Assessment of direct image processing methods to measure the apparent contact angle of liquid drops

Four methods to measure apparent contact angles of non-axisymmetric drops are analyzed critically. In particular, methods based on local edge interpolation or fitting are compared with direct image processing methods, based on image filtering with a suitable sampling mask. The methods performance is evaluated based on the dynamic contact angle of a dilute polymer solution drop impacting on a solid substrate. Results show that direct image processing methods yield an overall better performance, with a good compromise between measurement accuracy and computational cost, and have a limited dependence on the choice of parameters. Polynomial edge fitting yields the worst performance, both in terms of accuracy and in terms of computational cost.