Measurement of dynamic/advancing/receding contact angle by video-enhanced sessile drop tensiometry

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Abstract
A sessile drop tensiometer enhanced by video-image digitization is designed for the experimental measurement of dynamic/advancing/receding contact angle. A collimated light beam passes through the sessile drop of liquid and a silhouette of the drop is created. The equipment video images the silhouette, digitizes the image, and locates the edge coordinates of the drop. A new technique, replacing the classical selected plane method, is developed to obtain the values of capillary constant and the radius of curvature at apex from the edge coordinates of digitized drop profile. Four parameters (location of apex, radius of curvature at apex, and the capillary constant) are calculated from the best fit between the edge coordinates and the theoretical curve obtained from the Laplace equation. The contact angle is then obtained from the location of the air/solid interface and the best-fitted sessile drop profile. By controlling the humidity of air phase surrounding the drop, this technique can measure the advancing and receding contact angles and monitor the rate of advancing and receding of the three-phase line simultaneously. This technique works well on contact angle measurement for sessile drops with or without an equator. Preliminary studies on the dynamic contact angle have been made for water drops on paraffin, polymethylmethecrylate, and glass. The technique is capable of giving contact angle of 0.2° precision.