Kinetic friction of nonwetting drops

Abstract

Numerous engineering applications have been proposed to exploit the load-carrying and non-contact nature of noncoalescing and nonwetting systems. One such application is a lab-on-a-chip , or LOC, in which liquid samples would be delivered from point-to-point by sliding over a film of air without requiring either the large driving forces required to pump liquid through a microchannel or liquid-solid contact that could lead to sample-to-sample contamination. Due to the axisymmetry of the flow fields in both the lubricating gas and droplet associated with a stationary nonwetting droplet, such a situation has a vanishing coefficient of static friction. However, once motion is imparted, droplet deformation requires that a force be applied to sustain such motion.

The program of research in this dissertation focuses on investigating the lubrication force between a drop of silicone oil and a moving unwetted substrate due to the presence of a gas lubricating film driven by a rotating disk. The frictional (or lubrication) force was measured using an optical-lever technique as a function of: (1) linear velocity of the moving solid; (2) relative displacement of the drop toward the solid; (3) drop volume; and (4) viscosity. The data reveal an increase in magnitude of the measured force with either increasing relative squeezing of the drop against the glass or increasing speed of the rotating disk. Contrary to initial expectations, no pattern could be isolated regarding drop volume or viscosity of the oil. The experimental data collected will serve to validate numerical work as further models are developed.