### Session P24: Focus Session: Jamming: Rheology and Failure

 Wednesday, March 23, 2005 11:15AM - 11:27AM P24.00001: Avalanche behavior in yield stress fluids Daniel Bonn We show that above a critical stress, typical yield stress fluids (gels, clay suspensions) and soft glassy materials (the colloidal glass of Laponite) start flowing abruptly and subsequently accelerate, leading to avalanches that are remarkably similar to those of granular materials. Rheometrical tests reveal that this is associated to a bifurcation in rheological behavior: for small stresses, the viscosity increases in time: the material ages,'' and eventually stops flowing. For slightly larger stresses the viscosity decreases continuously in time: the flow accelerates and we observe a 1 rejuvenation'' of the material by the flow. We show that for the Laponite system, both the aging and the shear rejuvenation can be observed directly using Diffusive Wave Spectroscopy. We propose a simple physical model capable of reproducing the rheological observations. These results may have some implication in geophysics: they shed some light on certain landslides of clayey soils, and the way quicksand works. Wednesday, March 23, 2005 11:27AM - 11:39AM P24.00002: Three Dimensional Observation of Force Chain Formation in Emulsions Theodore Brzinski , Charlotte Hollinger , Raul Gonzalez , Eric Weeks The spatially heterogeneous distribution of forces may be a common feature of jammed granular, foam and emulsion systems. This is thought to be due to the presence of force chains, structures of particles, droplets or bubbles which bear the bulk of a force exerted on the system. In emulsions, adjacent droplets exert forces on one another, which results in the deformation of droplet interfaces. We use fast laser-scanning confocal microscopy to observe the local deformations of each droplet in a sample in three dimensions, in order to visualize the force chains. Furthermore by placing jammed emulsions in a sheer-cell we observe the dynamics of force chain formation in real-time. Wednesday, March 23, 2005 11:39AM - 11:51AM P24.00003: Collision times and stress in gravity driven granular flow John Drozd , Colin Denniston We investigate, using simulations, collision times and stress distributions in two and three-dimensional steady-state granular matter in jammed versus diffuse flows. Surprisingly we find little dependence on the dimensionality of space. Grains remain separated with similar power-law distributions of times between collisions and the origin of stress transfer follows a similar mechanism in both cases. We compare our simulations to experimental results. Wednesday, March 23, 2005 11:51AM - 12:27PM P24.00004: Microrheology and the jamming transition in colloidal suspensions Invited Speaker: Eric R. Weeks We study concentrated colloidal suspensions, a model system which has a jamming transition (the colloidal glass transition). We use an optical confocal microscope to view the motion of these colloidal particles in three dimensions. These are suspensions of small solid particles in a liquid, and exhibit glassy behavior when the particle concentration is high; the particles are roughly analogous to individual molecules in a traditional glass. This allows us to directly study the microscopic behavior responsible for the macroscopic viscosity divergence of glasses. In particular we use small magnetic particles to locally poke'' on the colloidal samples, a form of active microrheology. We find a yield force (below which there is no motion), which grows as the glass transition is approached. Above this force, the magnetic particle moves, disturbing the surrounding colloidal particles. The results are interpreted in the framework of microrheology. Wednesday, March 23, 2005 12:27PM - 12:39PM P24.00005: Stressed out colloids: The effects of gravity on sedimented suspensions Daniel L. Blair , Jacinta Conrad , Eric R. Dufresne , David A. Weitz Using laser scanning confocal microscopy we investigate dense hard-sphere colloidal sediments. Confocal microscopy allows for both high temporal resolution and real-space three dimensional imaging. By density mismatching the particles and solvent, and introducing slight polydispersity, we produce sediments without crystalline order that are dynamically arrested. Within these sediments, distinct clusters of particles with reduced local volume are observed. We will present our analysis of these structures, and postulate their equivalence to characteristic features of analogous jammed systems. Wednesday, March 23, 2005 12:39PM - 12:51PM P24.00006: Yield stresses and stress relaxation in latex dispersions Suresh Ahuja Static and dynamic yield stress as functions of temperature and time are used to study dynamics of jammed systems. Similar to hard spheres, inter-particle bonds which are function of volume fraction, crowding of clusters and aggregates can be used to and modulus is a powerful tool. Particle to particle interaction dominate in concentrated dispersions resulting in a complex microstructure due to ordering of particles. Extent of ordered and disordered regions in a microstructure depends on the magnitude of shear rate ( Peclet number ). Flow fields influence particle to particle collisions which are subject to Brownian, hydrodynamic, electrostatic and van der Waal forces moving a dispersion from having a dominant ordered structure to a disordered structure and then eventually into a dominant ordered region in the dispersion. Polydispersed particles of acrylic latexes were treated to both steady and oscillatory shear and their shear response was analyzed to determine shear rate dependence, dynamic and steady yield stress. The aggregate particles stemming from primary particle form fractal-like structures, which coupled with volume fraction of particles, relate to correlation length of the network. By considering chains from a bundle of primary particles in a network undergoing creation, evolution and annihilation, stress and relaxation modulus can be calculated with relaxation time constant as dependent on fractal size and volume concentration. The shear modulus and relaxation time constant from dynamic experiments are calculated and their dependence on aggregate size is determined and compared to network models. Wednesday, March 23, 2005 12:51PM - 1:03PM P24.00007: Universal Breakdown of Elasticity at the Onset of Material Failure Craig Maloney , Ana\"el Lema\^{I}tre We show that, in the athermal quasi-static deformation of amorphous materials, the onset of failure is accompanied by universal scalings associated with a \emph{divergence} of elastic constants. A normal mode analysis of the non-affine elastic displacement field allows us to clarify its relation to the zero-frequency mode at the onset of failure and to the crack-like pattern which results from the subsequent relaxation of energy. Wednesday, March 23, 2005 1:03PM - 1:15PM P24.00008: Sum rules for the visco-elastic response of disordered solids at zero temperature Anael Lemaitre , Craig Maloney We study exact results concerning the non-affine displacement fields observed by Tanguy et al [Europhys. Lett. {\bf 57}, 423 (2002), Phys. Rev. B {\bf 66}, 174205 (2002)] and their contributions to elasticity. A normal mode analysis permits us to estimate the dominant contributions to the non-affine corrections to elasticity, and relate these corrections to the correlator of a fluctuating force field. We extend this analysis to the visco-elastic response of the system. Wednesday, March 23, 2005 1:15PM - 1:27PM P24.00009: Jamming and dynamics in Confined Quasi-One Dimensional Systems R.K. Bowles , K.K. Mon , J.K. Percus Geometrically confining particles to a quasi-one dimensional arrangement so that they can only interact with their nearest neighbours simplifies the way the particles can pack to the extent that we can calculate the distribution of jammed packings exactly, making them ideal systems for exploring the connection between jamming and dynamics. We study the mean squared displacement (MSD) of a system of two dimensional hard discs subject to inertial motion and confined to a single file by two hard lines. At low densities the MSD of the discs increases linearly with time, consistent with the Einstein relation for normal diffusion. However, at high densities the system exhibits anomalous diffusion, where the MSD is proportional to $t^{1/2}$. We show how this dynamic transition is related to the nature and distribution of jammed structures. We also use this simple system to examine the role of dynamic heterogeneity in the motion of dense confined fluids. Wednesday, March 23, 2005 1:27PM - 1:39PM P24.00010: Signatures of dynamical heterogeneities in dense granular flows Bulbul Chakraborty , Allison Ferguson Recent interest in understanding the dynamical arrest in both thermal and athermal systems has led to questions about the nature of these jamming transitions (PRL {\bf 86}, 111 (2001), Nature {\bf 411}, 772 (2001)), as well as the role extended structures may play in determining the dynamics of the system (Science {\bf 287}, 627 (2000)). Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear chains of particles with a high collision frequency even at flow velocities well above the jamming transition (EPL {\bf 66}, 277 (2004)). These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these "frequently-colliding" particles. Several striking features develop which may be connected to the presence of the chains, including a strong anisotropy in the distribution of collision angles and an increase in collision frequency as the granular temperature is decreased. The granular temperature displays a different dependence on flow velocity than is predicted by current kinetic theory (PRE {\bf 65}, 011303 (2001)); the velocity fluctuations are observed to die out more rapidly than expected, as observed in experiments (Science {\bf 275}, 1920 (1997)). Understanding the effects that these long-lived dynamical stress chains have on dense, flowing granular materials can lead to further insight into the nature of these systems. Wednesday, March 23, 2005 1:39PM - 1:51PM P24.00011: Is a homogeneous'' description of dynamic heterogeneities possible? Grzegorz Szamel We study the simplest model of dynamic heterogeneities in glass forming liquids: the one-spin facilitated kinetic Ising model introduced by Fredrickson and Andersen [G.H. Fredrickson and H.C. Andersen, Phys. Rev. Lett. \textbf{53}, 1244 (1984); J. Chem. Phys. \textbf{83},5822 (1985)]. We previously showed that the low-temperature, long-time behavior of the excitation density autocorrelation function predicted by a scaling approach can be obtained from a self-consistent mode-coupling-like approximation. Here we use a similar approach to investigate diffusion of a test particle and the incoherent intermediate scattering function. Wednesday, March 23, 2005 1:51PM - 2:03PM P24.00012: Heterogeneous Dynamics in Thin Films of Glassy Polymers Arlette Baljon , Joris Billen , Rajesh Klare In this talk, we present an analysis of the heterogeneous dynamics in ultrathin polymeric films near the glass transition. Specifically, behavior of polymer films supported by an absorbing structured surface is studied using molecular dynamics simulation. A coarse-grained, bead-spring model is used for the polymer chains. We define a specific criterion to characterize the polymer bead mobility and use this to determine the mobile and immobile beads in the system. The immobile beads are found to occur throughout the film, but their distribution is inhomogeneous, with the probability of their occurrence decreasing with distance from the substrate. Still, enough immobile beads are located near the free surface to cause them to percolate in the direction normal to the substrate surface, at a temperature near the glass transition temperature. The immobile beads block or “jam” the overall molecular motion in the film and hence cause the type of dynamic arrest, typically associated with glass transition. This result is in agreement with a recent theoretical model of glass transition [D. Long, F. Lequeux, Eur. Phys. J. E 4, 371 (2001)]. Wednesday, March 23, 2005 2:03PM - 2:15PM P24.00013: Origins of jamming in the zero-temperature dynamics of the Sherrington-Kirkpatrick model Paul Eastham , Richard Blythe , Alan Bray , Mike Moore We consider zero-temperature dynamics of the Sherrington-Kirkpatrick spin-glass model. Such dynamics consistently converges to an energy above that of the ground state. We argue that this jamming cannot be explained soley by the presence of large numbers of metastable states. We elucidate the origins of the jamming by modelling the dynamics as a Markov process in the single-site energies. We discuss the features of this process which cause the jamming, and present an approximate derivation of the dynamics that captures these features.