Formation of microfibers and nanofibers by capillary-driven thinning of drying viscoelastic filaments

Abstract

Recent experiments have shown that it is possible to draw controlled networks of very uniform polymeric microfibers and nanofibers by exploiting elasto-capillary thinning and stretching of macroscopic liquid bridges (Harfenist et al., Nano Lett., 4(10),2004). We develop a model of this process that describes the simultaneous visco-elasto-capillary thinning, stretching and drying of cylindrical filaments of polymer solutions. A one dimensional formulation is developed using a slender body approximation to the inertialess equations of motion. The evolution in the kinematics, stress and composition of differential material elements are computed by numerical simulation using an explicit Eulerian scheme. The polymer rheology is described by a single mode Giesekus model with a concentration-dependent shift factor that accounts for compositional dependence of the zero shear rate viscosity and the relaxation time of the fluid. An averaged mass transfer coefficient accounts for evaporation by conduction. The numerical simulations are compared to capillary break-up extensional rheometer (CABER) experiments and stretching experiments using high molecular weight poly(methyl methacrylate) solutions in chlorobenzene with a range of mass fractions in the concentrated regime. Very large reductions in the radius of the thinning thread - spanning two to three orders of magnitude - are attainable by careful control of the mass transfer rate and the stretching/thinning dynamics.

(cont.) Simulations show that the fiber formation process can be conveniently parameterized by three dimensionless groups which compare, respectively, the rate of capillary thinning, the rate of elastic stress relaxation, the rate of stretching and the rate of solvent evaporation. Guidelines to design polymer solutions are provided and a theoretical prediction of the equilibrium diameter based on simple scaling is derived.