Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B21: Colloids II: Colloidal Crystals, Gels, and Glasses |
Hide Abstracts |
Sponsoring Units: DFD Chair: Maria Kilfoil, McGill University Room: Baltimore Convention Center 318 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B21.00001: Phase diagram and direct mechanical measurements of colloidal gels Na Wang, Maria Kilfoil We study the phase diagram of colloidal aggregates according to different salt concentration and particle volume fraction, in the absence of gravity. This phase diagram then helps us to locate the region to form strong colloidal gels, whose mechanical properties are studied with time-sharing optical tweezers. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B21.00002: Real Time Observations of Decaying Colloidal Clusters Marco Polin, Sang-Hyuk Lee, Willem Kegel, Andrew Hollingsworth, David Grier We have studied model colloidal dispersions characterized by long-range electrostatic repulsions and short-range entropically driven attractions. Through a combination of holographic optical trapping and precision digital video microscopy we can create nonequilibrium cluster configurations and track their decay fluctuations. These measurements reveal density and shape fluctuations that accompany the decay of the clusters. Such processes may cast additional light on decay of large atomic nuclei. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B21.00003: Length scale dependent relaxation in colloidal gels Emanuela Del Gado, Walter Kob Although gels are ubiquitous in fundamental science, technological applications and also in our daily life, their structural and dynamical properties are not well understood. In contrast to other systems that show a slow relaxation, such as glass-forming liquids, the structure of gels is given by an open network that is thought to be responsible for the unusual dynamical properties of these systems. We present the results of a recent study based on a simple model that does indeed have the characteristics of (colloidal) gel-forming systems at a finite temperature. By means of molecular dynamics computer simulations, we investigate the gel formation from the equilibrium sol phase. In particular we show that the strong length scale dependence of the dynamics in gel forming systems is tightly related to the formation of the gel structure and is therefore a general feature. This study allows for the first time to investigate on a microscopic level the relaxation processes in the incipient gel and to understand why they must strongly depend on the length scale investigated. In our model the mesh-size of the incipient gel network corresponds to a crossover length between dramatically different relaxation processes, from stretched to compressed exponentials. Moreover our results link the super-exponential relaxation at low temperature to the motion of pieces of the incipient gel structure. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B21.00004: Crossover from Intermittent to Continuum Dynamics for Locally Driven Colloids Charles Reichhardt, Cynthia J. Olson Reichhardt We simulate a colloid with charge $q_d$ driven through a disordered assembly of interacting colloids with charge $q$ and show that, for $q_d \approx q$, the velocity-force relation is nonlinear and the velocity fluctuations of the driven particle are highly intermittent with a $1/f$ characteristic. When $q_d \gg q$, the average velocity drops, the velocity force relation becomes linear, and the velocity fluctuations are Gaussian. We discuss the results in terms of a crossover from strongly intermittent heterogeneous dynamics to continuum dynamics. We also make several predictions for the transient response in the different regimes. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B21.00005: Evolving Bulk Properties of Collapsing Colloidal Gels Stephen Kamp, Maria Kilfoil We present a study of the time evolution of the elastic properties of colloidal depletion gels. Silica colloids were suspended in NaCMC solutions and homogenized. Both the colloid volume fraction and the interaction strength (polymer concentration) were varied. The time evolution of the elastic properties of the suspensions was studied with a bulk rheometer in a double-wall Couette cell throughout the gel lifetime. The early lifetime is characterized by an elastic shear modulus that increases logarithmically with time, following which the gels experience catastrophic failure and the elastic modulus drops dramatically. As the gel collapses, various complex behaviors are seen, including a temporary stabilization against collapse, and reformation of a new gel with its own elastic properties which then follows its own trajectory to collapse. Time-lapsed images were taken of identical samples in a separate transparent cell of identical dimensions and the gel height was used to calibrate the measured shear modulus values. The visual cell also allows us to see the sample-spanning collective rearrangement involved in the collapse. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B21.00006: Experimental characterization of the interface of a colloidal suspension. Jessica Hernandez-Guzman, Eric R. Weeks We investigate experimentally the interface between liquids and solids by using a suspension of colloids as our model. Their size and slow motion makes colloidal suspensions experimentally accessible using 3D high speed confocal microscopy. We track the position of the particles over time, and characterize the spatial structure using bond order parameters as has been done by previous workers. We additionally measure the mobility of each particle. We study the thickness of the interface both in terms of the static structure (which changes from liquid-like disorder to crystal-like order across the interface) and the mobility of the particles with respect to distance from the interface. We find a transition region with a thickness of a few particle diameters. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B21.00007: Freezing and Melting of Colloidal Crystals with Short-Range Attractive Potential J.R. Savage, D.W. Blair, R.A. Guyer, A.D. Dinsmore We study the kinetics of melting and freezing of colloidal crystals formed by a short-range attractive potential. We use aqueous suspensions of micron-sized latex spheres mixed with surfactant micelles, which create a depletion attraction among the spheres. Single- and multi-layer crystals appear on the glass surface. Upon uniformly heating or cooling, the micelles grow or shrink. Upon heating, the depletion attraction weakens by up to 0.7 kT, and the crystals melt. Optical microscopy is used to track the motions of hundreds of colloidal spheres for up to 8 hours, until crystals have melted. We initially observe a steady decrease in the size of the crystallites. When the size reaches approximately 20-30, however, crystallites rapidly shrink. Once the crystals have melted, we then supercool them and monitor the nucleation and growth of crystallites. The kinetics of individual bond-breaking events and the evolution of the crystalline order parameter in both melting and freezing will be presented. This work is supported by the NSF-DMR 0305395. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B21.00008: Melting of Temperature-Sensitive 3D Colloidal Crystals Ahmed Alsayed, Yilong Han, Arjun Yodh We employ thermally responsive monodisperse microgel colloidal spheres to study the melting mechanisms of colloidal crystals [1]. The particle diameter decreases with increasing temperature and leads to volume fraction changes that drive phase-transitions. We will describe observations of a variety of phenomena. Premelting, the localized loss of crystalline order near defects (e.g. grain boundaries) at volume fractions above the bulk melting transition, is directly observed by video microscopy, and is characterized by monitoring the first peak position of the particle pair correlation function. We find the position of the first peak shifts toward smaller particle separations at the onset of premelting. After Delaunay triangulation, mean square rotational and translational fluctuations of bonds were measured close to and away from defects. The behavior of all such quantities exhibits increased disorder near the defects. By locally heating the material within a crystal domain, we also studied the superheating and melting of a perfect 3D crystal. Finally, the introduction of weak attractions between spheres reveals free-floating 3D crystal `blobs' which can be made to melt and recrystallize by tuning the temperature. [1] A. M. Alsayed, M. F. Islam, J. Zhang, P. J. Collings, A. G. Yodh$,$ Science \textbf{309}, 1207 (2005). This work was supported by grants from NSF (DMR-0505048 and MRSEC DMR05-20020) and NASA (NAG8-2172). [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B21.00009: Evolution of Particle-Scale Dynamics in Suspensions of Weakly Attractive Colloids Undergoing Structural Arrest H. Guo, D. Liang, S. Ramakrishnan, C.F. Zukoski, J.L. Harden, R.L. Leheny Suspensions of colloids with weak, short-range attractions can undergo an ergodic to nonergodic transition (ENT) as the strength of the attraction or the particle concentration increases. At low densities the transition corresponds to gelation, while at high densities it is identified as an attractive glass transition. We employ x-ray photon correlation spectroscopy (XPCS) to investigate the slowing dynamics associated the ENT in suspensions of nanometer-scale silica colloids coated with octadecyl-hydrocarbon-chains at wavevectors corresponding to interparticle length scales. At high temperatures the chains form a solvated brush that stabilizes the colloids. At low temperature, the brush collapses leading to a short-range attraction between colloids. Following a quench in temperature, the intermediate scattering function displays two features, a plateau value and a terminal relaxation time, that increase with time since the quench. A comparison between suspensions with concentrations of $\phi=0.20 $ and $0.43$ shows qualitative differences in their temporal evolution, indicating a crossover from gelation-like to glass- like dynamical arrest. Further, a comparison with rheometry indicates how the slowing particle-scale dynamics correlates with the growth of the system's elastic modulus. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B21.00010: Microstructure and micromechanics of hard spheres with short-range attraction Myung Han Lee, Eric M. Furst We study the microscopic mechanical response of colloidal gels, such as yielding, compression and~rearrangement, by manipulating single probe particles within the network. For this work,~we use fluorescent polymethylmethacrylate (PMMA)~dispersed in mixtures of decalin and cyclohexylbromide, with a refractive index and~density close to that of PMMA. The strength of attraction is controlled by the~concentration of a non-adsorbing polymer, polystyrene, which induces a depletion attraction between particles. In the presence of sufficiently strong attractive~forces, particles form a colloidal gel.~Confocal microscopy is used to~observe the structural evolution in real time.~This enables us to investigate the role of the local structure~and particle interactions in the elasticity of the network. Specifically, we identify local elastic and plastic deformations in gels, which~depend on the probe oscillation amplitude and strength of attractive interactions. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B21.00011: Deformation of Colloidal Glasses Peter Schall, Itai Cohen, Frans Spaepen, David Weitz Amorphous colloidal suspensions are known to be powerful models for studying dynamical processes in glasses. Since the particles can be observed optically in real time, colloidal systems offer the unique opportunity of studying important mechanisms at the single particle level. We use confocal microscopy to investigate structural rearrangements in colloidal glasses under applied shear. Our setup allows us to track the motion of the individual particles in three dimensions while the suspension is sheared. We use the particle positions determined by confocal microscopy to calculate the local strain tensor and to identify regions of non-affine deformation. We observe ``shear zones'' that are accompanied by a complex strain field around them. Using the single particle data, we elucidate the particle rearrangements in the shear zones, and we investigate correlations between the location of these shear events and regions of high free volume. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B21.00012: Rotational perturbations of colloidal suspensions near the colloidal glass transition. Piotr Habdas, Eric R. Weeks By increasing the concentration of small solid particles in a liquid (colloidal suspension) a colloidal glass transition is approached. This is due to the fact that the system becomes increasingly crowded and when it reaches the glass transition it becomes essentially jammed. Therefore, a colloidal suspension is a model system that exhibits a glass transition. There are a few ways of locally perturbing a colloidal suspension. For example, one of the colloidal particles can be dragged through the colloidal suspension or a dimer made out of two small particles can be rotated. To perform the latter, we mix a small number of paramagnetic beads with dense PMMA particles. Some of the beads form dimers which can be put into rotation by rotating an external permanent magnet. Using confocal microscopy we study response of the system to such local perturbation. In particular, we investigate average spatial range of these perturbations and how it varies with the rotational rate and sample concentration. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B21.00013: Microscopy studies of the re-entrant glass transition. Andrzej Latka, Nora Graneto, Piotr Habdas Colloidal suspensions are a model system for studying the glass transition. At the volume fraction $\phi _g \approx 0.58$ a glass transition occurs and a hard sphere colloidal glass is formed. The formation of hard sphere glass is attributed to the ``caging'' effect, in which the particles form cages around each other that restrict their movement. Introducing an attractive depletion force between the particles surprisingly causes the hard sphere glass to melt and the system becomes a liquid. Interestingly, by further increasing the attractive force an attractive glass is formed. Our system is a suspension of nearly hard-sphere colloidal particles and nonadsorbing linear polymer which induces a depletion attraction between the particles. We study the dynamics of the colloidal particles using microscopy techniques. In particular, our research is focused on the transition into the attractive glass phase. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B21.00014: Scaling of non-linear viscoelastic transitions in soft colloidal glasses Matthew Helgeson, Norman Wagner, Dimitris Vlassopoulos Soft colloidal particles undergo a transition to a glass-like state at sufficient concentration, due to kinetic trapping, similar to the glass transition in hard sphere systems. In this work we explore the use of rheological measurements as a tool to probe the mechanisms that lead to structure formation and breakage under shear in the glassy state for a monodisperse solution of multi-arm star polymers in an athermal solvent. Maxima in the loss moduli are observed with increasing strain amplitude at the onset of shear melting. We show that the transition to non-linear viscoelastic behavior and the onset of flow follow a systematic, rate-dependent trend. Specifically the critical strain increases with frequency. We discuss this trend in terms of phenomenological understanding of the kinetic trapping of soft colloids in the glassy state, leading to characterization of the softness of the colloidal glass as well as mechanisms of shear melting. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700