Jayabrata Dhar

Alterations in streaming potential in presence of time periodic pressure-driven flow of a power law fluid in narrow confinements with nonelectrostatic ion-ion interactions.

We study the coupled effect of electrokinetic phenomena and fluid rheology in altering the induced streaming potential in narrow fluidic confinements, which is manifested by establishing a time periodic pressure‐driven flow in presence of electrical double layer phenomenon. However, in sharp contrast with reported literature, we take into account nonelectrostatic ion–ion interactions toward estimating the same in addition to electrostatic interactions and steric effects. We employ power law based rheological model for estimating the induced streaming potential. We bring out an intricate interaction between nonelectrostatic interactions and fluid rheology on the concerned electrokinetic phenomena, bearing immense consequences toward designing of integrated lab‐on‐a‐chip‐based microdevices and nanodevices.

Resolving Anomalies in Predicting Electrokinetic Energy Conversion Efficiencies of Nanofluidic Devices.

We devise a new approach for capturing complex interfacial interactions over reduced length scales, towards predicting electrokinetic energy conversion efficiencies of nanofluidic devices. By embedding several aspects of intermolecular interactions in continuum based formalism, we show that our simple theory becomes capable of representing complex interconnections between electro-mechanics and hydrodynamics over reduced length scales. The predictions from our model are supported by reported experimental data, and are in excellent quantitative agreement with molecular dynamics simulations. The present model, thus, may be employed to rationalize the discrepancies between low energy conversion efficiencies of nanofluidic channels that have been realized from experiments, and the impractically high energy conversion efficiencies that have been routinely predicted by the existing theories.

Ion-size dependent electroosmosis of viscoelastic fluids in microfluidic channels with interfacial slip

We report a study on the ion-size dependent electroosmosis of viscoelastic fluids in microfluidic channels with interfacial slip. Here, we derive an analytical solution for the potential distribution in a parallel plate microchannel, where the effects of finite sized ionic species are taken into account by invoking the free energy formalism. Following this, a purely electroosmotic flow of a simplified Phan-Thien-Tanner (sPTT) fluid is considered. For the sPTT model, linear, quadratic, and exponential kernels are chosen for the stress coefficient function describing its viscoelastic nature across various ranges of Deborah number. The theoretical framework presented in our analysis has been successfully compared with experimental results available in the literature. We believe that the implications of the considered effects on the net volumetric throughput will not only provide a deeper theoretical insight to interpret the electrokinetic data in the presence of ionic species but also serve as a fundamental ...

Electro-thermally driven transport of a non-conducting fluid in a two-layer system for MEMS and biomedical applications

Biomedical and biochemical applications pertaining to ion exchange or solvent extraction from one phase to another phase often deal with two-fluid flows, where one layer is non-conducting and the other layer is a biofluid. In the present study, we investigate the transport of two-layer immiscible fluids consisting of one non-conducting fluid and another conducting fluid layer in a micro-grooved channel, employing an alternating current electrothermal (ACET) mechanism. The conducting fluid, driven by the influence of ACET forces, transfers its induced momentum across the fluid-fluid interface allowing the movement of the non-conducting fluid layer. We use an order parameter based approach to track the interface of the two-layer fluid transport via the coupled Cahn-Hilliard-Navier-Stokes equation, while the potential and temperature distribution are solved using the Laplace equation and the thermal energy balance equation, respectively. The efficiency with which the non-conducting layer gets transported is ...

Spontaneous electrorheological effect in nematic liquid crystals under Taylor-Couette flow configuration

Electrorheological (ER) characteristics of Nematic Liquid Crystals (NLCs) have been a topic of immense interest in the field of soft matter physics owing to its rheological modulation capabilities. Here we explore the augmentation in rheological characteristics of the nematic fluid confined within the annular region of the concentric cylindrical space with an Electrical Double Layer (EDL) induced at the fluid-substrate interface due to certain physico-chemical interactions. Using a Taylor-Couette flow configuration associated with an EDL induced at the inner cylinder wall, we show that a spontaneous electrorheological effect is generated owing to the intrinsic director anisotropy and structural order of complex nematic fluids. We seek to find the enhancement in torque transfer capability due to the inherent electrorheological nature of the nematic medium, apart from exploiting the innate nature of such homogeneous media to remain free of coagulation, a fact which makes it an excellent candidate for the ap...

Electroosmosis of Viscoelastic Fluids: The Role of Wall Depletion Layer

We investigate electroosmotic flow of two immiscible viscoelastic fluids in a parallel plate microchannel. Contrary to traditional analysis, the effect of the depletion layer is incorporated near the walls, thereby capturing the complex coupling between rheology and electrokinetics. Toward ensuring realistic prediction, we show the dependence of electroosmotic flow rate on the solution pH and polymer concentration of the complex fluid. In order to assess our theoretical predictions, we have further performed experiments on electroosmosis of an aqueous solution of polyacrylamide (PAAm). Our analysis reveals that neglecting the existence of a depletion layer would result in grossly incorrect predictions of the electroosmotic transport of such fluids. These findings are likely to be of importance in understanding electroosmotically driven transport of complex fluids, including biological fluids, in confined microfluidic environments.

Oscillatory regimes of capillary imbibition of viscoelastic fluids through concentric annulus

In this study, we analyze the capillary filling dynamics of a viscoelastic fluid through a concentric annulus, which has far reaching consequences in practical applications and offers a distinct disparity in the dynamical characteristics as compared to the classical cylindrical capillary based paradigm. Such non-trivial characteristics are primarily attributed to a complex and intricate interplay between the intrinsic fluid rheology and the annular flow geometry, as is effectively manifested through distinctive features of the underlying oscillatory dynamics. We also estimate a criterion for the onset of oscillations, as a function of the Bond number. Our results predict remarkably attenuated oscillatory behavior and a higher capillary rise due to the presence of an annular geometry, as compared to a cylindrical one. We further relate the primary peak overshoot response with the Bond number that enables us to draw further physical insights into the oscillatory regime dynamics.

Energy-efficient generation of controlled vortices on low-voltage digital microfluidic platform

Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage ( ≤ 10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be generated inside the droplet within the standard regimes of operating parameters, bearing far-reaching consequences towards enhancing internal mixing in multifarious droplet based microfluidic applications. An inherent integrability with the existing electrowetting on dielectric platforms renders the process ideal to be used in conjunction with digital microfluidic technology.Generating controlled vortices in a sessile surface droplet configuration in an energy efficient manner is an outstanding research problem of interdisciplinary relevance, having implications in widely varying areas ranging from biomedical diagnostics, thermal management to digital microfluidic technology. Here, we experimentally and theoretically demonstrate a simple yet energy efficient strategy for generating controlled vortices inside a surface droplet, by deploying interacting electrical and thermal fields over inter-digitated electrodes on an electrically wetted platform. Unlike the traditional electrically driven mechanisms, this strategy involves significantly low voltage ( ≤ 10 V) to induce rotational structures inside the droplet, by exploiting the strong spatial gradient of electrical properties on account of the prevailing thermal field as attributable to intrinsically induced Joule heating effects. Our experiments demonstrate that fluid velocities typically of the order of mm/s can be genera...

Weak Anchoring and Surface Elasticity Effects in Electroosmotic Flow of Nematic Liquid Crystals Through Narrow Confinements

Advent of nematic liquid crystals flows have attracted renewed attention in view of microfluidic transport phenomena. Among various transport processes, electroosmosis stands as one of the efficient flow actuation method through narrow confinement. In the present study, we explore the electrically actuated flow of a nematic fluid with ionic inclusions taking into account the influences from surface induced elastic and electrical double layer phenomena. Influence of surface effects on the flow characteristics is known to get augmented in micro-confined environment and must be properly addressed. Towards this, we devise the coupled flow governing equations from fundamental free energy analysis considering the contributions from first and second-order elastic, dielectric, flexoelectric, ionic and entropic energies. We have further considered weak anchoring surface conditions with second order elasticity which helps us to more accurately capture the director deformations along the boundaries. The present study focuses on the influence of surface charge and elasticity effects in the resulting linear electroosmosis through a slit-type microchannel whose surface are considered to be chemically treated in order to display a homeotropic-type weak anchoring state. An optical periodic stripe configuration of the nematic director has been observed especially for higher electric fields wherein the Ericksen number for the dynamic study is restricted to the order of unity.Advent of nematic liquid crystal flows has attracted renewed attention in view of microfluidic transport phenomena. Among various transport processes, electro-osmosis stands as one of the efficient flow actuation mechanisms through narrow confinements. In the present study, we explore the electrically actuated flow of an ordered nematic fluid with ionic inclusions, taking into account the influences from surface-induced elasticity and electrical double layer (EDL) phenomena. Toward this, we devise the coupled flow governing equations from fundamental free-energy analysis, considering the contributions from first- and second-order elastic, dielectric, flexoelectric, charged surface polarization, ionic and entropic energies. The present study focuses on the influence of surface charge and elasticity effects in the resulting linear electro-osmosis through a slit-type microchannel whose surfaces are chemically treated to display a homeotropic-type weak anchoring state. An optical periodic stripe configuration of the nematic director has been observed, especially for higher electric fields, wherein the Ericksen number for the dynamic study is restricted to the order of unity. Contrary to the isotropic electrolytes, the EDL potential in this case was found to be dependent on the external field strength. Through a systematic investigation, we brought out the fact that the wavelength of the oscillating patterns is dictated mainly by the external field, while the amplitude depends on most of the physical variables ranging from the anchoring strength and the flexoelectric coefficients to the surface charge density and electrical double layer thickness.

Solvent‐mediated nonelectrostatic ion–ion interactions predicting anomalies in electrophoresis

We study the effects of solvent‐mediated nonelectrostatic ion–ion interactions on electrophoretic mobility of a charged spherical particle. To this end, we consider the case of low surface electrostatic potential resulting in the linearization of the governing equations, which enables us to deduce a closed‐form analytical solution to the electrophoretic mobility. We subsequently compare our results to the standard model using Henrys approach and report the changes brought about by the nonelectrostatic potential. The classical approach to determine the electrophoretic mobility underpredicts the particle velocity when compared with experiments. We show that this issue can be resolved by taking into account nonelectrostatic interactions. Our analysis further reveals the phenomenon of electrophoretic mobility reversal that has been experimentally observed in numerous previous studies.