Thermodynamics of contact angles on rough, heterogeneous surfaces

Contact angle is of fundamental importance in many solid-liquid interfacial phenomena. However, theoretical studies/modeling carried out to date have been limited to several idealized surface configurations, on either rough or heterogeneous surfaces. This thesis presents a systematic study on the thermodynamics of contact angles on general rough, heterogeneous surfaces. A concept of liquid front was used to simplify the thermodynamic treatments and a theoretical model based on this concept was proposed. Advancing, receding and system equilibrium contact angles were obtained and correlated to surface topography and heterogeneity. The system equilibrium contact angles (theta ES) can be generally expressed as a function of surface roughness factor (delta) and the Cassie contact angle (thetaC): costheta ES = deltacosthetaC. This expression can be reduced to thetaES = thetaw (Wenzel contact angle) for rough but homogeneous surfaces, and thetaES = theta C for heterogeneous but smooth surfaces.;A parameter called surface feature factor, o, was proposed to divide surfaces into three categories: roughness-dominated, heterogeneity-dominated and general.;The effect of irregular topography, multiple material components, and energy barriers on contact angles was also studied in this thesis. The concepts of major and minor energy barriers were defined and their values were calculated using the model proposed in this thesis. Several common observations in contact angle measurements, including the fluctuation of measured contact angles, the poor reproducibility of receding contact angles and the "stick-slip" phenomenon, were studied through an analysis on energy barriers. The effect of surface impurities on contact angles was also discussed.;The results obtained in the present study are more general than those from other existing models, and the idealized surfaces previously studied can be treated as special cases of the present model. The present model agrees well with the experimental results from the literature and can explain the common observations in contact angle phenomena. (Abstract shortened by UMI.).