arXiv subjects

Suman Chakraborty

Publications and source records attributed to Suman Chakraborty.

117 records · Page 7Linked to original sources

arXiv2015

Wettability Modulated Charge Inversion and Ionic Transport in Nanofuidic Channels

We unveil the role of substrate wettability on the reversal in the sign of the interfacial charge distribution in a nanochannel in presence of multivalent ions. In sharp contrast to the prevailing notion that hydrophobic interactions may trivially augment the effective surface charge, we demonstrate that the interplay between surface hydrophobicity and interfacial electrostatics may result in a decrease in the effective interfacial potential, and a consequent charge inversion over regimes of low surface charges. We also show that this phenomenon, in tandem with the interfacial hydrodynamics may non-trivially lead to either augmentation or attenuation or even reversal of the net streaming current, depending on the relevant physical scales involved. These results, supported by Molecular Dynamics simulations and experimental data, may bear far ranging consequences in understanding complex biophysical processes and designing nanofluidic devices and systems involving multivalent counterions.

physics.flu-dyn↗

arXiv2015

Electrically modulated dynamic spreading of drops on soft surfaces

The intricate interaction between the deformability of a substrate and the dynamic spreading of a liquid drop on the same, under the application of an electrical voltage, has remained far from being well understood. Here, we demonstrate that electrospreading dynamics on soft substrates is dictated by the combined interplay of electrocapillarity, the wetting line friction and the viscoelastic energy dissipation at the contact line. Our results reveal that during such electro-elastocapillarity mediated spreading of a sessile drop, the contact radius evolution exhibits a universal power law in a substrate elasticity based non-dimensional time, with an electric potential dependent spreading exponent. Simultaneously, the macroscopic dynamic contact angle variation follows a general power law in the contact line velocity, normalized by elasticity dependent characteristic velocity scale. Our results are likely to provide the foundation for the development of a plethora of new applications involving droplet manipulations by exploiting the interplay between electrically triggered spreading and substrate-compliance over interfacial scales.

physics.flu-dyn↗

arXiv2015

Electrohydrodynamic migration of a spherical drop in a general quadratic flow

We investigate the motion of a spherical drop in a general quadratic flow acted upon by an arbitrarily oriented externally applied uniform electric field. The drop and media are considered to be leaky dielectrics. The flow field affects the distribution of charges on the drop surface, which leads to alteration in the electric field, thereby affecting the velocity field through the Maxwell stress on the fluid-fluid interface. The two-way coupled electrohydrodynamics is central towards dictating the motion of the drop in the flow field. We analytically address the electric potential distribution and Stokesian flow field in and around the drop in a general quadratic flow for small electric Reynolds number (which is the ratio of the charge relaxation time scale to the convective time scale). As a special case, we consider a drop in an unbounded cylindrical Poiseuille flow and show that, an otherwise absent, cross-stream drop migration may be obtained in the presence of a uniform electric field. Depending on the direction of the applied electric field and the relative electrical properties of the drop and media, the drop may migrate towards or away from the centreline of the imposed Poiseuille flow.

physics.flu-dyn↗

arXiv2015

Uniform electric field induced lateral migration of a sedimenting drop

We investigate the motion of a sedimenting spherical drop in the presence of an applied uniform electric field in an otherwise arbitrary direction in the limit of low surface charge convection. We analytically solve the electric potential in and around the leaky dielectric drop, and solve for the Stokesian velocity and pressure fields. We obtain the drop velocity through perturbations in powers of the electric Reynolds number which signifies the importance of the charge relaxation time scale as compared to the convective time scale. We show that in the presence of electric field either in the sedimenting direction or orthogonal to it, there is a change in the drop velocity only in the direction of sedimentation due to an asymmetric charge distribution in the same direction. However, in the presence of an electric field applied in both the directions, and depending on the permittivities and conductivities of the two fluids, we obtain a non-intuitive lateral migration of drop in addition to the buoyancy driven sedimentation. These dynamical features can be effectively used for manipulating drops in a controlled electro-fluidic environment.

physics.flu-dyn↗

arXiv2015

The effect of interfacial slip on the motion and deformation of a droplet in an unbounded arbitrary Stokes flow

The motion and deformation of a droplet suspended in an unbounded fluid with an arbitrary, but Stokesian, imposed flow is investigated when there is a slip at the interface between the two liquids. The boundary condition at the interface is accounted by means of a simple Navier slip condition. Expressions are derived considering the effect of slip on the velocity and the shape deformation of the droplet for any arbitrary imposed flow field, and results are presented for the specific cases of shear flow and Poiseuille flow with the results of Hetsroni and Haber (J. Fluid Mech., 1970, vol. 41(04), pp. 689-705); and Ramachandran and Leal (J. Rheol., 2012, vol. 56(6), pp. 1555-1587) as the limiting cases of our general expressions. The modification to Faxén's law is also presented in the above perspective.

physics.flu-dyn↗

arXiv2014

Electrically Modulated Thin Film Dynamics Controlling Bubble Manipulation in Microfluidic Confinement

Thin film dynamics and associated instability mechanisms have triggered a wide range of scientific innovations, as attributed to their abilities of creating fascinating patterns over small scales. Here, we demonstrate a new thin film instability phenomenon governed by electro-mechanics and hydrodynamics over interfacial scales in a narrow fluidic confinement. We first bring out the essential physics of this instability mechanism, in consideration with the fact that under the action of axial electrical field in a confined microfluidic environment, perturbations may be induced on the interfaces of thin corner films formed adjacent to the walls of a microchannel, leading to the inception of ordered lateral structures. A critical electric field exists beyond which these structures from the walls of the confinement intermingle to evolve into localized gas pockets in the form of bubbles. These bubbles do not remain static with further changes in electric field, but undergo a sequence of elongation-deformation-breakup episode in a dynamically evolving manner. By elucidating the complex interplay of electro-hydrodynmic forces and surface tension, we offer further insights into a new paradigm of interfacial instability mediated controlled microbubble manipulation for on-chip applications, bearing far-ranging scientific and technological consequences in executing designed fluidic operations in confined miniaturized environment.

physics.flu-dyn↗

arXiv2014

Wettability Induced Crack Dynamics and Morphology

Substrate wettability alteration induced control over crack formation process in thin colloidal films has been addressed in the present study. Colloidal nanosuspension (53nm, mean particle diameter) droplets have been subjected to natural drying to outline the effects of substrate surface energies over the dry-out characteristics with emphasis on crack dynamics, crack morphology and underlying particle arrangements. Experimental findings indicate that number of cracks formed decreases with increase in substrate hydrophobicity. These physical phenomena have been explained based on the magnitude of stress dissipation incurred by the substrate. DLVO predictions are also found to be in tune with the reported experimental investigations.

physics.flu-dyn↗

arXiv2012

Electrohydrodynamics within electrical double layer in a pressure-driven flow in presence of finite temperature gradients

A wide spectrum of electrokinetic studies is modelled as isothermal ones to expedite analysis even when such conditions may be extremely difficult to realize in practice. As a clear and novel departure from this trend, we address the case of flow-induced electrohydrodynamics, commonly referred to as streaming potential, in a situation where finite temperature gradients do indeed exist. By way of analysing a model problem of flow through a narrow parallel plate channel, we show that the temperature gradients have a significant effect on the streaming potential, and, consequently, on the flow itself. We incorporate thermoelectric effects in our model by a full-fledged coupling among the electric potential, the ionic species distribution, the fluid velocity and the local fluid temperature fields without resorting to ad hoc simplifications. We expect this expository study to contribute towards more sophisticated future inquiries into practical micro-/nano-fluidic applications coupling thermal field focusing with electrokinetic effects.

physics.flu-dyn↗

arXiv2010

Implications of Interactions between Steric Effects and Electrical Double Layer Overlapping Phenomena on Electro-Chemical Transport in Narrow Fluidic Confinements

In this paper, we study the non-trivial interactions between the ionic Steric effects and Electric Double Layer (EDL) overlap phenomenon on the resultant electro-chemical transport in narrow fluidic confinements. Through a comprehensive mathematical model, we demonstrate that more prominent Steric effects may result in greater magnitudes of the channel centerline potential. However, since the magnitude of the zeta potential also gets perpetually enhanced with Steric interactions, this phenomenon cannot by be considered by itself as a trivial interpreter of an augmentation in the extent of the effective EDL overlap. Further investigations in this regard, however, do reveal an intricate coupling between EDL overlap phenomenon and finite ionic size effects, so as to result in an effective enhancement in the extent of EDL overlap, far beyond what is predicted by classical electrochemical considerations. Insights are also provided on the possible implications of the intricate interactions of the underlying physico-chemical mechanisms on the design of future- generation nanofluidic devices.

cond-mat.mes-hall↗