Static and Dynamic Properties of the Electrical Double Layer Near Amorphous Silica: Relevance for Device Design

B. Shi, H. Zhang, A. Hassanali, Y.K. Shin, C. Knight, S.J. Singer
Ohio State University, US

Keywords: amorphous silica, nanofluidics, microfluidics, electrical double layer, electroosmotic flow

Summary:

Amorphous silica (“glass”) is commonly used to fabricate micro- and nano-fluidic devices. Using an atomistic model, we have studied the static and dynamic properties of the electrical double layer near the water/amorphous silica interface. The heat of immersion (Delta-H to bring water in contact with a hydroxylated silica surface), EOF and ion current are in good agreement with experiment, and the model compares favorably with ab intio molecular dynamics. We obtain a microscopic picture of the Stern layer, elucidate local dielectric properties of the interface, and compare microscopic and continuum hydrodynamic views of electroosmotic flow. Our studies indicate 1) electroosmotic flow persists up to the silica surface, in contrast to the usual Stern model, 2) the layer of water polarization charge, which always exists when an applied electric field is not parallel to the surface, is about 1nm in thickness, comparable to the width of the double layer, 3) regions of variable hydrophilic and hydrophobic character depending on the local density of silanol groups (also verified using ab initio MD), and 4) rich interplay between electrokinetic and adverse pressure effects in complex geometries, such as a nano-nozzle.