is applied to simulate the fluid flow with phase transitions and the heat transfer. The cases without electric field, with initially uniform electric displacement field, and with non-uniform electric field are considered. A uniform field enhances the cooling slightly. Non-uniform field leads to the rupture of the film. In this case, the local heat flux in local regions increases significantly when the film becomes thin and effectively evaporates. After the rupture of the film, the heat flux from a dry spot decreases abruptly. The formation of a dry spot can be prevented by switching off the electric field before the film rupture. Thus, we demonstrate the possibility of enhanced cooling of local regions at a surface using pulses of non-uniform electric field acting on a thin film of dielectric liquid placed at the surface. If the inflow of liquid to the film could be provided, it is in principle possible to realize a periodic process of application of voltage pulses to electrodes and to enhance the cooling of surface by the evaporation of a film of dielectric liquid.

Electric control of dielectric droplets and films

We investigate the behavior of dielectric droplets and films placed onto a solid surface under the action of electric field of different configurations. The mesoscopic thermal multiphase lattice Boltzmann model [A. Kupershtokh, D. Medvedev, and I. Gribanov, “Thermal lattice Boltzmann method for multiphase flows,” Phys. Rev. E 98, 023308 (2018)] is used for simulation. Different configurations of electric field were produced by using dissected flat electrodes of various shapes. On a simple flat electrode, droplets elongate after the application of electric voltage. Quite different behavior was observed when the central round part of the electrode was made non-conductive. In this case, the droplet spreads under the action of a non-uniform electric field, and the breakup and the formation of an annular structure were observed. A film of dielectric liquid flowing along a solid surface made of conductive and non-conductive transversal stripes exhibits a variety of regimes. When the voltage is low, the action of electric field produces waves at the surface of liquid. At a high voltage, the liquid is pinned to the edges of stripes, and the flow may be stopped completely. The purpose of this article is precisely to attract experimenters to the study of this type of phenomena.

Effects of working temperature and fluid physical properties on heat transfer enhancement in conduction pumping: An experimental study

In the current study, heat transfer behavior in electrohydrodynamic conduction pumping of two different dielectric liquid films using flush electrodes has been experimentally studied, and effects of working temperature and fluid physical properties on heat transfer enhancement are investigated. The study is conducted for different liquid film thicknesses and working temperatures, while changing the magnitude of the applied electrical voltage. Results show that higher ion mobility difference and working temperature of the dielectric liquid, as well as lower density and viscosity increase flow rate and heat transfer of the pump. Also, it is seen that heat transfer coefficient is intensively enhanced for all different film thicknesses and working temperatures by using the conduction pump, which is due to the turbulent flow created in the vicinity of the electrodes. For 8 mm film thickness of n-hexane at35°C, the significant enhancement of heat transfer coefficient equal to 1040.9% was observed.

Nonlinear dynamics of thin liquid films subjected to mixed-frequency electrical field

The nonlinear dynamics of the interface between both perfect and leaky dielectric liquid films, interposed between two parallel electrodes, are investigated under the effect of mixed-frequency electric fields. A coupled system of evolution equations is derived in dimensionless form, by employing the long-wave approximation. The linear stability analysis is implemented in accordance with the characteristics of each specific case, namely, the constant (DC) and the altering (AC) fields. In particular, the response of the system to the multi-mode AC electrical field is analyzed. Assisted by the conclusions of the theoretical investigation, the initial-boundary-value problem associated with the coupled system of evolution equations is solved numerically for several parameter sets. The system behavior is studied by monitoring the evolution process and by examining the steady/quasi-steady pillar formations in the nonlinear regime. The possibility to generate interface profiles of diverse topological forms, to manipulate their features, and to control the time dependent progress and the film rupture by imposing different combinations of frequencies and/or amplitudes of the corresponding mode is confirmed.

Numerical investigation of conduction pumping of dielectric liquid film using flush-mounted electrodes

Electrohydrodynamic conduction pumping of dielectric liquid films using flush-mounted electrodes is investigated numerically. Two major factors consisting of the ion mobility difference and electrodes’ configuration can affect the conduction pumps. The relative importance of these factors on the hydrodynamic behavior has been studied at different configurations of flush-mounted electrodes for conduction pumping of various dielectric liquids with different electrical properties. Furthermore, the effect of heterocharge layer structure on the hydrodynamic behavior of conduction pump has been studied. The electrical behavior and flow patterns of dielectric liquids with real measured mobilities are compared with the experimental results, and new features of conduction pumping are found. The numerical results indicate that in the various operating conditions, the flow direction is dictated by the dominant factor.

Experimental study of heat transfer enhancement in electrohydrodynamic conduction pumping of liquid film using flush electrodes

Electrohydrodynamic conduction pumping of free surface dielectric liquid film, using flush electrodes, has been studied experimentally for various film temperatures. Volume flow rate, heat transfer and power consumption ratio and conduction pumping efficiency of free surface liquid film in different film thicknesses and temperatures have been investigated and then the best operating conditions have been presented. Also, the heat transfer coefficient on free surface liquid film passing on flush electrodes is compared with similar liquid film in absence of flush electrodes in different temperatures. Results show that as applied voltage increases, significant differences in volume flow rates have been observed by changing the temperature. Applied voltage related to the highest percentage of heat transfer coefficient enhancement demonstrates the reverse relation with temperature. Results confirm that there is a direct relationship between film thickness and the applied voltage related to the maximum heat transfer per pumps power consumption.

Experimental investigation into three-dimensional wavy liquid films under the influence of electrostatic forces

Three-dimensional interfacial waves that develop on the free surface of falling liquid films are known to intensify heat and mass transfer. In this context, the present paper studies the effect of electrostatic forces applied to a falling film of dielectric liquid on its three-dimensional nonlinear wave dynamics. Therefore, measurements of the local film thickness using a confocal chromatic imaging method were taken, and the complex wave topology was characterized through photography. The experiments show a complex interaction between the electric field and the hydrodynamics of the falling film, whereby electrostatic forces were found to both increase and decrease wave peak height in different regions of the wave. Additionally, an electrically induced breakup of the three-dimensional wave fronts, which leads to a locally doubled frequency in streamwise direction, is found. The ability to influence the wave topology demonstrated here opens the possibility to optimize heat transfer processes in falling liquid films.

Electrohydrodynamic conduction pumps with cylindrical electrodes for pumping of dielectric liquid film in an open channel

In the present study, electrohydrodynamic conduction pumping of n-hexane and n-decane liquid films in an open channel has been investigated experimentally. These two dielectric liquids have nearly the same electrical properties but with their different viscosities. The effects of film thicknesses, the arrangement of electrodes and the gap between pumps on the flow rate of liquid films have been also studied. The pumps with cylindrical electrodes have been installed in the channel. The best performance of the conduction pumps, revealing with experimental results, has been achieved at 8 mm thickness of liquid film with the gap size of 55 mm for both dielectric liquids in the present layout of the pumps.

Effect of tangential electric field on the evolution of the Rayleigh-Taylor instability of a dielectric liquid film

The effect of surface ponderomotive forces produced by a uniform tangential electric field on the evolution of the Rayleigh-Taylor instability of an isothermal film of a homogeneous incompressible dielectric liquid coating the lower surface of a horizontal insulating plate is studied in the long-wavelength approximation for the hydrodynamic equations and a simplified system of electrostatics equations. The lower boundary of the liquid is the interface with a stationary gas. It is shown in the framework of the linear theory that during the disruption of the continuous film, ponderomotive forces induce the formation of liquid billows extended along the applied field lines.

Selective Spreading and Jetting of Electrically Driven Dielectric Films

We study the electrically driven spreading of dielectric liquid films in wedge-shaped gaps across which a potential difference is applied. Our experiments are in a little-studied regime where, throughout the dynamics, the electrical relaxation time is long compared to the time for charge to be convected by the fluid motion. We observe that at a critical normal electric field the hump-shaped leading edge undergoes an instability in the form of a single Taylor cone and periodic jetting ensues, after which traveling waves occur along the trailing thin film. We propose a convection-dominated mechanism for charge transport to describe the observed dynamics and rationalize the viscosity dependence of the self-excited dynamics.

Flow pattern visualization of liquid film conduction pumping using flush mounted electrodes

Conduction pumping is one convenient method for pumping of dielectric liquid films. Flow visualization can be useful for understanding the hydrodynamic behavior of this kind of flow. In this study, the flow pattern of the liquid film conduction pumping using flush mounted electrodes has been visualized for various electrode arrangements. Symmetric and asymmetric configurations of electrodes have been examined for pumping of silicon oil. Two major factors affect the conduction pumps; ion mobility difference and electrodes configuration. For known ion mobility difference two cases have been considered: Case 1 in which electrode configuration strengthens the ion mobility difference pumping effect and Case2 in which electrode configuration has the opposite effect. The results show that, for one pair of electrodes acting as unit pump, a primary vortex is generated on the electrodes which arises from rotational part of the Coulomb force. For more than one pair of electrodes, secondary vortices appeared which are generated by the primary vortices. For Case 1, the vorticity of primary and secondary vortices increases with electrode width ratio because of the strengthening effect of the two factors but it decreases for Case 2 because of weakening effect of them.

Electrically Induced Dielectric Liquid Film Flow Based on Electric Conduction Phenomenon

Electrohydrodynamic (EHD) conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, generated by the process of dissociation of the neutral electrolytic species and recombination of the generated ions. The theoretical formulation for EHD conduction pumping of liquid film is presented and fundamentally analyzed with the aid of numerical solutions. This model includes fluid dynamics governing equations under laminar and isothermal conditions which are modified to account for the presence of electric body force. The model also includes charge transport equations which are related to the dissociation/recombination phenomenon along with Maxwell's relations that govern the electric field distribution. This paper determines how liquid film flow is generated based on the electric conduction phenomenon. Specifically, the role of controlling dimensionless parameters on the heterocharge layers and flow structures along with the impact of liquid film velocity on charge distribution are illustrated and fundamentally analyzed. In addition, the contribution of unique electrode designs toward electric body force distribution and flow pattern is investigated followed by the effect of interaction between adjacent electrode pairs in multi-pair configurations on generated flow rate. Further, a brief discussion of the conduction pumping efficiency is presented. Finally, the numerical results are verified against experimental data.

Equilibrium shapes and stability of a liquid film subjected to a nonuniform electric field

Stresses induced by a spatially nonuniform electric field acting on an initially flat fluid-fluid interface can (i) be exploited beneficially to pattern polymer microstructures without the use of resists, exposure, development, and etching, but (ii) cause undesirable nonuniformity in film thickness in precision coating processes. The equilibrium shape of an interface separating a liquid film from an ambient fluid subjected to a uniform electric field is flat so long as the field strength is below a critical value. A nonuniform electric field, however, results in the deformation of the interface no matter how small its strength, an important difference which previous theoretical studies have not addressed satisfactorily. Hence, whereas under a uniform field loss of stability occurs via a bifurcation from the flat film solution, under a nonuniform field destabilization may occur at a turning point at which the film profile already exhibits a finite-amplitude deformation. This deficiency in understanding is remedied here by analyzing a model problem in which a gas overlying a perfect dielectric liquid film is sandwiched between two electrodes wherein the top electrode is grounded and the electric potential of the bottom electrode varies sinusoidally with distance measured along it. The equilibrium shapes and stability of the liquid-gas interface are determined directly in the present work by simultaneously solving the augmented Young-Laplace equation governing the shape of the free surface and the Laplace equation governing electric potentials theoretically by domain perturbation analysis and numerically by finite element analysis. For small nonuniformities in the electric field, analytical solutions are reported for the profile of the free surface. The computational predictions are shown to be in excellent accord with these small-deformation results. Moreover, computations are used to extend the investigations into the nonlinear regime where nonuniformities in the electric field and deformations of the free surface are large, and loss of stability may occur. The variation of the equilibrium shapes and the limits of stability with the governing dimensionless groups are investigated thoroughly. It is shown that the rich response exhibited by the system can be rationalized by interrogating the computed solutions and scrutinizing the balance of stresses due to the normal component of the electric field, which are destabilizing, and those due to its tangential component, which are stabilizing.

Liquid transport generated by a flashing field-induced wettability ratchet

The authors develop a model for ratchet-driven macroscopic transport of a continuous phase. The transport relies on a field-induced dewetting-spreading cycle of a liquid film based on a switchable, spatially asymmetric, periodic interaction of the free surface of the film and the solid substrate. The concept is exemplified using an evolution equation for a dielectric liquid film under an inhomogeneous voltage. The authors analyze the influence of the various phases of the ratchet cycle on the transport properties. Conditions for maximal transport and the efficiency of transport under load are discussed.

Finite-amplitude surface waves on a thin film flow subject to a unipolar-charge injection

The interaction of an electric field with a dielectric liquid film is investigated as it drains under gravity down an inclined plane electrode emitting uniform positive ions into the liquid region. By applying long-wave approximation to the governing equations, the evolution equation for the free surface is derived up to the first order of a thin film parameter ξ To investigate the space charge effect on the development of a finite-amplitude surface wave, a neutral stability condition is obtained as a critical Reynolds number through a linear stability analysis, and the amplitude and velocity of a periodic disturbance are also calculated within a supercritically stable flow region. The presence of a unipolar space charge in the fluid makes a steady surface wave take on even higher amplitude and faster wave speed compared with the case of no space charge.

Dynamical Instability of Thin Liquid Films Between Conducting Media

It is shown that thin dielectric liquid films between electrically conducting media are dynamically unstable if the electronic work functions of the conductors are different. As a consequence, they are expected to dewet by a spinodal dewetting scenario, i.e., via amplification of surface waves with mode selection. The wave number of the fastest growing mode is discussed as a function of the strength of the electric and dispersive forces. In contrast to what seems to be widely believed, it turns out that electric forces dominate over dispersive forces in most experimental situations of the rather general type studied here.

Wave flows in a thin layer of a viscous liquid. Influence of a constant electric field

The influence of a constant transverse electric field on the dynamics of longwave, weakly nonlinear flow of a viscous dielectric liquid film down a vertical wall is studied. An amplitude integrodifferential equation in partial derivatives of the Kuramoto-Sivashinskii equation type, which describes the behavior of the free surface of the layer, is derived using the method of multiscale stretching. In the case considered, the potential energy of the electric field is a source of longwave perturbations, but, on the whole, secondary regimes are apparently nonlinearly steady. Probably, the electric polarization effects studied can be used as a factor that governs the dynamics of film flow.

SIMULTANEOUS MEASUREMENTS OF THICKNESS AND TEMPERATURE PROFILE IN A WAVY LIQUID FILM FALLING FREELY ON A HEATING WALL

A technique far measuring the thickness of liquid films was developed and successfully tested. The feasibility of this technique was demonstrated in stagnant liquid films as well as in liquid jets. A procedure for in-situ calibration of the thickness probe was developed, allowing the adaptation of the probe to measurements of wavy liquid films. The thickness probe was constructed from a platinum-rhodium wire that was stretched across the film. A constant DC current was supplied through the probe wire, and film thickness was determined from variations in the probe voltage drop resulting from the large differences in the electrical resistances of the wetted and unwetted segments of the wire. Unlike electrical admittance thickness probes, the new probe did not require dissolving an electrolyte in the liquid, making the new probe well suited to studies involving sensible heating of a film of pure dielectric liquid that is in duvet contact with a current-carrying wall. Also presented is a composite probe that ...

Sensible Heating and Boiling Incipience in Free-Falling Dielectric Liquid Films

Controlling boiling incipience is of paramount importance for reliable operation of liquid-cooled microelectronic heat sources during power transients. This study focuses on heat transfer from a simulated multichip module to a falling film. Experiments have been performed to develop an understanding of the influence of surface tension and wetting characteristics on sensible heat transfer and boiling incipience in free-falling dielectric (FC-72) liquid films. The boiling results reveal that the vanishingly small contact angle of FC-72 precludes the application of correlations currently employed to predict incipience. Also, the temperature excursion commonly encountered upon boiling incipience in wetting fluids was nonexistent in all the experimental runs.

High field conduction in dielectric liquid films in time range of 10 −5 − 10 −3 s

In order to develop more enough the investigation of electrical conduction in dielectric liquid films, partly reported previously by the authors, the transient conductivity at higher electric field has been studied. The measurement has been carried out under the conditions as follows; time duration (10 −5 − 10 −3 s), field strength (100 kV/cm - 1 MV/cm), temperature (room temperature - 150°C), a coefficient of kinematic viscosity (1 – 100 cSt) and sample thickness (3 – 5 μm). The results indicate that 1) the transient current i has been observed to decay according to i ∝ t -n where the exponent n is independent of the field strength, 2) the transient conductivity σ varies with field strength E as σ ∝ exp (bE) where b is a constant, and 3) the hopping distance of excess electrons obtained from conductivity-field curves depends on time and temperature, and these dependences correspond to time and temperature dependences of the activation energy respectively. The transient processes of excess electrons in dielectric liquids from these results are discussed.