Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D29: Focus Session: Colloids II |
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Sponsoring Units: DFD Chair: Mark Bowick, Syracuse University Room: Colorado Convention Center 303 |
Monday, March 5, 2007 2:30PM - 3:06PM |
D29.00001: Imaging the Dynamics of Freezing and Sublimation of Colloidal Crystals Invited Speaker: Anthony D. Dinsmore We study the kinetics of freezing and sublimating colloidal crystals with single-particle resolution. In experiments, a short-ranged depletion attraction between spheres leads to crystallites that are one to three layers thick. The spheres are tracked with optical microscopy and the sizes and bond-orientational order parameters of the crystallites are measured. The inter-particle attraction is reduced or increased by modest changes in temperature, which lead either to sublimation of crystallites or to formation of crystallites from a gas phase. The sublimation process is also investigated using Brownian Dynamics simulations. In both experiments and simulations of sublimation, we find a two-stage process: at first, large crystallites sublimate by escape of particles from the perimeter. The rate of crystallite shrinkage is then greatly enhanced as the size falls below a cross-over value that ranges between 20 and 50 in different regions of the phase diagram. Simultaneous with the enhanced sublimation rate, the crystallites transform to a dense amorphous structure, which then rapidly vaporizes. The two-step kinetics are also seen in freezing at sphere area fractions near 0.3, but not at substantially higher or lower area fractions. The two-step kinetics are attributed to a thermodynamically meta- or unstable amorphous phase (ten Wolde and Frenkel, Science \textbf{277}, 1975 (1997).). The results should be relevant in diverse systems including colloids, proteins, and atoms such as Argon. We gratefully acknowledge support from Research Corporation and from the NSF through grant DMR-0605839. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D29.00002: Dynamics and Instabilities of Defects in Curved Two-Dimensional Crystals Mark Bowick, Homin Shin, Alex Travesset Point defects play a fundamental role in determining the thermodynamic, elastic and mechanical properties of two-dimensional crystals. When such crystals are curved, finite length grain boundaries (scars) appear as basic structural features. We discuss an analytical determination of the elastic spring constants of dislocations bound within scars and compare them with existing experimental measurements from optical microscopy. We further show that vacancies and interstitials, which are stable defects in flat crystals, are generally unstable in curved geometries. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D29.00003: Two-Dimensional Melting of Microgel Colloidal Crystals Yilong Han, Na Young Ha, Ahmed Alsayed, Arjun Yodh We investigate the phase behavior of 2D colloidal crystals composed of NIPA (N-isopropyl acrylamide) microgel spheres whose diameters can be temperature-tuned. The measurement of a variety of densities of defects, order parameters and correlation functions (static and dynamic) are reported and are in agreement with KTHNY theory at least some of the time. In contrast to previous experiments we use the divergence of translational and rotational susceptibilities (i.e. fluctuations of the corresponding order parameters) to determine the phase transition points. This approach avoids some ambiguities inherent in the other analyses and clearly resolves the intermediate hexatic phase between the solid and liquid phases. Our measurements uncover a novel premelting stage in solid and suggest that traditional analysis methods can incorrectly associate the premelting stage with the hexatic phase. In separate measurements of the melting of two-layer square lattices, we also observed a 'middle' phase. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D29.00004: Angular rheology study of colloidal nanocrystals using Coherent X-ray Diffraction Mengning Liang, Ross Harder, Ian Robinson A new method using coherent x-ray diffraction provides a way to investigate the rotational motion of a colloidal suspension of crystals in real time. Coherent x-ray diffraction uses the long coherence lengths of synchrotron sources to illuminate a nanoscale particle coherently over its spatial dimensions. The penetration of high energy x-rays into various media allows for in-situ measurements making it ideal for suspensions. This technique has been used to image the structure of nanocrystals for some time but also has the capability of providing information about the orientation and dynamics of crystals. The particles are imaged in a specific diffraction condition allowing us to determine their orientation and observe how they rotate in real time with exceptional resolution. Such sensitivity allows for the study of rotational Brownian motion of nanocrystals in various suspensions and conditions. We present a study of the angular rheology of alumina and TiO2 colloidal nanocrystals in media using coherent x-ray diffraction. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D29.00005: Correlated Motion of Rods Diffusing in 3D Kenneth Desmond, Eric R. Weeks It's well known that micron size particles suspended in a fluid will undergo Brownian motion. This Brownian motion is the result of thermal fluctuations that cause the particles to exhibit both translational and rotational diffusion. Translational diffusion due to Brownian motion has been well studied in the past, but rotational diffusion has not received nearly as much investigation. In our experiments, we observe rotational diffusion using polystyrene ellipsoids suspended in a water glycerol mixture. We have developed an algorithm to detect both the center of mass and orientation of our ellipsoidal particles in 3 dimensions. We examine spatial correlations between rotational and translational motion of pairs of these particles. It's known that the spatial correlation between the translational motion of spherical particles decays as 1/r in a homogeneous solution where r is the separation distance between two particles. We are currently investigating the spatial decay of rotational correlation of the colloidal rods. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D29.00006: Two-Dimensional Phase Behavior of Colloidal Peanuts Sharon Gerbode, Angie Wolfgang, Stephanie Lee, Bettina John, Chekesha Liddell, Fernando Escobedo, Itai Cohen While the phase behavior of spherical colloidal suspensions has been well studied, the ordering of non-spherical colloidal particles remains a largely unexplored yet important problem. In this talk we will describe ongoing studies of one very simple extension of the spherical particle: the colloidal peanut. These peanuts have an aspect ratio that makes them comparable to dimer particles. Confining the colloidal peanuts to two dimensions, we find that the suspension can undergo a phase transition from a liquid to an ordered phase in which each individual peanut lobe resides on a triangular lattice site. The lobe packing is very similar to the hexagonally close packed crystalline arrangement formed by spheres in 2D. Unlike their spherical counterparts, however, the colloidal peanuts are not isotropic, and in particular, each peanut has a specific orientation, or director. In this talk we will describe the correlations between defects in the underlying triangular lattice and the local director field. We will also report on our measurements of long-range director correlations, and if time permits, we will describe ongoing work relating to phases formed by peanut particles with different aspect ratios. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D29.00007: Cage Trapping and Melting of Colloidal Suspensions under Confinement and Shear Excitation Prasad Sarangapani, Y. Elaine Zhu The details of the glass transition are still hotly debated. The unusual phenomenon where the viscosity of supercooled fluids diverges near the glass transition without marked structural change is often attributed to a growing length scale of cooperatively rearranging clusters (CRC) of molecules or particles. One way to probe the dynamics of CRC is through confinement, where a glass transition can be observed `sooner' as film thickness approaches a critical value while temperature and volume fraction remain constant. We study a hard-sphere poly(methyl methacrylate) colloidal suspension to model glassy materials. Using a home-designed micro-rheometer interfaced with a confocal microscope, we visualize the structure and dynamics of confined colloidal thin films between two surfaces at narrow gap spacing ranging from 50 $\mu $m to 1-2 $\mu $m. Recent experimental evidence has shown that the size of CRC grows dramatically as film thickness approaches an apparent critical dimension of 10-15 particle layers. In preliminary experiments by \textit{in situ} shear force measurements and microscopic characterization, we investigate the re-fluidization or `melting' of glassy colloidal thin films by applying large shear amplitude and frequency. This phenomenon consequently causes the $\alpha $ and $\beta $ relaxation regimes to occur sooner compared to un-sheared confined glassy thin films. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D29.00008: Phase transitions in charged colloidal suspensions Gregg Lois, Corey O'Hern Experiments on charged colloidal suspensions suggest that their structure and dynamics are sensitive to small variations in the amount of charge deposited on each particle. We numerically explore the phase diagram of charged colloidal suspensions for different values of temperature and charge polydispersity. For increasing charge polydispersity we find that the crystalline ground state is no longer accessible at low temperature and the system forms a glass. We compare the dynamic signatures of this state to the properties of hard-sphere colloids with size polydispersity. We also observe spatial and temporal inhomogeneities in the glassy state and examine the length and time scales over which they persist. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D29.00009: Coulomb Interactions of Colloidal Particles in Oil Sunil Sainis, Eric Dufresne We study the electrostatic interactions of microspheres (PMMA-PHSA) in solutions of surfactant (NaAOT) in oil (hexadecane). We directly measure the forces between isolated pairs of particles to extract the particle charge and solvent ionic strength. Over a wide range of surfactant concentrations, the interparticle forces are indistinguishable from unscreened Coulomb interactions. Far above the critical micelle concentration, however, the interactions assume the familiar screened Debye-Huckel form. Long-ranged interactions between micron-sized particles provide a window to study the structure and dynamics of strongly-correlated systems. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D29.00010: Computer Simulation of Colloidal Electrophoresis Burkhard Duenweg, Vladimir Lobaskin, Krishnan Seethalakshmy-Hariharan, Christian Holm We study the motion of a charged colloidal sphere surrounded by solvent, counterions, and salt ions, under the influence of an external electric field. The ions are modeled as particles which interact dissipatively with a lattice Boltzmann background, such that hydrodynamic interactions are taken into account. Similarly, the colloid is modeled as a spherical array of such point particles. Finite concentration values are taken into account by simulating the system in a box with periodic boundary conditions. In terms of dimensionless reduced parameters, the results compare favorably with experimental data. As a complementary approach, we solve the electrokinetic equations by a finite element method. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D29.00011: Structure of highly packed microgel particles Alberto Fernandez-Nieves, Johan Mattson, Hans Wyss, Lidia Rodriguez-Maldonado, Manuel Marquez, Enrique Lopez-Cabarcos, Antonio Fernandez-Barbero, David A. Weitz We study the structure of concentrated suspensions of ionic microgel particles. In the shrunken state, the particles are essentially charged hard spheres and crystallize at high enough volume fractions. When swollen, however, we find no sings of crystallization, as shown by light and neutron scattering experiments; this is the case irrespective of particle concentration. Instead, the scattered intensity is characterized by the presence of two distinct peaks at low and high scattering wave vectors. Surprisingly, we find that the shift of the peaks follow identical scaling laws with concentration both above and below random close packing. The scaling is different for both peaks indicating they have a different physical origin. While the first maximum seems to be related to the structure of the system, the second peak seems to arise from charge correlations inside the microgel particles. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D29.00012: Effective Interactions Between Like-Charged Colloids: The Role of Colloid Charge Stephen Barr, Erik Luijten We investigate the effect of colloid charge on the interactions between like-charged colloids in the presence of multivalent counterions by means of computer simulations. Because there is a large size asymmetry between the colloids and the counterions, conventional simulation methods are inefficient. In order to overcome this, we extend the generalized geometric cluster algorithm for colloidal suspensions [J. Liu and E. Luijten, Phys.\ Rev.Lett.\ \textbf{92}, 035504 (2004)] to allow for the efficient simulation of systems with electrostatic interactions. In the presence of multivalent counterions, like-charged attraction between the colloids is found to occur over a window of colloid charges. If the colloid charge is too low, the colloid-counterion attraction is too weak for like-charged attraction to occur, and if the colloid charge is too high, the direct electrostatic repulsion overwhelms the attraction induced by the counterions. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D29.00013: Diffusion of charged colloidal particles at aqueous interfaces Penger Tong, Wei Chen We report our recent experimental study of Brownian dynamics of weakly charged particles at a water-air interface. Optical microscopy and multi-particle tracking are used to measure the mean square displacement of the interfacial particles. The measured short-time self-diffusion coefficient $D^s_s$ has the form, $D^s_s /D_0 = \alpha (1 - \beta n)$, where $n$ is the area fraction occupied by the particles and $D_0$ is the Stokes- Einstein diffusion coefficient. The values of the fitting parameters $\alpha$ and $\beta$ are found to be different from those for the three dimensional (3D) colloidal suspensions, indicating that hydrodynamic interactions at the interface have interesting new features when compared with their 3D counterpart. *This work was supported by the Research Grants Council of Hong Kong SAR under Grant No. HKUST603305. [Preview Abstract] |
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