Dynamic Contact Angle Measurement on a Microscopic Area and Application to Wettability Characterization of a Single Fiber

Dynamic contact angles on a microscopic area were measured using a specially developed system. Combining pulse injection equipment, a high-speed image capture system set on a microscope, and precise positioning stages, contact angles of typically 2 nL water droplets were measured at a repetition rate of 130 ms. Thereafter, measuring the series of the contact angles of a droplet on a planar silicon surface, contact angle hysteresis, defined as the difference between the advancing and receding contact angles, was measured, and the effect of droplet size was clarified. The system was then applied to characterize a single fiber wherein the contact angles of droplets suspended on a polypropylene fiber, typically 19 μm in diameter, were measured. Plasma treatment is often adopted to modify wettability and has directionality. By fixing a fiber while applying torsion and changing the measurement position along the fiber, the contact angles at different circumferential positions can be characterized. This effect was unraveled by comparing the contact angles on the treated side to its opposite side as well as the effect of fiber diameter.