Surface free energy analysis of electrospun fibers based on Rayleigh-Plateau/Weber instabilities

Electrospinning is an increasingly common technique used to produce fibers with a range of diameters. These electrospun fibers are used extensively in applications that exploit the material’s high surface area to volume ratio, thus requiring detailed knowledge of the surface properties of the fibers. The surface free energy of individual free standing electrospun styrene-butadiene rubber (SBR) fibers was determined here from the time-dependent break-up of long fibers driven initially by Rayleigh-Plateau/Weber instabilities. Individual free standing electrospun rubber fibers were observed to change from a cylindrical fibrous geometry to semi-spherical droplets during a time period of several days when above the glass transition temperature of the polymer. A wave-like transition from fiber to droplet was attributed to a surface tension driven break-up process occurring over a time strongly influenced by the rubber's viscosity. The surface free energy for an electrospun rubber fiber was found using a Weber approach for the free standing fibers and Diez et al theory for dynamic fluid instability of fluid ridges. Both methods lead to similar values of fiber surface free energy and were confirmed from bulk measurements exploiting Owens-Wend theory. The approach presented here is powerful as the surface free energy, indicative of the physical and chemical behavior of the fiber surface, can be determined for any fiber diameter provided the geometric break-up of the fiber is observed.