Uddipta Ghosh

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.

Electroosmosis of viscoelastic fluids over charge modulated surfaces in narrow confinements

In the present work, we attempt to analyze the electroosmotic flow of a viscoelastic fluid, following quasi-linear constitutive behavior, over charge modulated surfaces in narrow confinements. We obtain analytical solutions for the flow field for thin electrical double layer (EDL) limit through asymptotic analysis for small Deborah numbers. We show that a combination of matched and regular asymptotic expansion is needed for the thin EDL limit. We subsequently determine the modified Smoluchowski slip velocity for viscoelastic fluids and show that the quasi-linear nature of the constitutive behavior adds to the periodicity of the flow. We also obtain the net throughput in the channel and demonstrate its relative decrement as compared to that of a Newtonian fluid. Our results may have potential implications towards augmenting microfluidic mixing by exploiting electrokinetic transport of viscoelastic fluids over charge modulated surfaces.

Contact line dynamics of electroosmotic flows of incompressible binary fluid system with density and viscosity contrasts

We consider electrically driven dynamics of an incompressible binary fluid, with contrasting densities and viscosities of the two phases, flowing through narrow fluidic channel with walls with predefined surface wettabilities. Through phase field formalism, we describe the interfacial kinetics in the presence of electro-hydrodynamic coupling and address the contact line dynamics of the two-fluid system. We unveil the interplay of the substrate wettability and the contrast in the fluid properties culminating in the forms of two distinct regimes—interface breakup regime and a stable interface regime. Through a parametric study, we demarcate the effect of the density and viscosity contrasts along with the electrokinetic parameters such as the surface charge and ionic concentration on the underlying contact-line-dynamics over interfacial scales.

Electro-osmosis over inhomogeneously charged surfaces in presence of non-electrostatic ion-ion interactions

In this study, we attempt to bring out a generalized formulation for electro-osmotic flows over inhomogeneously charged surfaces in presence of non-electrostatic ion-ion interactions. To this end, we start with modified electro-chemical potential of the individual species and subsequently use it to derive modified Nernst-Planck equation accounting for the ionic fluxes generated because of the presence of non-electrostatic potential. We establish what we refer to as the Poisson-Helmholtz-Nernst-Planck equations, coupled with the Navier-Stokes equations, to describe the complete transport process. Our analysis shows that the presence of non-electrostatic interactions between the ions results in an excess body force on the fluid, and modifies the osmotic pressure as well, which has hitherto remained unexplored. We further apply our analysis to a simple geometry, in an effort to work out the Smoluchowski slip velocity for thin electrical double layer limits. To this end, we employ singular perturbation and develop a general framework for the asymptotic analysis. Our calculations reveal that the final expression for slip velocity remains the same as that without accounting for non-electrostatic interactions. However, the presence of non-electrostatic interactions along with ion specificity can significantly change the quantitative behavior of Smoluchowski slip velocity. We subsequently demonstrate that the presence of non-electrostatic interactions may significantly alter the effective interfacial potential, also termed as the “Zeta potential.” Our analysis can potentially act as a guide towards the prediction and possibly quantitative determination of the implications associated with the existence of non-electrostatic potential, in an electrokinetic transport process.

Electrokinetic Maneuvering of Bubble-Driven Inertial Micro-Pumping Systems

The pumping of an aqueous electrolyte by means of an asymmetrically placed thermal resistor and electrodes is investigated in this work. This device has no moving parts and provides a continuous and controllable pulsating flow, which make it a very attractive and viable option for use on lab-on-a-chip devices. The electric field induced modulation provides a higher degree of control on the mass flow rate, by means of which one can achieve on-the-fly mass flow rate control. The pumping action is achieved by means of a high-pressure bubble generated by actuating a thermal resistor which is located asymmetrically between two reservoirs. The ends of the channel are connected to fluidic columns. The combined action of an applied electric field and a faster refilling of the shorter arm after bubble collapse essentially drive a net amount of electrolyte through the system. We study the influence of the geometric parameters like the location of the heater, channel width and the channel length apart from the physiochemical parameters like the Debye length and the applied field strength on the mass flow rate achieved through this device.

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.