Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G7: Shear-Induced Patterns in Complex Fluids and Granular Matter |
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Sponsoring Units: GSNP Chair: David Weitz, Harvard University Room: Baltimore Convention Center 307 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G7.00001: Subcritical shear-induced instabilities and turbulence of visco-elastic fluids Invited Speaker: While instabilities in normal Newtonian fluids are usually due to inertial effects, most visco-elastic instabilities are noninertial, i.e. happen at small Reynolds numbers. In polymeric fluids the viscoelastic instabilities and turbulence are due to shear-induced anisotropic elastic forces: when the so-called Weissenberg number is larger than about 1, a polymer fluid is very non-Newtonian: in this regime the fluid is anisotropic and elastic, and relaxation effects are important. After reviewing some recent experiments, I will discuss the recent progress on understanding the viscoelastic instabilities in parallel shear flows (planar Couette and Poiseuille flow). Contrary to common belief that such flows are absolutely stable in the small Reynolds number limit, our recent nonlinear amplitude analysis predicts that the transitions are subcritical. The critical Weissenberg number our analysis predicts is close to the values where in practice such flows are found to exhibit instabilities. The scenario suggested by these results shows strong resemblance to the transition to (weak) turbulence scenario in Newtonian shear flows. \\ [1] A. N. Morozov and W. van Saarloos, {\em Subcritical finite-amplitude solutions in plane Couette flow of visco-elastic fluids}, Phys. Rev. Lett. {\bf 95}, 024501 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G7.00002: Shear thickening, thinning and jamming in colloidal suspensions Invited Speaker: A microscopic theory for shear thinning behavior in glassy colloidal suspensions is presented. This theory is tested against extensive computer simulation. The theory is extended to study non-linear viscoelastic properties, and is shown to be in remarkable agreement with the measured strain sweep dependence of the loss modulus in colloidal gels. Lastly, we present numerical simulations to address shear thinning and possible shear thickening behavior of dense colloidal suspensions with short-ranged depletion-induced attractions. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:48AM |
G7.00003: Shear Alignment of Hexagonal and Striped Patterns in Block Copolymer Thin Films Invited Speaker: Diblock copolymers spontaneously self-assemble into nanodomain structures: simple repeating patterns with a periodicity (typically 20-100 nm) set by the polymer molecular weight and a symmetry controlled by the relative lengths of the two blocks. Similar patterns are obtained when block copolymers are deposited onto substrates, as films with thicknesses accommodating only a few or even only one layer of nanodomains: cylindrical nanodomains form a striped pattern when viewed from above, while spheres form a hexagonal pattern. In the absence of any applied field, however, these patterns exist as a polygrain structure, with a grain size which is typically microns at most. Recently, we have developed methods to shear these thin films, producing samples free from grain boundaries and with orientational order extending over centimeters. But while excellent orientational order can be achieved by shearing, translational order is still limited to the micron scale due to isolated dislocations which remain in the sheared films (though at a markedly reduced density compared with unsheared films). Shear can be applied either with an elastomeric pad, or with a viscous fluid; in the latter case, the patterns can be induced to follow the shape of fluid flow channels of arbitrary shape and millimeter-scale width. For the striped patterns, shear alignment is effective on films containing either a single layer or multiple layers of nanodomains, but for the hexagonal pattern, two or more layers are required due to the mechanical isotropy of a two-dimensional hexagonal lattice. A threshold stress is required to achieve the limiting quality of alignment, a stress which decreases as the block copolymer's order-disorder transition temperature is approached. A simple model appears to capture the principal features of the stress, time, and temperature dependence of the alignment quality. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:24AM |
G7.00004: Shear banding in complex fluids: Hints from colloidal crystals Invited Speaker: When sheared out of equilibrium, a variety of complex fluids exhibit an unusual behavior, where bands of high and low shear rates are spontaneously formed. This phenomenon is typically attributed to a nonlinear material rheology. In this talk I will describe a similar phenomenon observed in oscillatory shear experiment on dense colloidal crystal, where the data reflect a linear, rather than nonlinear material rheology. This observation suggests an alternative mechanism for shear banding, as a coexistence under nonequilibrium conditions between two linearly responding phases of a complex fluid. Some consequences will be discussed, such as the singular role played by near-wall pure solvent layers, and possible nonequilibrium mechanisms for selection of the observed band width. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 11:00AM |
G7.00005: Force fluctuations and shear banding near the jamming transition in granular materials Invited Speaker: What are the structural changes which turn an un-jammed system that flows like a liquid into a jammed, solid-like configuration or vice versa? For granular just as for molecular systems it turns out that these changes are so subtle that so far there still has been no clean way of identifying the transition based on direct measurements of the grains' or molecules' spatial arrangements. However, in macroscopic granular systems it is possible to measure also the forces between contacting grains. At high packing densities near jamming, where nearly all particles touch neighbors, contact force measurements pick up directly and sensitively what matters most, namely changes in relative grain position. They thus can act as magnifier for even minute structural rearrangements. This talk will discuss experiments [1] able to detect a structural signature of the jamming transition by analyzing changes in the shape of the distribution of contact forces. The second part of the talk will be devoted to discussing new experimental geometries in which shear bands, usually localized to within about 10 grain diameters at the onset of shear, can become much wider. This allows for detailed studies of the shear rate profile and the associated velocity fluctuations. I will compare data we recently obtained from direct imaging by video, magnetic resonance imaging of the interior, and molecular dynamics simulations [2]. \newline \newline [1] E. I. Corwin, H. M. Jaeger, S. R. Nagel, “Structural signature of jamming in granular media”, Nature 435, 1075 (2005). \newline [2] X. Cheng, J. B. Lechman, A. F. Barbero, G. S. Grest, H. M. Jaeger, G. S. Karczmar, M. E. M\"{o}bius, and S. R. Nagel, “Three- dimensional shear in granular flow”, cond-mat/0507469. [Preview Abstract] |
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