Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F17: Focus Session: Glass Formation and Crystallization in Anisotropic Particles |
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Sponsoring Units: GSNP DPOLY Chair: Corey O'Hern, Yale University Room: 402 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F17.00001: Odd-even Effects of Glass Transition Temperature with a Network-forming Ionic Glass Ke Yang, Madhusudan Tyagi, Jeffrey Moore, Yang Zhang Odd-even effects, the non-monotonic dependency of physical properties on odd/even structural units, are widely observed in homologous series of crystalline materials. However, such alternation is not expected for molecular amorphous materials. Herein, we report the synthesis of a class of network-forming ionic glasses (IG) using non-spherical multivalent ammonium cations and citrate anions. The glass transition temperatures of these amorphous solids show an alternating pattern with increasing backbone length. To understand the phenomenon's molecular origin, we performed incoherent elastic neutron scattering measurements of the nanosecond atomic dynamics. In addition, quasi-elastic neutron scattering measurements were performed to measure two very discrete relaxation process in this ionic glass. Our results suggest that the molecules' mobility, thus the glass transition temperature, correlates with their structural symmetry. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F17.00002: Phase behavior and crystal nucleation and growth in a system of short semi-flexible chains Bart Vorselaars, David Quigley A system of semi-flexible short chains is simulated to study its phase behavior and ability to crystallize, by using a combination of molecular dynamics and other techniques. For calculating the free energy of the liquid phase a new method is introduced. It is very simple to implement in practice and leads to accurate computation of the melting curve. Furthermore we determine the rate of nucleation and crystal growth in this system via a combination of path-sampling and brute-force simulation techniques. By comparing these quantities, we infer the initial microstructure of the solid phase. Due to the strong anisotropy in the crystal growth rate grains no thicker than a single chain are common, even at moderate supercoolings. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 9:00AM |
F17.00003: Effect of chain topology and angular interactions on the competition between crystallization and glass-formation Invited Speaker: Robert S. Hoy We investigate the role of chain topology and angular interactions on the competition between crystallization and glass formation by mapping out the phase diagram of model ``soft'' colloidal polymers as a function of temperature and bending stiffness $k_b$, spanning the range from fully flexible to rodlike chains. For small $k_b$, monomers occupy the sites of close-packed crystallites while chains retain random-walk-like order. For large $k_b$, for short chains, nematic chain ordering typical of lamellar precursors coexists with a high degree of close-packing, while for longer chains, close-packed chain-folded lamellae separated by amorphous regions are formed. At intermediate values of bending stiffness, the competition between random-walk-like and nematic chain ordering produces glass-formation, as indicated by both dynamical heterogeneity and the growth of icosahedral order near and below $T_g$. The kinetics of the ordering transition depend strongly on chain flexibility: in the flexible limit chains crystallize at lower temperatures and the disorder-order transition occurs very sharply, while as the rodlike limit is approached, the initial phase of ordering occurs at significantly higher temperatures but the crystallization rate is much slower. We also examine the crystallization behavior of short (unentangled) branched polymers as a function of $k_b$. Branching points significantly suppress crystallization, especially for stiffer chains, because their preferred bond angles are incompatible with close-packing. Finally, we discuss the degree to which colloidal polymers can serve as proxies for their microscopic counterparts. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F17.00004: Colloidal analogues of spin systems: Order and phase transitions in dense suspensions of magnetic ellipsoids Peter Schurtenberger, Ilya Martchenko, Jerome Crassous We have determined the phase diagram of magnetic colloidal ellipsoids as a function of both packing fraction $\phi$ and external magnetic field $B$. We use core-shell particles with a magnetic core where the magnetic moment of the core is sufficiently small to avoid additional dipole-dipole interactions, but high enough to induce preferential particle alignment with an external magnetic field. By using a combination of small-angle x-ray scattering, microscopy and magnetometry we have examined positional correlations of the charged ellipsoids (aspect ratio $p = 2.7$) and orientational order of their magnetic moments. We establish structural criteria for the different phase and arrest transitions and map distinct isotropic, nematic and crystalline phases over an extended range of $\phi - B$ coordinates. We demonstrate that upon crystallization of the ellipsoids, the bulk magnetic behavior of the suspensions switches from superparamagnetic to ferromagnetic. We extend the often-used atom-colloid analogy to spin systems and present a relationship between the structural topology of suspensions of magnetic colloids and their macroscopic magnetic response. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F17.00005: Structural signatures of dynamic heterogeneities in monolayers of colloidal ellipsoids Yilong Han, Zhongyu Zheng, Ran Ni, Feng Wang, Marjolein Dijkstra, Yuren Wang we discovered two relationships between dynamic heterogeneity and structure for both translational and rotational motion in monolayers of colloidal ellipsoids by video microscopy: (1) the onsets of translational and rotational dynamic heterogeneities coincide with the maximum density fluctuation and the maximum orientation fluctuation respectively at aspect ratio > 2.5; and (2) the dynamic slowest-moving clusters, the static glassy clusters and the low-entropy clusters are strongly correlated and their sizes diverge at the ideal glass transition point with the same power-law length scaling as a function of density, whereas the size of the fastest-moving clusters diverges at the mode-coupling critical point. These results show that the glass transition has a thermodynamic origin. In addition, we observed one-step and two-step glass transitions at different aspect ratios. All experimental results were confirmed by simulations. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F17.00006: Characterizing the local environment for self-assembly Wenbo Shen, Greg van Anders, Eric Harper, Matthew P. Spellings, Michael Engel, Sharon C. Glotzer Recent advances in synthesis techniques of nano- and micrometer sized colloids have produced a diversity of enthalpically and entropically patchy particles. Relating particle patchiness and structure is necessary for the successful self-assembly of these building blocks. We investigate the relationship between patchiness and local structure by quantifying variations in the relative alignment of particle pairs. Using these methods we compare the efficiency of different types of patchiness for inducing ordering and apply them to self-complementary and actively driven shapes. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F17.00007: Jamming transition in hierarchical networks Xiang Cheng, Stefan Boettcher Jamming transitions arise in disordered granular materials where the systems fall out of equilibrium due to an increase in the packing density. A kinetically constrained lattice gas model due to Biroli and Mezard (BM) has connected the jamming transition to an equilibrium phase transition.\footnote{F. Krzakala {\it et al.}, Phys. Rev. Lett. {\bf 101}, 165702 (2008).} In this description, before this equilibrium transition can be reached, any experiment or simulation would fall out of equilibrium at a Kauzmann transition. However, this analysis is based on a mean-field calculation which, for disordered systems, may have limited relevance in finite dimensions. We study the BM-model on a lattice-like network,\footnote{S. Boettcher and A. K. Hartmann, Phys. Rev. E {\bf 84}, 011108 (2011).} which mixes geometric and mean-field features, to reproduce such a phase transition. Computationally, we use the Wang-Landau algorithm which should be less affected by the jamming near the phase transition. The algorithm produces the density of states and, hence, the entropy directly, in addition to many critical properties, such as packing fraction, compressibility, etc. Also, lattice-like hierarchical networks conveniently allow exact or approximate renormalization group treatments, extending analytical results to the thermodynamic limit. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F17.00008: The Effect of Particle Shape on the Density of Acoustic Modes in Granular Materials Alex Mauney, Sara Berry, Theodore Brzinski, Karen Daniels Most granular simulations and experiments are conducted using circular particles. We report on the effect of particle shape on the density of vibrational modes in a real two dimensional granular material. We acoustically excite static packings composed of one of four shapes: circles, ellipses, pentagons, and concave stars. Using embedded piezoelectric sensors we measure the particle-scale vibrational response. We observe shape-dependence in the acoustic spectra and density of modes, particularly at low frequency. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F17.00009: Stabilizing Liquid Drops in Nonequilibrium Shapes by the Interfacial Jamming of Nanoparticles Mengmeng Cui, Todd Emrick, Thomas Russell Nanoparticles can assemble at the interface between two fluids into a 2-D, forming liquid-like arrays where the nanoparticles can diffuse laterally at the interface. By changing the shape of the liquid domain with an external field, the surface area increases and more nanoparticles adsorb to the interface. By releasing the field, the interfacial area decreases and the nanoparticles are jammed, arresting further change in the shape of the drop. The shapes of the liquid can be tailored and indefinitely remain trapped into shapes far different than spherical indefinitely. Limitations related to the inherent weak forces holding the nanoparticles at the interface are overcome by generating nanoparticle-surfactants in situ. The ability to generate and stabilize liquids with a prescribed shape poses unique opportunities for reactive liquid systems, packaging and delivery, and storage. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F17.00010: Tumbling Motion of Interacting U-Shaped Particles in a Uniform Shear Flow Near Jamming Theodore Marschall, Scott Franklin, Stephen Teitel We simulate a system of overdamped frictionless U-shaped particles (staples) in a uniformly sheared host fluid. An isolated staple in such a shear flow undergoes a tumbling motion due to its asymmetric shape, with average angular velocity proportional to the shear strain rate. We investigate how this tumbling motion is modified in a dense system of interacting staples as we approach the jamming transition. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F17.00011: Strain-rate and temperature-driven transition in the shear transformation zone Penghui Cao, Xi Lin, Harold S. Park We couple the recently developed self-learning metabasin escape algorithm, which enables efficient exploration of the potential energy surface (PES), with shear deformation to elucidate strain-rate and temperature effects on the shear transformation zone (STZ) characteristics in two-dimensional amorphous solids. In doing so, we report a transition in the STZ characteristics that can be obtained through either increasing the temperature or decreasing the strain rate. The transition separates regions having two distinct STZ characteristics. Specifically, at high temperatures and high strain rates, we show that the STZs have characteristics identical to those that emerge from purely strain-driven, athermal quasistatic atomistic calculations. At lower temperatures and experimentally relevant strain rates, we use the newly coupled PES + shear deformation method to show that the STZs have characteristics identical to those that emerge from a purely thermally activated state. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F17.00012: An experimental study of the phases of hard squares Lee Walsh, Narayanan Menon We study the phase diagram of hard squares in two dimensions using millimeter-sized square particles on a vibrated plate. The plate serves as a quasi-thermal noise source which generates translational and rotational diffusion of isolated particles. As area density increases, the spatial arrangement of the squares undergoes a transition from isotropic to phases with four- and six-fold ordering, and subsequently develops crystalline order. This succession of transitions in orientational and translational ordering is in qualitative agreement with recent simulations [C. Avenda\~no and F. Escobedo, Soft Matter 2012 (8) 4675]. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F17.00013: Tunneling percolation behavior and filling factors in metal-insulator nanocomposites Rupam Mukherjee, Zhi-Feng Huang, Boris Nadgorny We have studied conventional transport and tunneling in nanocomposite metal-insulator systems. Two types of percolation thresholds P$_{\mathrm{C}}^{1}$ and P$_{\mathrm{C}}^{2}$, associated with both conventional transport and quantum tunneling respectively are identified in the same nanocomposite systems and the functional dependence between the two thresholds is investigated. In addition, we have studied the relationship between filling factors and percolation thresholds, particularly the importance of geometric effects of nanoparticles of different sizes and shapes in metal-insulator composite systems. A non-monotonic dependence of filling factor as a function of filler volume fraction is established and possible implications of this non-trivial behavior is discussed. [Preview Abstract] |
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