Mechanism Analysis of Dynamic On-State Resistance Degradation for a Commercial GaN HEMT Using Double Pulse Test

The dynamic on-resistance (RON) behavior of one commercial GaN HEMT device with p-GaN gate is investigated under hard-switching conditions. The non-monotonic performance of dynamic RON with off-state voltage ranging from 50 to 400 V is ascribed to the “leaky dielectric” model. The highest normalized RON value of 1.22 appears at 150 and 200 V. The gradual increase and following maximum of dynamic RON are found when the device is exposed to a stress voltage for an extended stress time under 100 and 200 V, which is due to a much longer trapping time compared to detrapping time related to deep acceptors and donors. No obvious RON degradation, thanks to the suppressed trapping effect, is observed at higher VDS. From the multi-pulse test, the dynamic RON is seen to be insensitive to the frequency. It is demonstrated that the leakage, especially under source and drain contact, is a key issue in the dynamic resistance degradation.

Numerical Analysis on the Stability Conditions of an Electrohydrodynamic Jet

The Taylor cone jet is a well-known electrohydrodynamic flow (EHD), usually produced by applying an external electric field to a capillary liquid. The generation of this kind of flow involves a multi-phase and a multi-physics process and its stability has a specific operation window. This operating window is intrinsically dependent on the flow rate and magnitude of the applied electric voltage. In case high voltages are applied to the jet it can atomize and produce an electrospray. Our work presents a numerical study of the process of atomization of a Taylor cone jet using computational fluid dynamics (CFD). The study intents to assess the limit conditions of operation and the applied voltage needed to stabilize an electrospray. The numerical model was implemented within OpenFOAM, where the multi-phase hydrodynamics equations are solved using a volume-of-fluid (VOF) approach. This method is coupled with the Maxwell equations governing an electrostatic field, in order to incorporate the electric body forces into the incompressible Navier-Stokes equations. The leaky-dielectric model is used and, therefore, the interface between the two phases is subject to the hydrodynamic surface tension and electric stress (Maxwell stress). This allows a leakage of charge though the phase due to ohmic conduction. Thus, the permittivity and conductivity of the phases are taken into consideration. A two-fluid system with relevant electric properties can be categorized as, dielectric-dielectric, dielectric-conducting, and conducting-conducting considering the electrical conductivity and permittivities of the participating phases. Due to the usage of the leaky-dielectric model, it is possible to simulate any of this physical situations. By increasing the applied voltage reaches a value where the cone instability is verified, allowing a discussion on this effect. It is demonstrated that to adequately model the process of atomization a fine grid refinement is needed. The validation of the numerical model is made by comparing against diverse experimental data, for the case of a stable jet. The diameter and velocity of the droplet and the electric current of the jet are the main variables that are compared with previous results. The tests were performed with Heptane. The cone and the jet are strongly affected by the flow rate. The dimensionless diameter, as a function of the dimensionless flow rate, agrees with the scaling laws. The model predicts accurate results over a wide range of flow rates with an accuracy of around 10%. The results are obtained using structured meshes.

Electro-Hydrodynamics of Emulsion Droplets: Physical Insights to Applications

The field of droplet electrohydrodynamics (EHD) emerged with a seminal work of G.I. Taylor in 1966, who presented the so-called leaky dielectric model (LDM) to predict the droplet shapes undergoing distortions under an electric field. Since then, the droplet EHD has evolved in many ways over the next 55 years with numerous intriguing phenomena reported, such as tip and equatorial streaming, Quincke rotation, double droplet breakup modes, particle assemblies at the emulsion interface, and many more. These phenomena have a potential of vast applications in different areas of science and technology. This paper presents a review of prominent droplet EHD studies pertaining to the essential physical insight of various EHD phenomena. Here, we discuss the dynamics of a single-phase emulsion droplet under weak and strong electric fields. Moreover, the effect of the presence of particles and surfactants at the emulsion interface is covered in detail. Furthermore, the EHD of multi-phase double emulsion droplet is included. We focus on features such as deformation, instabilities, and breakups under varying electrical and physical properties. At the end of the review, we also discuss the potential applications of droplet EHD and various challenges with their future perspectives.

The Role of Surface-Charge Transport in Electrohydrodynamics and Electromechanics of a Dielectric Sphere

To simulate the electrokinetic processes in weakly-conducting dielectric media, the Taylor–Melcher leaky-dielectric model is widely used, though its applicability conditions are unknown. To define them, the electric-potential distributions inside and outside a dielectric sphere placed in an electric field are determined, by assuming the sphere and the environment are weakly conducting and by considering the electric and diffusion interfacial currents and the surface-charge decay. Earlier, an electric-field characteristic of a dielectric sphere, for example, the real part of the Clausius–Mossotti factor found for a direct current (DC) field was commonly thought to be a single-valued function of two parameters, the conductivities of the sphere and the environment. Now, it depends on a larger number of parameters and, in the dc case, can range from the perfect-dielectric to perfect-conductor values even for a particle of a good insulator. Using the proposed theory, a variety of the experimental results on the electrohydrodynamic (EHD) fluid circulation and dielectrophoretic (DEP) motion of microparticles in the dielectric drops are explained for the first time or in a new way. The dielectrophoretic inflection and cross-over frequencies are defined allowing for the decay of the surface charge. A dependence of the effective conductivity of a sphere on the angular field distribution is predicted for the first time.

Linear Instability of Liquid Sheets Subjected to a Transverse Electric Field

A temporal linear instability analysis was performed for a liquid sheet moving around the inviscid gas in transverse electrical field. The fluid was described by the leaky-dielectric model, which is more complex and more comparable to the liquid electrical properties than existing models. As a result, the sinuous and the varicose modes exist, in which the dimensionless dispersion relation between wave number and temporal growth rate can be derived as a 3 × 3 matrix. According to this relationship, the effects of liquid properties on sheet instability were performed. It was concluded that, as the electrical Euler number (Eu), the ratio of gas-to-liquid density (ρ), Weber number (We), Reynolds number (Re), and the relative relaxation time (τ) increased, the instability of the sheet was enhanced. This work also compared the leaky-dielectric model with the perfect conductor model and found that the unstable growth rate in the leaky-dielectric model was higher than the one in the perfect conductor model. Moreover, as the ratio of gas-to-liquid improved, this difference decreased. Finally, an energy approach was adopted to investigate the instability mechanism for the two models.

Electrohydrodynamics of deflated vesicles: budding, rheology and pairwise interactions

We develop a new boundary integral method for solving the coupled electro- and hydrodynamics of vesicle suspensions in Stokes flow. This relies on a well-conditioned boundary integral equation formulation for the leaky-dielectric model describing the electric response of the vesicles and an efficient numerical solver capable of handling highly deflated vesicles. Our method is applied to explore vesicle electrohydrodynamics in three cases. First, we study the classical prolate–oblate–prolate transition dynamics observed upon application of a uniform DC electric field. We discover that, in contrast to the squaring previously found with nearly spherical vesicles, highly deflated vesicles tend to form buds. Second, we illustrate the capabilities of the method by quantifying the electrorheology of a dilute vesicle suspension. Finally, we investigate the pairwise interactions of vesicles and find three different responses when the key parameters are varied: (i) chain formation, where they self-assemble to form a chain that is aligned along the field direction; (ii) circulatory motion, where they rotate about each other; (iii) oscillatory motion, where they form a chain but oscillate about each other. The last two are unique to vesicles and are not observed in the case of other soft particle suspensions such as drops.

The electrostatically forced Faraday instability: theory and experiments

The onset of interfacial instability in two-fluid systems using a viscous, leaky dielectric model is studied. The instability arises as a result of resonance between the parametric frequency of an imposed electric field and the system’s natural frequency. In addition to a rigorous model that uses Floquet instability analysis, where both viscous and charge effects are considered, this study also provides convincing validating experiments. In other results, it is shown that (a) the imposition of a periodic electrostatic potential acts to counter gravity and this countering effect becomes more effective if a DC voltage is also added, (b) a critical DC voltage exists at which the interface becomes unstable such that no parametric frequency is required to completely destabilize the interface and (c) the leaky dielectric model approaches a model for a perfect dielectric/perfect conductor pair as the conductivity ratio becomes large. It is also shown via experiments that parametric resonant instability using electrostatic forcing may be reliably used to estimate interfacial tension to sufficient accuracy.

Numerical simulation of electrospraying in the cone-jet mode

This article solves numerically the equations of the leaky-dielectric model applied to cone jets. The solution is a function of the properties of the fluid and its flow rate, universal in that it does not depend on the geometry and potential of the electrodes. This is made possible by the use of the potential field generated by a semi-infinite Taylor cone as a far-field boundary condition. The numerical solution yields the current emitted by the electrospray, which compares well with experimental data, and detailed information about the velocity, surface charge, electric field and the position of the free surface. These characteristics are generally inaccessible through experiments, and are needed to understand the relative importance of competing processes and the dominant physics. The simulations investigate the liquids tributyl phosphate and propylene carbonate (dielectric constants of 8.91 and 64.9 respectively), in a wide range of electrical conductivities and flow rates. The simulations show that the position of the surface, expressed in units of the characteristic length $r_{c}$, is largely invariant regardless of the physical properties and flow rates of the liquids. The surface charge falls below its equilibrium value along the transition from cone to jet, with a deficit that increases with the ratio between the electrical relaxation and flow residence times. Several characteristics of the cone jet are functions of the dielectric constant, which is consistent with the importance of charge relaxation effects (i.e. with the absence of surface charge equilibrium). The electric energy transferred to the transition region is largely transformed into viscous and ohmic dissipation, and conversion into kinetic energy only dominates once most of the current is fixed on the surface.

Deformation of an encapsulated bubble in steady and oscillatory electric fields

In this paper, we investigate the dynamics of an encapsulated bubble in steady and oscillatory electric fields theoretically, based on a leaky dielectric model. On the bubble surface, an applied electric field generates a Maxwell stress, in addition to hydrodynamic traction and membrane mechanical stress. Our model also includes the effect of interfacial charge due to the jump of the current and the stretching of the interface. We focus on the axisymmetric deformation of the encapsulated bubble induced by the electric field and carry out our analysis using Legendre polynomials. In our first example, the encapsulating membrane is modelled as a nearly incompressible interface with bending rigidity. Under a steady uniform electric field, the encapsulated bubble resumes an elongated equilibrium shape, dominated by the second- and fourth-order shape modes. The deformed shape agrees well with experimental observations reported in the literature. Our model reveals that the interfacial charge distribution is determined by the magnitude of the shape modes, as well as the permittivity and conductivity of the external and internal fluids. The effects of the electric field on the natural frequency of the oscillating bubble are also shown. For our second example, we considered a bubble encapsulated with a hyperelastic membrane with bending rigidity, subject to an oscillatory electric field. We show that the bubble can modulate its oscillating frequency and reach a stable shape oscillation at an appreciable amplitude.

Electrohydrodynamics of viscous drops in strong electric fields: numerical simulations

Weakly conducting dielectric liquid drops suspended in another dielectric liquid and subject to an applied uniform electric field exhibit a wide range of dynamical behaviours contingent on field strength and material properties. These phenomena are best described by the Melcher–Taylor leaky dielectric model, which hypothesizes charge accumulation on the drop–fluid interface and prescribes a balance between charge relaxation, the jump in ohmic currents from the bulk and charge convection by the interfacial fluid flow. Most previous numerical simulations based on this model have either neglected interfacial charge convection or restricted themselves to axisymmetric drops. In this work, we develop a three-dimensional boundary element method for the complete leaky dielectric model to systematically study the deformation and dynamics of liquid drops in electric fields. The inclusion of charge convection in our simulations permits us to investigate drops in the Quincke regime, in which experiments have demonstrated a symmetry-breaking bifurcation leading to steady electrorotation. Our simulation results show excellent agreement with existing experimental data and small-deformation theories.