The fuzzy interface of a drop

A novel image processing algorithm to measure the interface curvature and the contact angle of sessile drops is presented. The algorithm is based on the analysis of the local intensity gradient, instead of fitting the image edges with curves; this results into a set of convolution masks, tailored to the specific feature that is being sought for (curvature, contact angle). The algorithm was used to analyse the impact of a water droplet onto a hydrophobic substrate (a glass slide coated with a fluoropolymer solution); the proposed algorithm was shown to perform well, even when frames were characterized by very low contrast. Dynamic contact angle measurements were carried out by tracking the three-phase-point position along the fluid-fluid interface without previous substrate detection, using the mirror reflection of the drop on the substrate as a reference. Finally, a generalization of the algorithm can be used as corner detector.