Physical origins of apparently enhanced viscosity of interfacial fluids in electrokinetic transport

Abstract

A key concept in classical electrokinetic theories is that the viscosity of interfacial fluids is much higher than that of bulk fluids, and this concept is indirectly supported by experimental evidence and molecular dynamics simulations. However, a universal mechanism that encompasses the breadth of experimental evidence is still lacking. Here we show, using molecular dynamics simulations, that the "apparent" thickening of interfacial fluids in electrokinetic transport near molecularly smooth surface originates mainly from the fact that ion-wall interactions are not accounted for in the hydrodynamic model of classical electrokinetic theories. Specifically, strong ion-wall interactions cause intermittent adsorption of ions on charged walls, and this in turn leads to loss of driving force for flow and screening of fluid flow by the adsorbed ions. Although not considered in the classical electrokinetic theories, these effects can significantly suppress electrokinetic transport. Consequently, when the classical theories are used to interpret the electrokinetic data, the viscosity of interfacial fluids appears to be greatly enhanced even if their material viscosity is similar to that of the bulk fluids. This mechanism for the apparent thickening of interfacial fluids is applicable to electrokinetic transport near any type of charged surface.

Bridging the gap between atomistic phenomena and continuum behavior in electrochemical energy storage processes.

“…Otherwise those properties must be inferred via other means, such as the Widom insertion method for the chemical potential [108]. In addition, previous MD simulations of the EDL [27,75,94,76,49,111,24,89,16,113,110] have generally dealt with large ion concentrations and low surface charge densities, hence narrow EDLs with a single dominant "Stern layer" peak more nearly consistent with PB theory. In the current work, MD simulations were performed for a large range of ion concentrations and for surface charge densities many times greater than is typical of EOF.…”

“…As such, it has been used rather extensively in modelling of electro-osmotic flow (EOF) [27,75,94,76,49,111] and to treat the higher charge densities of importance in EDL capacitors [17,24,103]. However, MD simulations entail considerable computational cost, making them impractical for treatment of time and length scales found in many applications.…”

“…This step is in the direction of the more general MD studies of EOF [27,75,94,76,49,111] which also include the additional complexity of polar solvents, atomistically structured walls, and cross flow. An added complication for EOF is the apparent enhanced visocity within the EDL [111].…”

“…This step is in the direction of the more general MD studies of EOF [27,75,94,76,49,111] which also include the additional complexity of polar solvents, atomistically structured walls, and cross flow. An added complication for EOF is the apparent enhanced visocity within the EDL [111]. In our earlier effort to model EOF [67], we achieved favorable velocity-profile comparisons with MD simulation [75] by assuming a uniform viscosity, and even better results when applying the modified Chapman-Enskog model for densitydependent viscosity [10].…”

Theoretical and experimental studies of electrified interfaces relevant to energy storage

“…23,24 However, the assumption of a uniform dielectric constant in the 3CM is potentially problematic close to the surface. While MD studies of electrolyte solutions near electrified interfaces have been performed using both the 3CM 8,24 and MSM, 3-7,10,24,37−40 the consequence of the 3CM approximation has not been quantified. In the work discussed here, the 3CM is compared against the higher fidelity MSM using various metrics, such as capacitance, spatial variations in density, electric field, and electric potential, and nearwall fluid structure.…”