Reliable determination of contact angle from the height and volume of sessile drops

Contact angle is an important parameter in characterizing the wetting properties of fluids. For this reason, accurate measurement of the contact angle at liquid-solid interfaces is of great importance in industrial and scientific applications. The most common method for measuring the contact angle is to obtain it directly from the profile of a sessile drop. Though convenient, this method suffers from certain inherent drawbacks. Here, we describe an alternative method that uses the height and volume of a sessile drop as constraints to construct its profile by numerical integration of two parametric differential equations. The integration yields, self consistently, the average value of the contact angle along the entire contact line as well as the footprint radius of the drop and its crown radius of curvature. As a test case, the new method is used to obtain the contact angle of pure water on two different substrates, Teflon and Lucite. For each substrate, four drops ranging in volume from 10. to 40 mu l are used. The computed contact angles are consistent across the four different drop sizes for each substrate and are in good agreement with the literature values. (C) 2019 American Association of Physics Teachers.