The microstructure of glass fiber surfaces treated with silane and fluorocarbon monolayers has been investigated by use of dynamic contact angle analysis. Advancing and receding contact angles on single glass fibers as small as 10 nm in diameter were measured at low meniscus velocities by using the Wilhelmy method. A unique aspect of this work is that the dynamic wetting of fibers, in the limit of small diameters, takes the form of contact line pinning. This is induced by surface chemical inhomogeneities with domain sizes comparable to the fiber diameter, giving direct evidence regarding the surface heterogeneity of coated fibers. Several fiber surface treatments were investigated including a silane, a fluorochemical surfactant, and oxidative cleaning agents. Clean glass fibers of varying diameters were prepared from molten glass. From the fiber diameter dependence of contact line pinning, we determined that monolayers of octadecyltrichlorosilane (OTS) adsorb in patches with domains on the order of microns. To confirm our analysis, larger diameter fibers were studied, leading to a suppression of contact line pinning due to an averaging effect over the larger perimeter, although the classical contact angle hysteresis was still present. The results for OTS treated fibers are direct experimental evidence of the free energy barriers predicted to be the thermodynamic basis of contact angle hysteresis caused by surface chemical inhomogeneity. Analysis of the domain sizes and other surface features in terms of wettability should be possible for a wide variety of fiber coatings and surface treatments using this method. © 1990, American Chemical Society. All rights reserved.
