Date of Award
Master of Science in Material Science Engineering (MSMatSE)
Prof Rashid Chebbi
Dean of Graduate studies
In view of young’s equation, in which the contact angle “ϑ” and the liquid surface tension γlv are the only measurable parameters, the contact angle has been widely conceived as a thermodynamic quantity. Accordingly “ϑ” has been the main focus of most theoretical and experimental investigations. The rate of change in the contact angle has been commonly used as the relevant parameter of spreading dynamics in spite of difficulties associated with contact angle measurements that are well recognized in the literature. Considering that the velocity of the contact line is the pertinent quantity for spreading, it is therefore reasonable to regard the change in the contact area as the flux of the process. It is only recently that a rational link has been made between the contact angle and the contact area using spherical cap approximation. We have introduced a new measuring parameter for wettability based on the liquid/solid contact area. The term “contact ratio” has been coined to denote the new parameter. The contact ratio is defined as the ratio of the spreading contact area between the liquid and the solid surface to the surface area of the spherical drop before spreading. Measurements of low-rate dynamic contact radius and low-rate dynamic contact angles for various liquid/solid systems were carried out using the ADSA-P technique. The liquid drop volume and solid/liquid contact radius were also measured simultaneously. Subsequently, the contact ratio is determined. The theoretical relation between the contact ratio and the contact angle is based on spherical cap approximation. The results show that even with a high value of contact angle, there is a good agreement between the theoretical relation and the experimental values. Since the contact angle is well recognized as a wettability parameter because of its unique value, hence the contact ratio may also be considered as a wettability parameter for its unique value of a specific system. Therefore, contact ratio can be presented as a new concept in measuring wettability alternative to contact angle. Moreover, contact ratio may provide a precise measure of wettability than contact angle.
The spreading kinetics (in terms of dynamic constant ratio) of different viscosity (100 cP, 500 cP, 1000 cP, 10000 cP) silicon oils (PDMS) on three different solid surfaces, namely soda-lime glass, poly(methyl methacrylate) (PMMA), and polystyrene (PS) have been studied. A fixed liquid volume of 1.5 μL was used. The kinetics measurements of silicon oils with various viscosities were compared with theoretical models presented by Diez et al. (1994) and Chebbi (2000). The results show that Chebbi’s model fits the experiment data better than that of the Diez et al. model. The results show, for the spreading kinetics of silicon oils with various viscosities on the same type of solid surface, that the lower liquid viscosity, the faster the spreading. Overall, the results suggested that as the silicon oil viscosity increases, the viscous force becomes more effective where the liquid-solid intermolecular force seems to not have effect on the kinetics of spreading. As silicon oil viscosity decreases, other spreading forces, especially the liquid-solid intermolecular forces, might play a significant role in the spreading kinetics.
Adnan” Herbawi, Abdelhafiz “Moh’d, "Investigations of the kinetics of liquid droplets on soil substrates using a novel approach: Contact ratio" (2005). Theses. 344.