Viscous jets and filaments in electric fields: stability analysis and role of viscoelasticity
2017-02-28T05:02:37Z (GMT) by
Viscous liquid jets and filaments are important in a number of industrial and domestic applications as well as in several natural processes. Application of an electric field is found to have a remarkable effect on the behaviour of these jets, mainly in controlling the propagation of instabilities and breakup dynamics. Such electrified jets have been exploited in a number of industrial applications such as electrospraying, electrospinning, electroseparations, ink-jet printing, etc. This thesis presents five theoretical and computational studies on different electrified viscous jet/filament systems. The first study is on electrospinning, a simple technique to generate polymeric nanofibers by taking advantage of distinctive flow instabilities in electrified jets of polymer solutions. This process, though easy to perform is quite complex to model mainly due to coupling of a number of physics together and the large number of dependent parameters. Here it is attempted to derive a relation between the final fiber diameter and the various process parameters. A scaling analysis of an approximate equation for the motion of a bent jet is performed and two new dimensionless numbers describing viscous moment and surface charge repulsion effects are identified. Experimental data for a wide range of polymer solutions are all shown to have a common slope, when expressed in terms of these new dimensionless ratios. Using this correlation between the dimensionless numbers, a new scaling expression is obtained for the final fiber diameter as a function of various process parameters. In the next study, stability of immersed viscous liquid threads subjected to radial or axial electric fields is investigated using linear stability analysis. Axisymmetric (m = 0) and asymmetric (m = 1) modes of perturbation are studied for arbitrary viscosity ratios. The viscosity ratio, in general, is shown to have a damping effect on the two modes of perturbations, and the effect is more pronounced for the m = 1 as compared to m = 0 perturbation. Investigating the effect of both the electric field and the viscosity ratio simultaneously, an operating diagram is generated, showing the predominance of the two modes at any given value of operating parameters. The above analysis is extended to understand the occurrence of the unique “pearling” stability on lipid bilayer cylindrical vesicles under electric field. It is shown that a certain critical axial electrical field needs to be applied to induce pearls on a bilayer vesicle. The maximum growth rate and the wavenumber of the pearling instability were found to in- crease with increasing electric field. While growth rate continues to increase, the maximum wavenumber reaches a steady value at higher electric fields. Like electric field, viscoelasticity induced by dissolved polymer molecules also plays a significant role in controlling the dynamics of breakup of jets and filaments. In the fourth study, capillary thinning of viscoelastic liquid bridges is investigated using an efficient hybrid method that combines a 1-D slender-filament approximation for the full profile of a liquid bridge with a 0-D stress balance to predict the temporal evolution of the filament “neck”. In addition, an advanced constitutive model for polymeric stresses is used to study the anomalous concentration dependence of break-up dynamics in polymer solutions that are nominally regarded as being dilute. The microstructural constitutive model incorporates changes in the friction coefficient of polymer molecules as they unravel, stretch and begin to experience significant intermolecular interactions in strong extensional flows due to a phenomenon known as “self-concentration”. The hybrid simulation technique is used with this new constitutive model to predict dynamics of liquid-bridge necking that compares well with experimental observations reported in the literature on dilute polymer solutions. In the last study, the importance of relaxation time and self-concentration on electrospinning of dilute polymer solutions is investigated by considering the steady region of an electrified jet of a polymer solution. It is shown that elastic stresses increase exponentially with Deborah number (De). For each concentration there exists a critical De below which the elastic stresses at the end of the steady jet region are insufficient to overcome capillary stresses and lead to an unstable jet in the whipping region. However, above the critical De, the elastic stresses may be sufficiently dominant to lead to more uniform fibers, thus pointing to the possibility of improved “electrospinnability” even with dilute polymer solutions. Also, it is suggested that self-concentration may play an important role in electrospinning of polymer solutions with higher relaxation time and high conductivity. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.