ABSTRACT
To control wettability and adhesion of polymers numerous surface modification techniques are used, such as exposure to flames and low-pressure non-equilibrium
plasma and chemical modification (grafting) [1-3]. In many cases, the wettability is regulated by changes in the chemical composition of the surfaces. But, it has also been long recognized that surface roughness can be important for wettability. Wenzel was the first who discussed the influence of surface roughness on contact angle [4]. He introduced a roughness factor, rw, into the Young equation (eq. 1) because he argued that in the case of a rough solid surface, the interfacial tensions ysv and ysl should not be referred to the geometric area, but to the actual surface area. Thus,
(1)
or
(2) For the contact angle on a rough surface he obtained
(3) where the Wenzel contact angle, 9W, is, therefore, the equilibrium contact angle on a rough solid surface having the intrinsic angle 9. It corresponds to the absolute minimum in the free energy of the system. Based on this equation it can be predicted that roughness should have a major effect on the contact angle and, hence, on the wettability of surfaces. However, one has to take into account that the Wenzel equation does not describe contact angle hysteresis and hence the relation between roughness and the phenomenon of hysteresis.
