Contact Angles of Liquid Drops on Low-Energy Solid Surfaces

There have been many experiments that have determined the wetting behavior of liquids on low-energy solid surfaces. These experiments show that, in general, the contact angle, theta, measured through the liquid, increases with increasing liquid-vapor surface tension, sigmalv. Surprisingly, in experiments with some such surfaces, the contact angle of a drop appears to depend almost entirely on the liquid-vapor surface tension and very little on the other properties of the liquid. To gain some insight into this phenomenon, we have used a generalized van der Waals theory to model a diverse series of liquids on a low-energy substrate and to calculate the corresponding surface tensions and contact angles. We find that it is indeed possible to reproduce behavior similar to that seen in experiments, with the data for cos theta plotted versus sigmalv falling in a narrow band which could almost be interpreted as a smooth curve. The width of the band in our calculation depends on the ranges of the model parameters and on the details of the molecular interactions.