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. (C) 2016 Elsevier Inc. All rights reserved.