BITS Faculty Publications

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Subject: search.filters.subject.Electrohydrodynamics

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    Influence of electric potential boundary condition on the electrospraying process
    (Elsevier, 2025-08) Rao, Venkatesh K.P.; Yadav, Shyam Sunder
    In the current work, we perform three dimensional numerical simulations of the electrospraying process. Our aim is to investigate the effect of electric potential boundary condition on the electrospraying process of a liquid. We observe a steady electrospraying process in the cone jet mode for the case of uniform electric potential boundary condition. On the other hand, we observe a highly unsteady, violent electrospraying process for the case of non-uniform boundary condition. We provide explanation of this widely different behavior of the electrospraying process.
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    Oscillations of a Sessile Droplet in Contact and Non-contact Modes Under an AC Electric Field
    (Springer, 2016-09) Yadav, Shyam Sunder
    In the present work we numerically investigate the oscillations of a sessile conducting droplet in the contact and non-contact modes under an alternating electric field. We show that the oscillations in the non-contact mode, where the needle electrode remains away from the drop, are caused by the electric forces due to charge accumulation at the apex of the drop. In the contact mode case, where the needle remains dipped inside the drop, the electric charge accumulates at the drop surface just above the dielectric coating with a maximum value near the three phase contact line. These charges push the three phase contact line outwards with an oscillatory force which leads to drop oscillations. We also observe that higher needle voltage (~1 kV) is required for the non-contact mode while considerably less potential (~10 V) is enough for the contact mode to cause drop oscillations.
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    Numerical simulations of bubble formation from a submerged orifice and a needle: The effects of an alternating electric field
    (Elsevier, 2016) Yadav, Shyam Sunder
    In many applications, such as bubble column reactors, electric field is employed to provide a greater control on the sizes of bubbles forming at orifices and needles. In this study, we investigate the effects of an alternating electric field on the bubble dynamics. We perform numerical simulations of an alternating electric field coupled with two-phase flow using a Coupled Level-Set and Volume-of-Fluid method. We show that bubbles forming at orifices and needles decrease in size (up to ) only for a range of applied frequency and for other frequencies, the size of bubbles can be much bigger compared to the bubbles forming in the corresponding DC electric field case. The oscillating electric forces excite capillary waves on the bubble interface resulting in applied frequency dependent bubble oscillations. The numerically observed resonance for the needle case corresponds to , where is the frequency of the oscillation of the electric field force at the interface and is the capillary time scale, indicating that the resonance behavior is indeed governed by the interactions between the capillary and electric field force. A decomposition of bubble profile shapes into Legendre modes shows that for orifice as well as the needle case, second mode is most dominant followed by the fourth mode.
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    Numerical investigation of a conducting drop’s interaction with a conducting liquid pool under an external electric field
    (Elsevier, 2020-06) Yadav, Shyam Sunder
    A charged conducting drop suspended in an insulating medium shows non-coalescence with an interface under high strength of an externally applied electric field. We perform numerical simulations of the non-coalescence phenomenon to understand the underlying physical mechanisms and the effect of electric field strength and fluid conductivity on the coalescence behavior of a conducting drop with a conducting liquid pool under highly viscous conditions. We show that two factors primarily govern the coalescence or non-coalescence of the drop with the interface. First, the magnitude of the charge transfer time scale (which governs the rate of charge transfer during contact between the drop and the pool) relative to the time scale of the capillary waves. Second, the strength of the electric forces compared to the viscous forces. We further show that for the case of macro drops ( mm), charge transfer by fluid convection dominates charge conduction at lower electric conductivities ( S/m) only. Finally, we explain the non-dependence of secondary droplet’s size and charge on the fluid’s electric conductivity as observed in the experiments.
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    Interplay of high zeta potential and steric factor on the slip thermofluidics of power law fluids through narrow confinements
    (Elsevier, 2022-04) Harikrishnan, A.R.
    Thermal transport in a non-Newtonian fluid flow through a parallel plate microchannel is investigated with an emphasis on the effects of high zeta potential and steric factor. The fluid viscosity is modelled using the power law and the flow is driven by a combination of electric field and pressure gradient in the form of a dimensionless flow actuation coefficient. The effects of joule heating, viscous dissipation and slip length at the wall are considered. A semi-analytical formulation for the temperature profile is developed which is solved numerically using the Galerkin Finite Element method. We find that the Nusselt number increases as the flow behaviour index is increased. Velocity slip is also found to have a positive impact on convective heat transfer. However, as the zeta potential and steric factor increase, the Nusselt number begins to decline as there is an increase in viscous dissipation. We highlight the importance of considering variable electrical conductivity in evaluating the effect of joule heating at high zeta potential and moderate steric factor. In this regime, joule heating dominates advective transport and heat flow is reversed which may be detrimental or even damaging to microfluidic devices. A comprehensive thermal map is developed for a wide range of zeta potentials and steric factors.
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    Electrohydrodynamic fibrillation governed enhanced thermal transport in dielectric colloids under a field stimulus
    (RSC, 2018-05) Harikrishnan, A.R.
    Electrorheological (ER) fluids are known to exhibit enhanced viscous effects under an electric field stimulus. The present article reports the hitherto unreported phenomenon of greatly enhanced thermal conductivity in such electro-active colloidal dispersions in the presence of an externally applied electric field. Typical ER fluids are synthesized employing dielectric fluids and nanoparticles and experiments are performed employing an in-house designed setup. Greatly augmented thermal conductivity under a field's influence was observed. Enhanced thermal conduction along the fibril structures under the field effect is theorized as the crux of the mechanism. The formation of fibril structures has also been experimentally verified employing microscopy. Based on classical models for ER fluids, a mathematical formalism has been developed to predict the propensity of chain formation and statistically feasible chain dynamics at given Mason numbers. Further, a thermal resistance network model is employed to computationally predict the enhanced thermal conduction across the fibrillary colloid microstructure. Good agreement between the mathematical model and the experimental observations is achieved. The domineering role of thermal conductivity over relative permittivity has been shown by proposing a modified Hashin–Shtrikman (HS) formalism. The findings have implications towards better physical understanding and design of ER fluids from both ‘smart’ viscoelastic as well as thermally active materials points of view.
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    Slip hydrodynamics of combined electroosmotic and pressure driven flows of power law fluids through narrow confinements
    (Elsevier, 2021-10) Harikrishnan, A.R.
    Electroosmotic flows (EOF) in microfluidic devices can be greatly enhanced over superhydrophobic surfaces because the high shear rates within the electrical double layer can drive large slip velocities at the interface. Using the power law fluid model, we derive a novel formulation for the Helmholtz–Smoluchowski (HS) velocity and use it to examine the effect of slip on the hydrodynamics of a coupled pressure driven and EOF. Semi analytical relations for the velocity gradient are obtained for cases of a favourable pressure gradient but exact solutions of the velocity can be found only for certain power law indices. Cases of adverse pressure gradient and fractional power law indices are investigated using numerically using the Galerkin Finite Element Method. The validity of the semi analytical relations verified by comparison with the numerical method. The presence of velocity slip at the wall leads to an enhancement of the HS velocity that is most pronounced in shear thinning fluids. Adverse pressure gradients are observed to generate an inflection in the velocity profile and even a two-way flow for certain flow parameters. The strength of the adverse pressure gradient needed to setup a reverse flow at the channel centre reduces as the slip length is increased. The location of the point of inflection is found to depend on the channel height, pressure gradient, electric field, slip length, Debye length and non-Newtonian behaviour.