Iterated stretching and multiple beads-on-a-string phenomena in dilute solutions of highly-extensible flexible polymers

The dynamics of elastocapillary thinning in high molecular weight polymer solutions are re-examined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and competition of elastic, capillary and inertial forces leads to the formation of a periodic array of beads connected by axially-uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and ligaments. This iterated process results in the development of multiple generations of beads in agreement with predictions of Chang et al. (1999), although experiments yield a different recursion relation between successive generations. At long times, finite extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to progressive coalescence before rupture of the fluid column.Comment: Submitted to Physics of Fluids. Contains 15 pages, including 6 figures and 1 tabl

Nonlinear Shear and Extensional Flow Dynamics of Wormlike Surfactant Solutions

Accepted for publication in JNNFM.Nonlinear shear and extensional flow dynamics of rheological properties of a wormlike micellar solution based on erucyl bis (2-hydroxyethyl) methyl ammonium chloride, EHAC, are reported here. The influences of surfactant (EHAC) and salt (NH4Cl) concentrations on the linear viscoelastic parameters are determined using small amplitude oscillatory shear experiments. The steady and time-dependent shear rheology is determined in a double gap Couette cell, and transient extensional flow measurements are performed in a Capillary Breakup Extensional Rheometer (CABER). In the nonlinear shear flow experiments, the micellar fluid samples show strong hysteretic behavior upon increasing and decreasing the imposed shear stress due to the development of shear-banding instabilities. The non-monotone flow curves of stress vs. shear rate can be successfully modeled in a macroscopic sense by using the single-mode Giesekus constitutive equation. The temporal evolution of the flow structure of the surfactant solutions in the Couette flow geometry is analyzed by instantaneous shear-rate measurements for various values of controlled shear-stress, along with FFT analysis. The results indicate that the steady flow bifurcates to a global time-dependent state as soon as the shear banding/hysteresis regime is reached. Increasing the salt/surfactant ratio or the temperature is found to stabilize the flow, as also confirmed by the decreasing values of anisotropy factor in the Giesekus model. Finally we have investigated the dynamics of capillary breakup of the micellar fluid samples in uniaxial extensional flow. The filament thinning behavior of the micellar fluid samples is also accurately predicted by the Giesekus constitutive equation. Indeed quantitative agreement between the experimental and numerical results can be obtained providing that the relaxation time of the wormlike micellar solutions in extensional flows is a factor of three lower than in shear flows.NSF, Schlumberger, and the Turkish Scientific and Technical Research Institut

Iterated Stretching, Extensional Rheology and Formation of Beads-on-a-String Structures in Polymer Solutions

Accepted for publication in JNNFM, December 2005.The transient extensional rheology and the dynamics of elastocapillary thinning in aqueous solutions of polyethylene oxide (PEO) are studied with high-speed digital video microscopy. At long times, the evolution of the thread radius deviates from self-similar exponential decay and competition between elastic, capillary and inertial forces leads to the formation of a periodic array of beads connected by axially-uniform ligaments. This configuration is unstable and successive instabilities propagate from the necks connecting the beads and ligaments. This iterated process results in multiple generations of beads developing along the string in general agreement with predictions of Chang et al. [Phys Fluids, 11, 1717 (1999)] although the experiments yield a different recursion relation between the successive generations of beads. At long times, finite extensibility truncates the iterated instability, and slow axial translation of the bead arrays along the interconnecting threads leads to progressive coalescence before the ultimate rupture of the fluid column. Despite these dynamical complexities it is still possible to measure the steady growth in the transient extensional viscosity by monitoring the slow capillarydriven thinning in the cylindrical ligaments between beads.NASA and the Portuguese Science Foundatio

Instabilities of jets of non-Newtonian fluids impacting a plate

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (p. 106-111).The problem of buckling and coiling of jets of viscous, Newtonian liquids impacting a plate has received a substantial level of attention over the past two decades, both from experimental and theoretical points of view. Nevertheless, most industrial and everyday life fluids are non-newtonian, and their rheological properties affects their behavior in this problem. The present work aims at studying the instabilities of jets of such fluids falling on a plate, via both phenomenological descriptions and theoretical analysis of jet motion and shape. Several fluids with different rheological properties, including viscous Newtonian oil, model non-Newtonian fluids, and commercial shampoos, are used and different dynamical regimes are documented. A special focus is placed on viscoelastic, shear-thinning cetylpyridinium (CPyC1) solutions. In concentrated solutions, CPyCl surfactant molecules have been shown to assemble in long wormlike micellar structures, which gives the fluid its non-Newtonian properties. Jets of CPyCl solution show several novel shapes and dynamical regimes not observed in the case of Newtonian fluids. The present study provides quantitative experimental measurements and mechanisms for these novel features.by Matthieu Varagnat.S.M

Heterogeneous flow inside threads of low viscosity fluids leads to anomalous long filament lifetimes

Formation and breakup of fluid threads is pervasive in nature and technology, where high extensibility of fluid filaments and extended filament lifetimes are commonly observed as a consequence of fluid viscoelasticity. In contrast, threads of low viscous Newtonian fluids like water rupture quickly. Here, we demonstrate that a unique banding instability during filament thinning of model surfactant solutions, with a viscosity close to water and no measurable elasticity, leads to extremely long filament lifetimes and to the formation of remarkably long threads. Complementary measurements in planar extension as well as in shear reveal that this flow instability is characterized by a multivalued stress, arising beyond a critical strain rate, irrespective of flow kinematics. Our work reports the first observation of such phenomena during extensional deformation and provides a unifying view on instabilities in complex flow fields