Molecular and Continuum Perspectives on Intermediate and Flow Reversal Regimes in Electroosmotic Transport

Abstract

Electroosmotic slip flows in the Debye−Huckel regime were previously investigated using molecular dynamics and continuum transport perspectives (J. Phys. Chem. C 2018, 122, 9699). This continuing work focuses on distinct electrostatic coupling regimes, where the variations in electroosmotic flows are elucidated based on Poisson−Fermi and Stokes equations and molecular dynamics simulations. In particular, aqueous NaCl solution in silicon nanochannels are considered under realistic electrochemical conditions, exhibiting intermediate flow and flow reversal regimes with increased surface charge density. Electroosmotic flow exhibits plug flow behavior in the bulk region for channel heights as small as 5 nm. With increased surface charge density, constant bulk electroosmotic flow velocity first increases and then it begins to gradually decrease until flow reversal is observed. In order to capture the flow physics and discrete motions within electric double layer accurately, the continuum model includes overscreening and crowding effects as well as slip contribution and local variations of enhanced viscosity. After extraction of the continuum parameters based on molecular dynamics simulations, good agreement between simulation results and continuum predictions are obtained for surface charges as large as −0.37 C/m 2 .

Slip Effects on Ionic Current of Viscoelectric Electroviscous Flows through Different Length Nanofluidic Channels

“…Second, velocity slip also depends on the near surface hydrodynamics that earlier mentioned electroviscosity and viscoelectic effects inside the EDL are expected to strongly alter, e.g., the boundary slip. Specifically, recent molecular dynamic (MD) simulations measure substantial decrease in slip length with increasing surface charge, , opposing earlier literature presenting negligible slip length change at the corresponding low surface charges. , Just recently, Rezaei et al presented that such discrepancy is due to neglecting local viscosity and the resulting velocity gradient near the surface, which is different than the bulk behavior. Slip length calculated from bulk velocity gradient is found to decrease, but if the local gradient is considered, slip length remains mostly constant at low surface charge values.…”

“…For such a case, we incorporated the above given different mechanisms into the single parameter of f based on eq , similar to earlier literature. In Figure , we presented the viscosity distribution estimated by eq using corresponding parameters from the MD results , ( d = 0.4 nm, β = 4, and γ = 1.6) with the most common f = 1 × 10 –15 m 2 /V 2 based on the local charge density calculated by PNP for the 10 nm height channel at 10 mM. Next, we calculated the f value for eq to estimate a similar viscosity variation for the same charge density distribution.…”

The importance of specifically adsorbed ions for electrokinetic phenomena: Bridging the gap between experiments and MD simulations

“…S2 shows that the resulting flow is still within the linear response regime). 31,34,51,52 Note that simulating EOF is preferred over SC for our purpose since the relation between the streaming current and the ζ-potential depends on the channel height [Eq. (4)], which is not uniquely defined for atomistic surfaces with a finite roughness.…”

Overscreening, Co-Ion-Dominated Electroosmosis, and Electric Field Strength Mediated Flow Reversal in Polyelectrolyte Brush Functionalized Nanochannels

“…For example, changing the sign of the zeta potential or the surface charge density of the channel wall , or controlling the buffer pH value may help to control the EOS flow direction in some simplified systems. Reversed and patterned flows inside a nanochannel can also be induced by suitable surface modifications. , These experimental studies have been supplemented by atomistic simulation studies on modulating nanochannel EOS flows by changing the surface charge density or hydrophobicity of the channel wall. , All of these modifications eventually change the mobile ion distribution in the nanochannel, which in turn leads to different flow directions inside the nanochannel. Therefore, in order to control the direction of the EOS transport in a nanochannel by merely changing the strength of the driving electric field, one needs to trigger a situation where the ion distribution (or more generally, the mechanism that drives the EOS transport) gets altered by the variation in the strength of the electric field.…”