Analytical investigations on the effects of substrate kinetics on macromolecular transport and hybridization through microfluidic channels

Transient alternating current electroosmotic flow of a Jeffrey fluid through a polyelectrolyte-grafted nanochannel

Comparison of the velocity amplitude for Newtonian, Maxwell and Jeffrey fluids in PE-grafted and rigid nanochannels.

Filling of charged cylindrical capillaries

We provide an analytical model to describe the filling dynamics of horizontal cylindrical capillaries having charged walls. The presence of surface charge leads to two distinct effects: It leads to a retarding electrical force on the liquid column and also causes a reduced viscous drag force because of decreased velocity gradients at the wall. Both these effects essentially stem from the spontaneous formation of an electric double layer (EDL) and the resulting streaming potential caused by the net capillary-flow-driven advection of ionic species within the EDL. Our results demonstrate that filling of charged capillaries also exhibits the well-known linear and Washburn regimes witnessed for uncharged capillaries, although the filling rate is always lower than that of the uncharged capillary. We attribute this to a competitive success of the lowering of the driving forces (because of electroviscous effects), in comparison to the effect of weaker drag forces. We further reveal that the time at which the transition between the linear and the Washburn regime occurs may become significantly altered with the introduction of surface charges, thereby altering the resultant capillary dynamics in a rather intricate manner.

Electric double-layer interactions in a wedge geometry: change in contact angle for drops and bubbles

In this paper, we provide a theory to pinpoint the role of electric double layer (EDL) interactions in governing the contact angle of an electrolyte drop on a charged solid in air or a bubble on a charged surface within an electrolyte solution. The EDL interactions are analytically solved by representing the three phase contact line as a wedge edge, with the wedge being formed by the solid-liquid and the air-liquid interfaces, and calculating the corresponding Maxwell stresses. We demonstrate that the EDL effects induce an "electrowetting-like" behavior, resulting in a lowering of the contact angle. As a specific example, we use this model to analyze the effect of added salt on preformed surface nanobubbles, and find, in contrast to what has been reported earlier, that even for most moderate conditions, added salt may have remarkable effect in altering the contact angle in preformed surface nanobubbles.

Influence of streaming potential on the transport and separation of charged spherical solutes in nanochannels subjected to particle-wall interactions

In this work, we theoretically investigate the implications of streaming potential on the transport and size-based separation of charged solutes in nanoscale confinements. By employing a regular perturbation analysis, we demonstrate that the consideration of streaming potential establishes a new paradigm of size-based separation of charged solutes in nanochannels. Depending on the sizes of the particles being handled, we establish two distinctive separation regimes. For smaller particles with significantly large electrophoretic mobilities, electrophoretic transport mechanisms predominantly influence the solutal transport characteristics, whereas for larger particles the combined pressure-driven and back electroosmotic transport mechanisms essentially dictate the resultant separation characteristics. The extent of improvement in separation characteristics, on account of the consideration of streaming effect, largely depends on the consideration of particle-wall interactions. For cases without wall effects, the streaming potential may induce dramatic enhancements in the resolution of separation for small particles by exploiting optimal combinations of zeta potential values and relative thicknesses of the electrical double layers. However, with wall effect considerations, similar combinations of zeta potential and electrical double-layer thicknesses may give rise to dramatic improvements in the separation characteristics over a much wider range of particle sizes by interacting nontrivially with the streaming potential effects. Such confluences may be exploited in practice for designing efficient nanofluidic separation systems.

Transport and separation of charged macromolecules under nonlinear electromigration in nanochannels

In this work, we theoretically investigate the implications of nonlinear electrophoretic effects on the transport and size-based separation of charged macromolecules in nanoscale confinements. By employing a regular perturbation analysis, we address certain nontrivial features of interconnection among wall-induced transverse migrative fluxes, electrophoretic and electroosmotic transport, confinement-induced hindered diffusive effects, and hydrodynamic interactions in detail. We demonstrate that there occurs an optimal regime of influence of the nonlinear electrophoretic effects, within which high values of separation resolution may be achieved. This size-based optimal regime, however, can be effectively exploited only for nanochannel flows, as attributed to the strong electric double layer interactions prevalent within the same.

Generalization of interfacial electrohydrodynamics in the presence of hydrophobic interactions in narrow fluidic confinements

We show that the interfacial electromechanics in narrow fluidic confinements exhibits a universal dependence with the intrinsic surface-wettability characteristics, independent of the details of the bulk flow actuating mechanisms. Towards this proposition, we develop a generalized mesoscale model, which is extensively tested for combined electro-osmotic and pressure-driven nanochannel flows. Agreement with the molecular dynamics simulations is found to be quantitative.