Robert J. Fisher

Bio: Robert J. Fisher is an academic researcher from University of Delaware. The author has contributed to research in topics: Spinning & Instability. The author has an hindex of 4, co-authored 6 publications receiving 351 citations.

A theory of isothermal melt spinning and draw resonance

Abstract: The mechanics of isothermal melt spinning are studied for a viscoelastic liquid with a power law viscosity and a constant shear modulus. Steady state velocities and stresses are in agreement with experiment. The onset of the draw resonance instability, the magnitude of diameter fluctuations in the unstable region, and a second stable region at high draw ratio are predicted accurately.

Mechanics of nonisothermal polymer melt spinning

Abstract: Spin line tension, diameter attenuation, and the onset of the draw resonance instability are determined for a viscoelastic polymer melt, taking into account the changes in physical properties resulting from cooling of the filament. The theory predicts the experimentally observed stabilization of very short and very long filaments.

Instabilities in viscoelastic flows

Abstract: Viscoelastic instabilities are of practical importance, and are the subject of growing interest. Reviewed here are the fresh developments as well as earlier work in this area, organized into the following categories: instabilities in Taylor-Couette flows, instabilities in cone-and-plate and plate-and-plate flows, instabilities in parallel shear flows, extrudate distortions and fracture, instabilities in shear flows with interfaces, instabilities in extensional flows, instabilities in multidimensional flows, and thermohydrodynamic instabilities. Emphasized in the review are comparisons between theory and experiment and suggested directions for future work.

The stretching of an electrified non-Newtonian jet: A model for electrospinning

Abstract: Electrospinning uses an external electrostatic field to accelerate and stretch a charged polymer jet, and may produce ultrafine “nanofibers.” Many polymers have been successfully electrospun in the laboratory. Recently Hohman et al. [Phys. Fluids, 13, 2201 (2001)] proposed an electrohydrodynamic model for electrospinning Newtonian jets. A problem arises, however, with the boundary condition at the nozzle. Unless the initial surface charge density is zero or very small, the jet bulges out upon exiting the nozzle in a “ballooning instability,” which never occurs in reality. In this paper, we will first describe a slightly different Newtonian model that avoids the instability. Well-behaved solutions are produced that are insensitive to the initial charge density, except inside a tiny “boundary layer” at the nozzle. Then a non-Newtonian viscosity function is introduced into the model and the effects of extension thinning and thickening are explored. Results show two distinct regimes of stretching. For a “mild...

Simulation of melt spinning including flow-induced crystallization: Part I. Model development and predictions

Abstract: A mathematical model based on the formalism of Doufas et al. [A.K. Doufas, I.S. Dairanieh, A.J. McHugh, J. Rheol. 43 (1999) 85–109] was developed for the simulation of both low- and high-speed melt spinning including the combined effects of flow-induced crystallization (FIC), viscoelasticity, filament cooling, air drag, inertia, surface tension and gravity. Both an amorphous phase, simulated as a modified Giesekus fluid, and a semi-crystalline phase, approximated as rigid rods that grow and orient in the flow field, are coupled through the stress and momentum balance and the feedback of crystallinity to the system relaxation times. Since the onset of crystallization occurs at the equilibrium melting point, the freeze point arises naturally. The model is robust over a wide range of processing conditions and input parameters and exhibits material behavior consistent with that observed for semi-crystalline polymers under all spinning conditions. The model predicts neck-like deformation and associated strain softening in high-speed spinning, as well as the related velocity-, diameter-, temperature-, tensile stress-, apparent elongational viscosity-, orientation- and crystallinity-profiles. Calculations for the systems studied indicate that extensional softening followed almost immediately by FIC provides the primary mechanism responsible for neck formation, in agreement with experimental observations. The model provides a framework for the simulation and optimization of melt spinning involving FIC.

Stretching of a straight electrically charged viscoelastic jet

Abstract: A charged polymer jet may be accelerated and stretched by an external electric field, and this process is relevant to electrospinning for making nanofibers. The stretching of an electrified jet is governed by the interplay among electrostatics, fluid mechanics and rheology, and the role of viscoelasticity has not been systematically explored before. This paper presents a slender-body theory for the stretching of a straight charged jet of Giesekus fluid. Results show strain-hardening as the most influential rheological property. It causes the tensile force to rise at the start, which enhances stretching of the jet. Further downstream, however, the higher elongational viscosity tends to suppress jet stretching. In the end, strain-hardening leads to thicker fibers. This confirms the main result of a previous study using empirical rheological models. The behavior of the electrically driven jet forms an interesting contrast to that in conventional fiber spinning.