Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F21: Polymer Rheology - Flexibility, Charge and ExtensibilityInvited
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Sponsoring Units: DPOLY Chair: Vivek Sharma, University of Illinois at Chicago Room: 281-282 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F21.00001: Linear Viscoelasticity of Ionic Polymers: Ionomers and Polyelectrolytes Invited Speaker: Ralph Colby Polymers with covalently bonded ionic groups exhibit interesting viscoelasticity. In polar media, significant fractions of the unattached counterions dissociate, leaving the polyelectrolyte chain with a net charge that has both conformation and dynamics dominated by charge repulsion. In less polar surroundings, the same polymer is termed an ionomer, with nearly all ions paired and the neutral pairs attract each other and associate to create temporary crosslinks. Solutions of flexible polyelectrolytes with no added salt have a wide range of concentration that is semidilute but not entangled, with dynamics described by the Rouse model. In contrast, ionomers have strong dipolar attractions between neighboring chains and are instead described by either sticky-Rouse or sticky-reptation models. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:27PM |
F21.00002: Rheology of polyelectrolyte complex materials Invited Speaker: Matthew Tirrell Fluid polyelectrolyte complexes, sometimes known as complex coacervates, have rheological properties that are very sensitive to structure and salt concentration. Dynamic moduli of such viscoelastic materials very many orders of magnitude between solutions of no added salt to of order tenth molar salt, typical, for example of physiological saline. Indeed, salt plays a role in the rheology of complex coacervates analogous to that which temperature plays on polymer melts, leading to an empirical observation of what may be termed time-salt or frequency salt superposition. Block copolymers containing complexing ionic blocks also exhibit strong salt sensitivity of their rheological properties. Data representing these phenomena will be presented and discussed. [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 1:03PM |
F21.00003: Single polymer dynamics in semi-dilute unentangled and entangled solutions: from molecular conformation to normal stress Invited Speaker: Charles Schroeder Semi-dilute polymer solutions are encountered in a wide array of applications such as advanced 3D printing technologies. Semi-dilute solutions are characterized by large fluctuations in concentration, such that hydrodynamic interactions, excluded volume interactions, and transient chain entanglements may be important, which greatly complicates analytical modeling and theoretical treatment. Despite recent progress, we still lack a complete molecular-level understanding of polymer dynamics in these systems. In this talk, I will discuss three recent projects in my group to study semi-dilute solutions that focus on single molecule studies of linear and ring polymers and a new method to measure normal stresses in microfluidic devices based on the Stokes trap. In the first effort, we use single polymer techniques to investigate the dynamics of semi-dilute unentangled and semi-dilute entangled DNA solutions in extensional flow, including polymer relaxation from high stretch, transient stretching dynamics in step-strain experiments, and steady-state stretching in flow. In the semi-dilute unentangled regime, our results show a power-law scaling of the longest polymer relaxation time that is consistent with scaling arguments based on the double cross-over regime. Upon increasing concentration, we observe a transition region in dynamics to the entangled regime. We also studied the transient and steady-state stretching dynamics in extensional flow using the Stokes trap, and our results show a decrease in transient polymer stretch and a milder coil-to-stretch transition for semi-dilute polymer solutions compared to dilute solutions, which is interpreted in the context of a critical Weissenberg number $Wi$ at the coil-to-stretch transition. Interestingly, we observe a unique set of polymer conformations in semi-dilute unentangled solutions that are highly suggestive of transient topological entanglements in solutions that are nominally unentangled at equilibrium. Taken together, these results suggest that the transient stretching pathways in semi-dilute solution extensional flows are qualitatively different than for both dilute solutions and for semi-dilute solutions in shear flow. In a second effort, we studied the dynamics of ring polymers in background solutions of semi-dilute linear polymers. Interestingly, we observe strikingly large fluctuations in steady-state polymer extension for ring polymers in flow, which occurs due to the interplay between polymer topology and concentration leading to chain `threading' in flow. In a third effort, we developed a new microfluidic method to measure normal stress and extensional viscosity that can be loosely described as passive yet non-linear microrheology. In particular, we incorporated 3-D particle imaging velocimetry (PIV) with the Stokes trap to study extensional flow-induced particle migration in semi-dilute polymer solutions. Experimental results are analyzed using the framework of a second-order-fluid model, which allows for measurement of normal stress and extensional viscosity in semi-dilute polymer solutions, all of which is a first-of-its-kind demonstration. Microfluidic measurements of extensional viscosity are directly compared to the dripping-onto-substrate or DOS method, and good agreement is generally observed. Overall, our work aims to provide a molecular-level understanding of the role of polymer topology and concentration on bulk rheological properties by using single polymer techniques. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:39PM |
F21.00004: Configurations and Dynamics of Semi-Flexible Polymers in Good and Poor Solvents Invited Speaker: Ronald Larson We develop coarse-graining procedures for determining the conformational and dynamic behavior of semi-flexible chains with and without flow using Brownian dynamics (BD) simulations that are insensitive to the degree of coarse-graining. In the absence of flow, in a poor solvent, we find three main collapsed states: torus, bundle, and globule over a range of dimensionless ratios of the three energy parameters, namely solvent-polymer surface energy, energy of polymer folds, and polymer bending energy or persistence length. A theoretical phase diagram, confirmed by BD simulations, captures the general phase behavior of a single long chain (\textgreater 10 Kuhn lengths) at moderately high (order unity) dimensionless temperature, which is the ratio of thermal energy to the attractive interaction between neighboring monomers. We also find converged results for polymer conformations in shear or extensional flow in solvents of various qualities and determine scaling laws for chain dimensions for low, moderate, and high Weissenberg numbers Wi. We also derive scaling laws to describe chains dimensions and tumbling rates in these regimes. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 2:15PM |
F21.00005: Flow of Polymeric Solutions: Instabilities & Microstructure Invited Speaker: Paulo Arratia Solutions of long and flexible polymer molecules do not flow like water. These fluids exhibit hydrodynamic instabilities and a new type of turbulence -- the so-called\textit{ elastic turbulence} -- even at low Reynolds numbers (Re). These phenomena, driven by the anisotropic elasticity of the fluid, are experimentally observed only in geometries with sufficient curvature. In this talk, I will discuss recent results on the flow of polymeric solutions in parallel shear geometries. I will present experimental evidence that parallel shear flows of polymer solutions, like flow in a straight pipe or channel, can be in fact non-linearly unstable even at low Re. We perform particle-tracking velocimetry in a long, straight microfluidic channel where we perturb the flow by placing a variable number of obstacles at the channel entrance. Above a critical flow rate and a critical size of the perturbation, a sudden onset of large velocity fluctuations indicates presence of a subcritical instability. Furthermore, Velocimetry measurements show non-periodic fluctuations in the wake of curved cylinders as well as in a parallel shear flow region. The flow in these two locations of the channel is excited over a broad range of frequencies and wavelengths, consistent with the main features of elastic turbulence. Together with the previous observations of hydrodynamic instabilities in curved geometries, our results suggest that any flow of polymer solutions becomes unstable at sufficiently high flow rates. [Preview Abstract] |
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