Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N7: Nonlinear Rheology and Dynamics of Soft Materials |
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Sponsoring Units: DCMP Chair: David Weitz, Harvard University Room: Colorado Convention Center Korbel 4A-4B |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N7.00001: Elasticity of Crosslinked Biopolymer Networks Invited Speaker: Crosslinked networks of biopolymers exhibit an enormous variety of nonlinear elastic behaviors depending on the rigidity of constituent polymers and the geometry and topology of the network. This talk will present a brief review of the general theory of nonlinear elasticity. It will then discuss the phenomenon of strain stiffening in networks of semiflexible polymers and present a theory [1] of this phenomena based on the nonlinear force-extension curve of these polymers and the simplifying assumption of affine response. The nonlinear stress-strain curves predicted by this theory agree remarkably well with experiments on a number of different polymer networks. Limitations and extensions of the simple theory including extensions to nonaffine behavior will also be discussed. \newline \newline [1] Storm, Cornelis, Jennifer J. Pastore, Jennifer J., Fred C. MacKintosh, Fred C., T.C. Lubensky, T.C., and Paul A. Janmey, Paul A., \textit{Nature} \textbf{435}, 191-194 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N7.00002: Strain-Rate Frequency Superposition (SRFS) - A rheological probe of structural relaxation in soft materials Invited Speaker: The rheological properties of soft materials such as concentrated suspensions, emulsions, or foams often exhibit surprisingly universal linear and nonlinear features. Here we show that their linear and nonlinear viscoelastic responses can be unified in a single picture by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new approach to oscillatory rheology, which keeps the strain rate amplitude fixed as the oscillation frequency is varied. This allows for a detailed study of the effects of strain rate on the structural relaxation of soft materials. Our data exhibits a characteristic scaling, which isolates the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques. Our approach is reminiscent of a technique called time-temperature superposition (TTS), where rheological curves measured at different temperatures are shifted onto a single master curve that reflects the viscoelastic behavior in a dramatically extended range of frequencies. By analogy, we call our approach strain-rate frequency superposition (SRFS). Our experimental results show that nonlinear viscoelastic measurements contain useful information on the slow relaxation dynamics of soft materials. The data indicates that the yielding behavior of soft materials directly probes the structural relaxation process itself, shifted towards higher frequencies by an applied strain rate. This suggests that SRFS will provide new insight into the physical mechanisms that govern the viscoelastic response of a wide range of soft materials. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N7.00003: Probing the nonlinear response of soft materials by active microrheology Invited Speaker: In passive microrheology, the linear viscoelastic properties of complex fluids are inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing may also be used to obtain such linear-response information. More significant forcing may drive the material significantly out of equilibrium, thus potentially providing a window into the nonlinear response properties of the material. In leaving the linear-response regime, however, the theoretical underpinning for passive microrheology is lost, and a variety of issues arise. Most generally, what exactly can be measured, and how can such measurements be interpreted? Using a model system (a large colloidal probe pulled through a dilute suspension of small bath particles), we examine the different sources of stress upon the probe particle (e.g. direct probe-bath collisions vs. microstructural deformations within the bulk suspension) and discuss their analog in the corresponding macro- rheological measurement (or lack thereof). Several crucial issues emerge for the interpretation of nonlinear microrheology: 1) how to interpret the inhomogeneous and non-viscometric nature of the deformation field around the probe, 2) the distinction between of direct and bulk stresses and their deconvolution, and 3) the (Lagrangian) time-dependent nature of the stress histories experienced by material elements as they advect past the probe. Having identified these issues, we discuss several adaptations of the basic technique/interpretation, both to more faithfully recover bulk rheology as well as to measure properties inaccessible to macro- rheology. While we specifically discuss a model colloidal suspension, we ultimately envision a technique capable of measuring the nonlinear rheology of general materials. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N7.00004: Nonlinear structural dynamics in metal nanowires Invited Speaker: Most atoms in a metal nanowire are surface atoms with low coordination number. Classically, surface effects are expected to dominate their stability and structural dynamics, leading in particular to wire break-up due to the Rayleigh instability. On the other hand, long gold [1] and silver [2] nanocylinders have recently been observed using transmission electron microscopy, pointing to the presence of an additional stabilizing mechanism. Evidence of electron-shell filling effects [3] have been found in conductance histograms for various metals, suggesting that this mechanism comes from the transverse confinement of the electrons within the nanowire. Using the nanoscale free-electron model, a continuum model of the structural dynamics of simple-metal nanowires, I will discuss how the interplay of surface and electron-shell effects explains the stability and long lifetimes of nanowires, and favors the formation of kinks connecting cylindrical segments of the wire. A rich dynamics involving kink interactions and kink/antikink pair-creation and annihilation is uncovered, and is shown to explain the observed step-by-step thinning mechanism of Au nanowires [4]. \newline \newline [1] Y. Kondo et al., Science 289, 606 (2000). \newline [2] V. Rodrigues et al., Phys. Rev. Lett. 85, 4124 (2000). \newline [3] A. I. Yanson et al., Phys. Rev. Lett. 84, 5832 (2000). \newline [4] Y. Oshima et al., J. Electron Microsc. 52, 49 (2003). [Preview Abstract] |
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