Session J48: Focus Session: Gelation and Glass Transition in Colloids and Soft Matter Systems II

Kinetic arrest and activated relaxation in dense suspensions of attractive nonspherical colloids

The coupled translation-rotation activated dynamics in dense suspensions of uniaxial short-range attractive homogeneous (hDC) and Janus (JDC) dicolloids are studied using the microscopic nonlinear Langevin equation theory based on the concept of a dynamic free energy surface. For larger aspect ratios, three activated regimes, repulsive glass (RG), attractive glass (AG) and gel (G), are predicted where translation and rotation are always strongly coupled. Physical clustering in the JDC system suppresses the re-entrant RG-fluid(F) process. For hDCs of small enough aspect ratio, multiple kinetic arrest transitions, novel re-entrant phenomena, and a dynamical quadruple point emerge associated with F, G, RG, AG, and plastic glass (PG) states. Colloid translational and rotational motions are strongly coupled in the gel, but decoupled (slower translation) in the other activated regimes due to the presence of two distinct hopping processes. Both translation and rotation activated relaxation rates are non-monotonic functions of attraction strength at high volume fractions. Translation-rotation decoupling universally weakens as attraction strength grows, and exhibits a different volume fraction dependence at low, intermediate, and high attraction strengths.   

Rearrangements, Vibrations and Microscopic Glassy Dynamics and Structure in Quasi-2D Dense Colloidal Gels

In this work, we investigate the microscopic dynamics of quasi-2D dense attractive colloidal systems. We confine bidisperse polystyrene spheres between glass coverslips in a suspension of water and 2,6-lutidine; as we increase the temperature of the sample into a critical regime, lutidine wets the colloids, creating a strong attractive interaction ($>$ 4kT). We specifically study suspensions in the ``dense gel'' regime, i.e., at a volume fraction high enough that the attractive particles form a spanning cluster, yet just low enough that there exists some structural heterogeneity larger than the individual particle size. We track the particle locations via bright-field video microscopy and analyze the dynamics of both lower-volume-fraction gel states and higher-volume-fraction glassy states. Specifically, we make correlations between local structure, rearrangement-prone regions, and low-frequency vibrational modes. In doing so, we not only characterize the structural and dynamical differences and similarities between colloidal gels and glasses, but we also gain further insight into the origins of dynamic heterogeneity in glassy systems.   

Dynamical Phases and Rheology of Rod-Sphere Nanoparticle Mixtures

Colloidal mixtures involving sticky nonspherical particles that form glasses and gels are relatively unexplored compared to their single component analogs. We develop and apply a microscopic statistical dynamical approach (mode coupling and nonlinear Langevin equation theories) at the center-of-mass level for dense isotropic mixtures of spheres and rods as a function of (short range) attraction strength, aspect ratio, and composition. Based on the mixture structural pair correlations as input, up to seven dynamical phases (transiently localized states with activated dynamics) are predicted corresponding to fluid, repulsive glass, attractive glass, gel, a mixed coexisting glass-gel state, and several partially localized states. The dynamical complexity increases with aspect ratio, and reflects a rich competition between repulsive force caging, physical bond formation and rod interpenetration. Removal of the nanosphere attraction destroys the double gel state but otherwise has minor consequences. The elastic shear modulus and absolute yield stress are also studied, and order of magnitude changes are found along various trajectories in the dynamical phase diagram. Rods are found to generically impart greater bulk rigidity than spheres.   

Phonons and Rearrangements in Disordered Colloidal Glasses

I will describe experiments which explore the behaviors of disordered particle suspensions at high packing fraction (e.g, see references [1-5]). Recent systems studied include two-dimensional glasses composed of temperature-sensitive microgel particles, and colloidal glasses in binary fluid which permit interaction potential to be switched from attractive to repulsive. Displacement correlation matrix techniques are employed to derive phonon modes and phonon density of states, and video microscopy is employed to study particle rearrangements. Connections between phonons and stress-induced rearrangements will be described, and detailed observations of the properties of cooperative rearrangement events in attractive versus repulsive glasses will be presented. \\[4pt] [1] Chen, K., Manning, M.L., Yunker, P.J., Ellenbroek, W.G., Zhang, Z., Liu, A.J., and Yodh, A.G., \textit{Phys Rev Lett} 107, 108301 (2011).\\[0pt] [2] Yunker, P.J., Chen, K., Zhang, Z., and Yodh, A.G., \textit{Phys Rev Lett} 106, 225503 (2011).\\[0pt] [3] Chen, K., Ellenbroek, W.G., Zhang, Z.X., Chen, D.T.N., Yunker, P.J., Henkes, S., Brito, C., Dauchot, O., van Saarloos, W., Liu, A.J., and Yodh, A.G., \textit{Phys Rev Lett} 105, 025501 (2010).\\[0pt] [4] Yunker, P., Zhang, Z.X., and Yodh, A.G., \textit{Phys Rev Lett }104, 015701 (2010).\\[0pt] [5] Yunker, P., Zhang, Z.X., Aptowicz, K.B., and Yodh, A.G., \textit{Phys Rev Lett} 103, 11, 115701 (2009).   

Characterization of local shear zones that govern the deformation of colloidal glasses

Colloidal glass provides a unique experimental system with which to study the structure, defects, and dynamics of a generic amorphous material. We report experiments on 1.55-$\mu$m-diameter, hard-sphere silica colloidal glasses under conditions of uniform shear. We use confocal microscopy to follow the 3D, real-time trajectories of roughly 100,000 particles during homogeneous deformation and explore the roles of both glass density and applied strain rate. In this way, we probe the elastic, anelastic, and plastic response of the system, with particular focus on identifying specific mechanisms of deformation. In plastic deformation, we directly observe ``shear defects'' or ``shear transformation zones'' (STZs) as clusters of particles that behave as Eshelby inclusions, undergoing highly localized plastic strain. These clusters can be identified as regions that are best fits to the Eshelby strain field throughout the sample. We correlate the development of these regions with local density-related properties, including the Voronoi volume and the free volume of both individual particles and connected clusters of particles.   

Elasticity and microstructure of colloidal gels undergoing strain-induced yielding

The mechanism of yielding in colloidal gels, particularly for high strain-rate deformations that are typical of materials applications, is not fully understood. Here, we examine the oscillatory response and stress relaxation of a colloidal gel formed by short-range depletion interactions to find a correlation between the macroscopic rheological behavior and their microstructural features. We use confocal laser scanning microscopy to directly observe the strain-dependent evolution of 3D gel structure within a shearing device. The gels are made up of monodisperse poly(methyl methacrylate) spheres of different sizes and volume fractions dispersed in refractive index matched solvents. We impose simple shear flows of various strains on the gel and observe the 3D structural change after deformation. This is done using a UV light-activated photopolymer, which allows particle configurations to be locked in place rapidly ($<$0.6s) after yielding. We characterize the transition from a dense network to interconnected clusters using the contact number distribution. Our results show that rigid, stress-bearing clusters play an important part in contributing to the gel elasticity at large strains.   

Complex oscillatory yielding in model hard sphere glasses

The yielding behaviour of hard sphere glasses under large amplitude oscillatory shear has been studied by experimental rheology and Brownian Dynamics simulations. Here we focus on varying the frequency of oscillation probing the interplay between Brownian motion and shear-induced diffusion. Stress, structure and dynamics are followed by rheology and BD simulations Two frequency regimes are revealed: At low frequencies Brownian motion is dominant, assisting particles to escape their cage during a single step yielding process, identified by a peak of G'' at around the G'=G'' crossover. At high frequencies shear induced particle collisions causes cage breaking with the maximum energy dissipation marked by the G'' peak taking place beyond the G'=G'' crossover. Intermediate frequencies present a complex yielding behaviour influenced by both mechanisms that leads to a double peak in G'' that has not been reported before in HS glasses. The nonlinear response is quantified by the higher harmonics present in the stress signal. While at low frequencies the strength of higher harmonics reaches a constant value at high strains, in the intermediate and high frequency regime a non-monotonic behaviour is detected with characteristic large amplitude strains exhibiting apparent harmonic response.   

Interactions of Dislocations with Twin Boundaries in Hard-Sphere Colloidal Crystals

The interaction of dislocations with twin or grain boundaries is a key factor in understanding the strength and strain hardening of crystals. Confocal tracking of particles in a colloidal crystal allows a detailed look at the mechanisms of such interactions at the particle scale, analogously to what happens on the atomic scale in conventional crystals. The grain boundaries are prepared by sedimentation onto templates in the [110] $\Sigma $3 orientation. A complex set of interactions is observed: additional twinning, the emission of a pair of partial dislocations, and the formation of kinks in the original twin boundary.   

Gel formation and aging in weakly attractive nanocolloid suspensions

We present combined x-ray photon correlation spectroscopy (XPCS) and rheometry studies of the evolution of concentrated suspensions of nanometer-scale colloids undergoing thermo-reversible gelation and aging. After a quench through the gel point, suspensions display a protracted latency period in which they remain fluid followed by a gelation regime in which the shear modulus grows rapidly. The XPCS intermediate scattering function displays two features, a plateau value that provides information about constrained local dynamics and a terminal decay related to relaxation of residual stress. From the wave-vector dependence of the plateau value, a localization length can be extracted. At intermediate colloidal volume fractions ($\phi\simeq 0.20$), the relationship between the localization length and the shear modulus agrees quantitatively with a prediction based on a simplified mode coupling theory, while deviations from the predicted scaling at a higher volume fraction ($\phi\simeq 0.43$) are observed near the gel point. While some features of slow strain from stress relaxation correlate with the evolving rheology, others appear decoupled from the macroscopic behavior.   

Phase-Change Dynamics in Attractive, Polydisperse Colloidal Suspensions

Understanding the single-particle dynamics of phase changes in polydisperse suspensions is important for predicting the structures that arise in these systems. In this talk, we discuss the results of experiments on the melting and freezing of polydisperse colloidal particles. In these experiments, micron-sized colloidal particles are sedimented in water onto a glass coverslip to form a quasi two-dimensional gas. The particles experience an attractive interaction due to a size-tunable depletant added to the mixture. This allows both melting and freezing to be probed in the same experiment. Optical images with single-particle resolution are recorded and each particle is tracked through the duration of the experiment. Interestingly, we find that the increase in polydispersity may stabilize the fluid phase, which has previously been shown to be metastable in experiments on monodisperse particles. In particular, we explore the stability, structure, and dynamics of this fluid phase and how the structures in this phase solidify over time.   

The role of picosecond dynamics in relaxation of molecular glasses

The importance of the relative amplitude of elemental relaxation processes, specifically of the fast $\beta $ relaxation process ($<$u$^{2}>)$, to long-time relaxation processes have been emphasized in a number of theoretical models. In this talk I will focus on transport properties and their relationship to $<$u$^{2}>$ in supercooled liquids and glasses. In these studies we use antiplasticization to systematically tune molecular packing. These results provide insights into the role that localization and the concomitant emergence of an excess density of states plays in relaxation of and transport in molecular glasses.   

AC Impedance Study of the Structural Transformation of Graphite Suspensions

Recently graphite suspensions have been demonstrated to possess very high thermal conductivity enhancements with high stability. However, the thermal conductivity is quite sensitive to the internal structures of the suspensions or the different states of the particulates in the liquids. At low graphite loadings, the graphite particulates can only form evenly distributed isolated clusters in the host material. As the graphite loading increases, the clusters will start merging together to form a 3D percolation network and the graphite suspension becomes gel-like. For the first time, we have observed a sharp kink behavior in the thermal conductivity of suspensions at the percolation threshold. Combined microstructural and AC impedance spectroscopy studies suggest that this kink arises from the change in the bonding strength between graphite flakes as the suspensions go through a transition from isolated clusters to percolated structures. Our studies shed light on the heat conduction mechanisms of nanofluids, suspensions and composites.   

Large colloidal crystals grown by centrifugation onto a template

Colloidal crystals are commonly formed by sedimenting a colloidal solution at 1g onto a patterned template. Slow sedimentation was previously believed to be a requisite for growing large, perfect crystals without crossover to an amorphous sediment. By increasing the relative gravitational force applied to a monodisperse sample of hard-sphere, 1.55$\mu $m diameter silica colloids, we examined the effect of increased sedimentation velocity on the growth of face-centered cubic crystals on a (100) template. We varied relative centrifugal force up to 3000g, time of centrifugation, lattice parameter, and crystal thickness to assess their effect on crystal quality. Single crystals up to 52 $\mu$m thick were grown for all centrifugation speeds. Crystal defects were predominantly stacking faults (bounded by partial dislocations), most of which formed after the critical thickness was reached. The critical thickness, which is a function of the lattice mismatch between crystal and template, was measured directly by varying the crystal thickness. Final stacking fault and vacancy concentrations were independent of centrifugal force and time. We also examined samples centrifuged onto other templates to elucidate the critical role of the template design in directing crystal versus glass formation.