Study of static contact angle measurement method based on least squares fitting

In order to get accurate measurement of static contact angle of silicone rubber, the method based on the least squares fitting is proposed to obtain static contact angle through fitting the globule edge. This method fits the globule edge by the least square method according to the rule that the globule edge is round if the globule only has little water. Then, Levenberg-Marquardt algorithm is used to calculate the least squares fitting, the formula used in the fitting procedure is deduced theoretically, the algorithm flow chart is given, and the program is implemented. For simulated globule images and real globule figures, the protractor method and the fitting method proposed by the paper is used respectively to get the static contact angle. The results show that the proposed fitting method can effectively reduce the subjective influence by the protractor method of the measurement results, and it can also improve the measurement accuracy of the static contact angle in great degree. This paper studies the effect of the water volume on the accuracy of this paper´s algorithm, the results show that the error increases gradually with the increased volume.