Contact angle hysteresis on smooth and homogenous surfaces in gravitational fields

In an experiment conducted on a space shuttle flight, contact angle hysteresis was observed for the water-glass system. Surface heterogeneities have usually been taken as the cause of the observed hysteresis. In order to examine if other factors such as adsorption could be the cause of the observed hysteresis, contact angles on homogenous, smooth surfaces of glass were examined in ground-based experiments. Ten glass cylinders with different inside diameters were constructed with a capillary tube in the centre of each cylinder. The cylinders were filled with prepared water and the meniscus heights inside the capillary tube and in the glass cylinder were measured. Using the values of these parameters, the pressure in the vapour and the contact angle at the three-phase line of each interface were calculated from the equilibrium conditions. In each of the cylinders, a difference in the contact angle at the three-phase line of the capillary and the three-phase line of the cylinder was observed. This contact angle hysteresis cannot be explained by surface heterogeneity or line tension effect. The observed contact angle hysteresis is predicted to be the result of the pressure profile in the system. This suggests adsorption and its extraordinary sensitivity to vapour-phase pressure as the source of the contact angle hysteresis.