A video-enhanced plate method for simultaneous measurements of surface tension and contact angle

The assumption of zero contact angle of the Wilhelmy plate method causes error for the surface pressure-area characteristics of some spread monolayers. To avoid this problem, a video-enhanced plate method is developed to monitor the dynamic surface tension and contact angle simultaneously. According to this method, the profile of the air-water interface adjacent to a flat plate is captured by an image digitization technique. The surface tension and contact angle are obtained from the best fit between the edge coordinates of the captured image and a theoretical equation derived from the Young-Laplace equation. Preliminary results show satisfactory agreement between the curve of the air-water interface obtained from experimental data and the theoretical curve for obtained values of surface tension and contact angle. Twenty tests for pure water at 25°C give an average surface tension 72.0±0.2 mN/m, which confirms the accuracy of this method. A preliminary dynamic test also indicates that the effect of the contact angle in the Wilhelmy method can be corrected by dividing the apparent surface tension obtained from the Wilhelmy method by the cosine of contact angle measured by the present method.