Contact Angle Determination on Hydrophilic and Superhydrophilic Surfaces by Using r-θ-Type Capillary Bridges.

To avoid the restrictions of the captive bubble and the Wilhelmy plate techniques, a method was introduced for contact angle measurements under equilibrium conditions. It enables to determine even ultralow contact angles with high precision without prewetting the investigated surface because in this case, the capillary bridge of the test liquid is formed from a pendant drop and used as a probe. The contact angle is determined from the measured capillary force and liquid bridge geometry by using Delaunay's analytical solution. The method was experimentally proved to be valid. As a demonstration, contact angles less than 1° were measured with the uncertainty down to 0.1° on lightly corroded glass surfaces. Moreover, a new observation was obtained in complete wetting situations: the receding contact line starts to advance again during the increase of the bridge length. The contact angle is much lower in this readvancing phase compared to the advancing and receding values because the contact line finds prewetted surface in front of itself. Further advantage of the method is that the existing contact angle goniometers can be developed further into the presented measurement setup.