A goniometric mask to measure contact angles from digital images of liquid drops

A computational scheme to measure the inclination angle of apparent edge curves in binary images is proposed. The main concept of the scheme is the emulation of a goniometer tool, which does not require edge fitting with analytic curves for the angle calculation. The algorithm produces an output matrix with same size of the input binary image, where the computed angle values are placed in the same locations of edge pixels. The scheme is tested both on a synthetic binary image displaying an object with irregular contour, and on a stack of images displaying a water droplet impacting on a hydrophobic substrate. The comparison between analytical and calculated angles shows that results depend both on the mask size and, in particular, on the image resolution.