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

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.