A Static Contact Angle Algorithm and Its Application to Hydrophobicity Measurement in Silicone Rubber Corona Aging Test

To improve the accuracy of the static contact angle measurement for the corona aged silicone rubber, a static contact angle algorithm based on the least-squares circle fitting is presented, and the least-squares model, the optimization algorithm, the determination of initial values and the convergence criterion are introduced in a systematic way and at the same time the best selections are given by the systematical comparison. With regards to the numerically generated water drop profiles and the water drop images of the corona aged silicone rubber, the accuracy and the computation time of the goniometry, the theta/2 method, the circle fitting algorithm and the ADSA-P (axisymmetric drop shape analysis-profile) algorithm are analyzed and compared. The influence of the water drop volume/contact angle/profile point number on the accuracy of the circle fitting algorithm is analyzed. The results reveal that the contact angle errors increase with the water drop volume/contact angle/noise. The goniometry has the characteristics of relative low accuracy, expensive computation and large workload. The computed result of the circle fitting algorithm is very close to that of the theta/2 method, and both the two methods have relative less computation time. In the meantime, the fitted water drop edges can be obtained by the circle fitting algorithm, which can be taken as an indicator of the computation accuracy, while the theta/2 method cannot give such indicator. The ADSA-P algorithm has high accuracy and high computational complexity, and is very easy to be interfered by the noise. Therefore, it is not suited to the case of small contact angle and volume, and high interference. The circle fitting algorithm has less computation time, and is intrinsically resistant to noise. Therefore, for the case of small contact angle and volume, not the ADSA-P algorithm but the circle fitting algorithm should be selected. Therefore, the circle fitting algorithm is selected to measure the static contact angle of the corona aged silicone rubber. The critical water drop volumes and the critical contact angles of different liquids are given, which can be guaranteed for high accuracy of the static contact angle measurement.