Control of Nanoscale Flow in Thin Wetting Films by Electric Field

Jairus Kleinert, Sejong Kim, and Orlin D. Velev. Chemical and Biomolecular Engineering, North Carolina State University, Campus Box 7905, Raleigh, NC 27695-7905

We report a novel method for manipulating flow in wetting films of nanoscale thickness using electric fields. Aqueous films of tens of nanometers thickness were formed on hydrophilic mica surfaces under saturation humidity conditions. A linear relationship was established between the fluorescence intensity of dye in a film and film thickness. On this basis, we estimated the films formed to be less than 100 nm thick. Applying an electric field engendered tangential flow in the film by DC electroosmosis. The direction and speed of the film flow, visualized with fluorescent markers, were readily controlled by the external DC electric field. The film flow rate was characterized as a function of ionic strength, pH of the fluid, and external electric field. The resulting flow characteristics were consistent with the theory of electroosmosis. To manipulate the flows in a controlled geometry, we selectively hydrophobized the substrate by microcontact printing to make two-dimensional “virtual wall” channels. Flow velocity profiles of these confined nanoflows appear to be complicated and different from the profiles seen in conventional microfluidics and are currently being analyzed. This nanofluidics methodology will provide a basis for single molecule transport and manipulation as well as an improved nanoscale and molecular level understanding of the interactions of fluids with surfaces.