Contact angle and wetting hysteresis measurements by digital image processing of the drop on a vertical filament

The paper presents a novel approach to measure contact angles, wetting hysteresis and time evolution of drop parameters for the geometry of the drop on a vertical filament. The receding and advancing contact angles are determined by solving the Laplace equation of capillarity numerically and using as input parameter the liquid surface tension. The accuracy in contact angle measurement on well prepared filaments with the same diameters was not worse than 0.5°. At the same time, the difference of contact angle values between the different runs for the same filament was not higher than 0.08°, illustrating the high reproducibility of the experiment. The technique described allowed to detect the considerable effect of filament diameter on the advancing contact angle and consequently on the wetting hysteresis. The method performs particularly well for small contact angles below 20°, where other techniques, such as the sessile drop method, produce rather large errors.