Control of Gas Flow in Narrow Channels Using an Electric Field To Modify the Flow Boundary Condition

Abstract

We demonstrate that the resistance to the flow of gas in a narrow channel can be controlled in situ by using an electric field to modify a thin organic surface film. The flow was determined indirectly by measurement of the lubrication forces between a glass sphere and a gold-coated plate. The plate was coated with a self-assembled monolayer of ω-COOH thiol. Application of the field to the ω-COOH plate caused a large decrease in lubrication force whereas application of the field to a ω-CH3 plate did not. The lubrication force reverts back to the no-field value when the field is removed. We show that the field causes a larger decrease in the roughness of the ω-COOH plate compared to the ω-CH3 plate. The observed correlation between a decrease in lubrication force and roughness suggests that the mechanism for increasing the flow is decreasing the surface roughness: the roughness affects the tangential momentum accommodation coefficient, which in turn affects the boundary condition for gas flow at the solid. The proposed mechanism for the change in roughness is that the field applies a torque to the dipole on the ω-COOH group, causing a change in average tilt, which decreases the roughness. This research demonstrates a simple alternative method to mechanical valves in microscopic devices: gas flow is controlled in situ by molecular scale changes to the wall.