Session A53: Disordered and Glassy Systems I

The dependence of fraglity of glass forming liquids on interparticle interactions and density

The fragility of a glass forming liquid quantifies the rapidity of the change in viscosity and relaxation times with temperature and is an important material property. We study the influence of interparticle interactions on the fragility of a set of model glass formers using computer simulations. We consider both the kinetic fragility, given by the temperature variation of relaxation times, and the thermodynamic fragility obtained by the temperature variation of the configurational entropy. The Adam-Gibbs relation describes the temperature variation of relaxation times in terms of the variation of the configurational entropy, and thus we expect the kinetic and thermodynamic fragilities to be consistent with each other. We however find that the kinetic fragility increases with increasing softness of the interaction potential, with thermodynamic fragility showing the opposite trend. We rationalize our results by considering the full form of the Adam-Gibbs relation, which requires knowledge in addition of the high temperature activation energies, and explore the role of recent ideas on the scaling of temperature and density in systems exhibiting behavior akin to those with inverse power law interactions.   

Frequency and Wavevector Dependence of the Atomic Level Stress-Stress Correlation Function in a Model Supercooled Liquid

Temporal and spatial correlations among the local atomic level shear stresses were studied for a model liquid iron by molecular dynamics simulation [PRL 106,115703]. Integration over time and space of the shear stress correlation function $F(r,t)$ yields viscosity via Green-Kubo relation. The stress correlation function in time and space $F(r,t)$ was Fourier transformed to study the dependence on frequency, $E$, and wave vector, $Q$. The results, $F(Q,E)$, showed damped shear stress waves propagating in the liquid for small $Q$ at high and low temperatures. We also observed additional diffuse feature that appears as temperature is reduced below crossover temperature of potential energy landscape at relatively low frequencies at small $Q$. We suggest that this additional feature might be related to dynamic heterogeneity and boson peaks. We also discuss a relation between the time-scale of the stress-stress correlation function and the alpha-relaxation time of the intermediate self-scattering function $S(Q,E)$.   

Measuring Dynamical Facilitation in Supercooled Liquids and Related Materials

We provide a physical interpretation for excitation dynamics in kinetically constrained lattice models in the context of supercooled liquids and granular materials. Several physical quantities such as instanton times, onset temperatures, and particle displacement fields are derived. These quantities are used to interpret measurements of dynamical facilitation previously performed for atomistic and molecular supercooled liquids and granular materials. We show that these previous measurements provide strong evidence that dynamical facilitation plays a key role in glassy materials.   

Excitations are localized and relaxation is hierarchical in glass-forming liquids

For several atomistic models of glass formers, at conditions below their glassy dynamics onset temperatures, $T_{\mathrm{o}}$, we use importance sampling of trajectory space to study the structure, statistics and dynamics of excitations responsible for structural relaxation. Excitations are detected in terms of persistent particle displacements of length $a$. At supercooled conditions, for $a$ of the order of or smaller than a particle diameter, we find that excitations are associated with correlated particle motions that are sparse and localized, occupying a volume with an average radius that is temperature independent and no larger than a few particle diameters. We show that the statistics and dynamics of these excitations are facilitated and hierarchical. Excitation energy scales grow logarithmically with $a$. Excitations at one point in space facilitate the birth and death of excitations at neighboring locations, and space-time excitation structures are microcosms of heterogeneous dynamics at larger scales. This nature of dynamics becomes increasingly dominant as temperature $T$ is lowered. We show that slowing of dynamics upon decreasing temperature below $T_{\mathrm{o}}$ is the result of a decreasing concentration of excitations and concomitant growing hierarchical leng   

Anomalous properties of liquids for a family of models with short range tetrahedral interactions

Liquids with tetrahedral symmetry of the first coordination shell often display anomalous thermodynamic and dynamic behavior. The main reason for these anomalies is that pressurizing such liquids leads to the disordering of this local symmetry by the particles migrating from the second to the first coordination shell. This in some case may lead to the increase of entropy upon pressurizing and consequently to the volume increase upon cooling. Molecular simulations of various models with tetrahedral symmetry are able to reproduce this anomalous behavior. We study a family of simple models in which we can gradually change the degree of tetrahedrality and investigate the associated changes of the phase diagram by discrete molecular dynamics. A molecule in these models consist of a hard sphere and four point particles attached to the center of the hard sphere by directional bonds arranged in tetrahedral geometry. Each of these four particles has a narrow attractive square well so that the particles belonging to different molecules can attract to each other. We also impose a condition which does not allow a point particle in one molecule to include in its attractive well more than one point particle belonging to different molecules. We investigate how the phase diagram of the system depends on the parameters of the models. None of these models has a liquid -liquid phase transition in the accessible region of the phase. However, adding weak attractive square well to the hard sphere, or wider weak attractive square wells to the point particles can create a liquid-liquid critical point. A comparison with other simple models of the anomalous liquids is made.   

Stretched-exponential relaxation and hidden power laws in a solidifying 2D liquid

In a 2D Lennard-Jones liquid, the number of particles keeping their memorized nearest neighbors is found to decay stretched-exponentially; the probability for a particle to keep the same 6 nearest neighbors for a time t can be fitted with a power law. Using the lists of nearest neighbors (\textit{nn}-lists) as a topological order parameter, we studied the dynamics of the structure underlying these signature features of complexity in materials. The \textit{nn}-changes randomly appear along the boundaries of better ordered blocks at a time scale of the order of particles vibration period; these boundaries, and the shapes of the blocks, perform a next time-scale random motion. Particles diffusion includes periods of slow and fast diffusion. We discuss the feed-back interactions between nn-changes, block boundaries motion, and orientation relaxation in the system.   

Sheared athermal soft-particle suspensions near jamming: dependence of effective diffusion on packing density and system size

We perform numerical simulations to study diffusion in a model bi-disperse frictionless athermal soft-particle suspension of disks in two dimensions (2D) using the so-called ``mean field'' version of Durian's bubble model. We measure the effective transverse diffusion coefficient $D_{\rm eff}$ in shear flows at various volume fraction $\phi$ and shearing rate $\dot{\gamma}$. For $\phi>\phi_c$, where $\phi_c$ is identified with the random close packing limit, in the quasi-static limit, $D_{\rm eff}$ shows a pronounced linear system size dependence with very weak dependence on $\phi$. For $\phi<\phi_c$, $D_{\rm eff}$, in the quasi-static limit, increases with increasing $\phi$ and shows very little system size dependence. We discuss how the behavior of $D_{\rm eff}$ is related to non-trivial correlations in the spatial structure of the displacement fields \emph{at long times} in the Fickian regime.   

Decoupling of Rotational and Translational Diffusion in Supercooled Colloidal Fluids

Using high-speed confocal microscopy, we directly observe the three-dimensional rotational dynamics of rigid clusters of microspheres suspended in dense colloidal suspensions. The clusters are highly ordered packings of fluorescently-labeled PMMA particles, fabricated using a recently developed emulsification technique. Our colloidal suspensions serve as an excellent model of hard spheres, perhaps the simplest system with a glass transition, while the clusters probe the system's local rotational and translational dynamics. Far from the colloidal liquid's glass transition, both rotational and translational motion of the clusters are purely Brownian. However, in the liquid's supercooled regime, we observe a decoupling between the two types of motion: as the glass transition is approached, rotational diffusion slows down even more than translational diffusion. The nature of the decoupling is in good agreement with theoretical predictions and experiments with molecular glass formers. Our observation supports the notion that supercooled liquids are not merely liquids with large viscosities but that diffusion takes place by fundamentally changed mechanisms.   

Thermodynamics versus network topology of network glasses

Under cooling, the thermodynamics and the dynamics of super-cooled liquids are strongly correlated. The thermal evolutions of these quantities, characterizing the liquid fragility, depend greatly on the specific liquid considered. To date, there is no understanding of what controls these properties at a microscopic level. In chalcogenide glasses, the coordination of the covalent network can be changed continuously by varying their composition. Experiments show that as the coordination is increased, the jump of specific heat varies non-monotonically and is minimal at coordination near the Maxwell threshold where the covalent network becomes rigid. At such a composition the liquid is strong. We introduce a simplified model for the thermal evolution of networks that captures this observation.   

Elastic properties of compressed emulsions

Visualizing the packing of a dense emulsion in 3D as a function of the external pressure allows us to characterize the geometry and the local stress distribution inside this jammed system. We first test the scaling laws of the pressure and average coordination number over two orders of magnitude in density. We find deviations from theoretical exponents due to the non-affine motion of the particles. Second, we observe that the distribution of forces changes from a broad exponential at the jamming point to a narrower Gaussian-like distribution under high compression. Finally, we calculate the density of states from the measured force network in the approximation of a harmonic potential. Close to jamming, the number of low frequency modes is high, while the application of pressure shifts the distribution to higher frequencies, indicative of a rigid network. The confocal images reveal the structural features associated with the low frequency modes, as well as their localization within the packing. These data are then compared with published results from numerical simulations.   

Glassy Dynamics of Charge Density Waves in Chromium

Charge-density waves provide theoretically tractable systems for exploring longstanding questions posed by the physics of elastic media in the presence of quenched disorder. Interaction of quenched pinning fields and phase elasticity of CDWs results in a complex energetic landscape of metastable states, which in turn gives rise to ``glassy'' phenomena such as aging and hysteresis. Using synchrotron x-rays we have observed aging of CDW order parameter Q in bulk chromium following thermal quench to out-of-equilibrium configuration. Although temperature stabilization occurs in under two minutes, Q relaxes exponentially over the course of hours toward metastable configurations that depend on sample history.   

Local collapse of the atomic cage in a liquid flow

The local structure of a model glass under steady shear was studied by molecular dynamics simulation for both high (T$>$Tg) and low (T$<$Tg) temperature ranges. The local structure was presented in terms of the anisotropic pair-density function (PDF). We found that the local structure was strained over a limited range of distances, and the length-scale of the strained region was dependent on the strain rate, extrapolating to zero at a critical strain rate. A strong correlation between the local collapse, represented by cutting of the atomic bond, and the structural strain in the PDF was found. At low temperatures local failure happens in a serrated manner, caused mechanically by shear. At high temperatures the local failure occurs more randomly, which is governed by thermal fluctuation. An anomalous behavior was observed as temperature approaches to Tg. The results suggest that except for the supercooled state above Tg local failure occurs by cutting of a single bond. Only in the supercooled state multiple bonds have to be cut for flow to occur. A possible relation to the dynamic heterogeneity is discussed.   

Local Perturbation of Quasi Two-Dimensional Colloidal Glasses

Colloids are promising and widely used model systems to investigate the phase behavior of matter at length scales and timescales accessible to optical microscopy. We utilize disordered quasi two-dimensional colloidal systems in the jammed state to study the properties of glasses. It was recently found in simulation and experiment that so-called 'soft spots', where low frequency quasi-localized modes concentrate, are the locations in glasses prone to rearrangements. Therefore, we utilize short laser pulses to perturb colloidal glasses locally. By varying the pulse intensity of the laser, the strength of the perturbation is tunable, which allows us to induce either elastic or plastic deformations in the glasses. With this experimental geometry in combination with video microscopy, we investigate the correlation between locally induced rearrangements in the colloidal glass and the location of soft spots determined from analysis of the system's vibrational eigenmodes. The dependence of the mechanical response of a glass on the local environment in terms of these dynamic heterogeneities and their persistence is discussed.   

X-ray investigation of colloidal glasses under shear

Understanding glassification or dynamical arrest is one of the grand challenges of material science and is a topic of great current interest. It is a central observation in soft matter systems as well as glass forming molecular systems that -- with increasing density or decreasing temperature -- the motion of the particles or molecules slows down and eventually becomes arrested. Understanding this dynamical arrest as well as relaxations in the arrested state are fundamental problems, which to a large degree remain unanswered. We use a novel combination of rheological measurement and small angle x-ray scattering (at the synchrotron DESY in Hamburg) to study structure factor of dense suspensions under shear and during relaxation. The suspensions consist of silica particles 50nm in diameter. We observe clear changes of inter particles distances and configurations due to the different shear rates. Together with future dynamic x-ray measurements we aim to develop a universal scale-bridging understanding of dynamic arrest.   

Boundaries Matter for Confined Colloidal Glasses

We confine dense colloidal suspensions within emulsion droplets to examine how confinement and properties of the confining medium affect the colloidal glass transition. Samples are imaged via fast confocal microscopy. By observing a wide range of droplet sizes and varying the viscosity of the external continuous phase, we separate finite size and boundary effects on particle motions within the droplet. Suspensions are composed of binary PMMA spheres in organic solvents while the external phases are simple mixtures of water and glycerol. In analogy with molecular super-cooled liquids and thin-film polymers, we find that confinement effects in colloidal systems are not merely functions of the finite size of the system, but are strongly dependent on the viscosity of the confining medium and interactions between particles and the interface of the two phases.