Dynamic contact angles: Temperature dependence and the influence of the state of the adsorption film

Abstract

The reason for the non-zero value of contact angles (θ) of water on the surface of hydrophilic solid bodies is discussed. A process of wetting with water is considered as the process of activated transition of water molecules from the state of adsorption film (adsorbed from water vapor) to the state of bulk water near the surface of a polar solid body, the concept of Blake and Haynes being used to describe the dynamic wetting process. The results of experimental investigations of the temperature dependence of dynamic contact angles are presented. The dependence of contact angle on the velocity (cos θ - lg V) is calculated from the experimental data on capillary rise of water in cylindrical glass capillaries with a cross-section of 104–108 μm. The experiments on capillary rise were performed at temperatures from 20°C to 70°C for juvenile (dehydrated) and hydrated capillaries with the adsorption film of water on the surface of glass in a different state. The temperature dependence of dynamic contact angles was used to calculate the activation enthalpy and entropy for the wetting. The activation entropy for wetting was negative in sign in all cases. This fact shows that the structure of water on the glass surface is destroyed in the wetting process. The enthalpy of activation for wetting is significant and depends (as the entropy of activation does) on the state of the adsorption film of water on the glass surface. The results obtained are in good agreement with the activation model of dynamic wetting by Blake and Haynes.