Surface Tension by Image Analysis: Fast and Automatic Measurements of Pendant and Sessile Drops and Bubbles

Abstract

New developments in the method of axisymmetric drop shape analysis by image analysis have reduced the computational requirements of this method to the PC level. In this work, an edge detection procedure that gives sub-pixel resolution is combined with general interpolation and smoothing techniques into a PC program that is able to measure pendant and sessile drops and bubbles in typically 5 seconds. To obtain good initial estimates for the shape parameter, β, and size parameter, R 0, of a drop, two alternative methods are used, dependent on the drop's contact angle with the horizontal plane. These methods employ polynomial expressions determined from theoretical profiles. A second order optimization procedure gives a minimized residual of 0.1 to 0.4 pixel units in 1-2 runs. The surface tension of water was determined by both pendant and sessile drops and sessile bubbles, and gave satisfactorily consistent results. A major source of error in sessile drop measurements was found to be uneven wetting of the substrate, leading to lack of axisymmetry.