Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K15: Complex phases: Colloids and QuasicrystalsFocus
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Sponsoring Units: GSNP DMP GSOFT Chair: Peter Olmsted, Georgetown Universtity Room: 274 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K15.00001: Universal crystal structures in hard and soft matter systems Julia Dshemuchadse, Michael Engel, Sharon C. Glotzer Increasingly complex structures, from large cluster-based unit cells to quasicrystals, are being discovered in soft-matter systems in both experimental and computational studies. Several of these were previously known from compounds on the atomic scale, while others are novel. We perform high-throughput molecular-dynamics simulations of interacting particles to screen large sections of phase space. After analyzing the resulting data crystallographically, we investigate the conditions under which complex structures with large unit cells and diverse symmetries form in different kinds of soft-matter systems. These insights enable the targeted design of soft matter structures by an intelligent choice of building blocks. On the other hand, the agnostic treatment of the interaction potentials will help us distill the decisive factors that lead to the formation of any particular crystal structure. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K15.00002: Three-dimensional Icosahedral Phase Field Quasicrystal Priya Subramanian, Andrew Archer, Edgar Knobloch, Alastair Rucklidge Complex quasiperiodic structures are observed in a variety of soft matter systems, including dendrimers and star block copolymers. We investigate the formation and stability of such quasicrystals in three dimensions using a phase field crystal model. In the model, two length scales (in the golden ratio) are selected and can be controlled independently. In addition to regular crystals, one-, two- and three-dimensional quasicrystals can be found. We compute the minima of the free energy of the competing structures to determine the phase diagram and show that the icosahedral quasicrystal can be the global minimum free energy state.$^*$ We find that strong nonlinear coupling between density waves at the two length scales is responsible for stabilizing the three-dimensional icosahedral quasicrystal.\\ $^*$P. Subramanian, A. J. Archer, E. Knobloch and A. M. Rucklidge. Three-dimensional icosahedral phase field quasicrystal. Phys. Rev. Lett. 117, 075501 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 9:00AM |
K15.00003: Formation of ordered, nonperiodic structures: Icosahedral quasicrystals, hexagonal limit-periodic systems, and confined hard spheres Invited Speaker: Joshua Socolar The discovery in the early 1980’s that certain metallic alloys can form quasicrystalline phases challenged our understanding of the formation of long range translational order in atomic (or colloidal) systems. Both the thermodynamic stability and the kinetic accessibility of ordered, nonperiodic structures have received considerable attention, but fundamental questions remain open. I will present recent results on the emergence of nonperiodic order in three systems: tiling models that achieve nearly perfect growth of an icosahedral quasicrystal~\footnote{C.~T.~Hann, J.~E.~S.~Socolar, and P.~J.~Steinhardt, {\it Phys.~Rev.~B} {\bf 94}, 014113 (2016)}; a one-dimensional quasiperiodic structure that maximizes the density of hard spheres confined to a cylinder~\footnote{L.~Fu, W.~Steinhardt, H.~Zhao, J.~E.~S.~Socolar, and P.~Charbonneau {\it Soft Matter}, {\bf 12}, 2505-2514 (2016)}; and model systems composed of a single structured particle with only nearest neighbor interactions in which a slow quench induces formation of a limit-periodic phase~\footnote{C.~Marcoux, T.~W.~Byington, Z.~Qian, P.~Charbonneau, and J.~E.~S.~Socolar {\it Phys.~Rev.~E} {\bf 90}, 012136 (2014)}. For the icosahedral system, there are known materials that motivate the questions about growth dynamics. For the limit-periodic system, the theory suggests new possibilities for colloidal phases that may have a novel hierarchy of vibrational modes~\footnote{C.~Marcoux and J.~E.~S.~Socolar, {\it Phys.~Rev.~E} {\bf 93}, 174102 (2016)}. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K15.00004: Structure of Ultrathin Films of Barium Titanate Eric Cockayne BaTiO$_3$ is one of the best-known perovskite ferroelectrics, and has been extensively studied in its bulk and thin-film forms. Remarkably, a variety of new structures (including a dodecagonal quasicrystal) have recently been observed in ultrathin Ba-Ti-O films fabricated via various multistep heat treatments of an initial thin film perovskite BaTiO$_3$ layer deposited on a Pt(111) surface. We have found a tiling decoration model for the ultrathin film Ba-Ti-O structures that is fully consistent with experimental observations. The structures consist of rumpled Ti-O networks with each Ti threefold coordinated with O, and Ba nestled in the larger, mainly Ti$_7$O$_7$, pores. Simulated STM images show that the circular protrusions observed experimentally are the Ba ions. Ab initio molecular dynamics calculations demonstrate the stability of our model with respect to competing models for these thin film structures. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K15.00005: Engineering Entropy for Colloidal Design Yina Geng, Greg van Anders, Paul M. Dodd, Sharon C. Glotzer The inverse design of target material structures is a fundamental challenge. Here, we demonstrate the direct inverse design of soft materials for target crystal structures using entropy alone. Our approach does not require any geometric ansatz. Instead, it efficiently samples 92- or 188-dimensional building-block parameter spaces to determine thermodynamically optimal shapes. We present detailed data for optimal particle characteristics and parameter tolerances for six target structures. Our results demonstrate a general, rational, and precise method for engineering new colloidal materials, and will guide nanoparticle synthesis to realize these materials. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K15.00006: Efficient Parameter Searches for Colloidal Materials Design with Digital Alchemy Paul M. Dodd, Yina Geng, Greg van Anders, Sharon C. Glotzer Optimal colloidal materials design is challenging, even for high-throughput or genomic approaches, because the design space provided by modern colloid synthesis techniques can easily have dozens of dimensions. In this talk we present the methodology of an inverse approach we term "digital alchemy" to perform rapid searches of design-paramenter spaces with up to 188 dimensions that yield thermodynamically optimal colloid parameters for target crystal structures with up to 20 particles in a unit cell. The method relies only on fundamental principles of statistical mechanics and Metropolis Monte Carlo techniques, and yields particle attribute tolerances via analogues of familiar stress-strain relationships. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K15.00007: Self-assembly of colloidal spheres on a cylinder Nabila Tanjeem, Henry Wilkin, Vinothan N. Manoharan We study crystal growth on a cylindrical surface in order to understand how the geometrical constraints give rise to different types of structures. In our experimental system, submicron-sized colloidal spheres assemble into hexagonal lattices on a drawn silica fiber a few micrometers in diameter. The assembly is driven by a short-ranged depletion interaction between the spheres and between the spheres and fiber. Because a cylinder has zero Gaussian curvature and can accommodate a finite number of particles around its circumference, different crystalline structures assemble, depending on the ratio of the particle diameter to the cylinder circumference. In particular, we observe crystals of different chirality and crystals with defects that arise from packing constraints imposed by the cylindrical geometry. We demonstrate how these crystals grow and how the defects form. We also show preliminary simulations and experiments on crystal growth on tapered cylinders. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K15.00008: Assembly of hard spheres in a cylinder: a computational and experimental study Lin Fu, Ce Bian, Wyatt Shields, Daniela Cruz, Gabriel Lopez, Patrick Charbonneau Arrangements of hard spheres confined to a hard cylinder have been used to model various experimental systems, such as fullerenes in nanotubes. The study of the densest packings offers a rich set of targets, and the study of their assembly dynamics hints at what may be achievable in experiments. We used enhanced optimization schemes to identify the former, and both simulations and experiments to study the latter. The equilibrium and out-of-equilibrium assembly of hard spheres of diameter $\sigma$ within cylinders of diameter $\sigma\le D\le 2.82\sigma$ reveals that although phase transitions formally do not exist in a quasi-one-dimensional system, marked structural crossovers can nonetheless be observed. The origin of this effect is studied by a transfer matrix approach for small D. We also find that the sequence of equilibrium assemblies echoes the densest packing sequence at equilibrium, but that out-of-equilibrium self-assembly offers a rather rich control over the final morphology. Crossovers for which no continuous line-slip exists, for instance, are found to be dynamically unfavorable. Results from colloidal sedimentation experiments at high P\'eclet number are found to be consistent with the results of fast compressions, as long as appropriate boundary conditions are used. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K15.00009: Phase behavior of charged colloids on spherical surfaces Colm Kelleher, Rodrigo Guerra, Paul Chaikin For a broad class of 2D materials, the transition from isotropic fluid to crystalline solid is described by the theory of melting due to Kosterlitz, Thouless, Halperin, Nelson and Young. According to this theory, long-range order is achieved via elimination of the topological defects which proliferate in the fluid phase. However, many natural and man-made 2D systems posses spatial curvature and/or non-trivial topology, which require the presence of defects, even at $T=0$. In principle, the presence of these defects could profoundly affect the phase behavior of such a system. In this presentation, we describe experiments and simulations we have performed on repulsive particles which are bound to the surface of a sphere. We observe spatial structures and inhomogeneous dynamics that cannot be captured by the measures traditionally used to describe flat-space phase behavior. We show that ordering is achieved by a novel mechanism: sequestration of topological defects into freely-terminating grain boundaries (``scars''), and simultaneous spatial organization of the scars themselves on the vertices of an icosahedron. The emergence of icosahedral order coincides with the localization of mobility into isolated ``lakes'' of fluid or glassy particles, situated at the icosahedron vertices. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K15.00010: Diverse assembly behavior in colloidal Platonic polyhedral sphere clusters Ryan Marson, Erin Teich, Julia Dshemuchadse, Sharon Glotzer, Ronald Larson We simulate the self-assembly of colloidal ``polyhedral sphere clusters (PSCs)'', which consist of equal-sized spheres placed at the vertices of a polyhedron such that they just touch along each edge. These colloidal building blocks have recently been experimentally fabricated (\textbf{DOI: }10.1021/acsnano.5b03272)\textbf{;} here we predict crystal structures that would appear in the phase diagram of resulting particle assemblies. We use Brownian dynamics (BD) simulations of rigid body clusters performed in the open-source GPU-based HOOMD-Blue particle simulation package to show the assembly behavior of the 5 Platonic PSCs. The simulations contain as many as 4096 individual polyhedra, across over 30 different densities per cluster geometry, with some ordered phases possessing unit cells with 20 or more particles. We observe the formation of not only traditional cubic structures such as BCC and FCC, but also more complex phases having structure symmetries with Pearson symbols - hP7, cP20, cI2, mP6, and hR3. The observations reported here will serve as a guide for future colloidal assembly experiments using an expanded library of PSCs, consisting of other regular and irregular polyhedra, allowing researchers to target specific arrangements of ``halo'' and ``core'' particles for technologically relevant applications including photonics and structural color. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K15.00011: Realizing non-close-packed crystal structures through directional binding of DNA-functionalized colloidal clusters Talid Sinno, Mehdi Zanjani, John Crocker A promising approach for engineering assembling entities that exhibit anisotropy is to create small clusters out of spherical colloidal particles. Here, we study computationally the self-assembly of large numbers of colloidal clusters formed by a recently-introduced crystal templating method. We study the assembly of clusters mediated by the addition of spherical `bond' particles. In particular, the `bond' spheres are DNA-functionalized so that they interact attractively with the clusters. In this construct, the differing number of particle-particle contacts available between a cluster and bond particle as a function of orientation creates directional bonding. A variety of simulation techniques are employed to study the thermodynamics and kinetics of the assembly process. Specifically, we compute nucleation barriers for several crystalline configurations using tetrahedral, octahedral, and cubic clusters. We show that some cluster types lead to facile growth of crystalline superstructures, while others lead to structures that are highly susceptible to defect formation. Crystal growth kinetics are probed using molecular dynamics and Brownian dynamics simulations and again demonstrate a wide range of kinetic limitations depending on the cluster geometry. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K15.00012: Topological Defects and Structure Prediction in Nanoparticle Superlattices Alex Travesset, Nathan Horst, Curtis Waltmann, Surya Mallapragada, Honghu Zhang, Wenjie Wang, David Vaknin Materials whose fundamental units are nanoparticles, instead of atoms or molecules, are gradually emerging as major candidates to solve many of the technological challenges of our century. Those materials also display unique structural, dynamical and thermodynamical properties, often reflecting deep underlying geometry and topological constraints. In this talk, I will focus on crystalline assemblies of nanoparticles, i.e. supercrystals. I will discuss the challenges to predict the structure and dynamics of supercrystals and discuss our computational and analytical approach to predict two successful experimental strategies for the rational design of nanoparticle materials: evaporation of organic solvents with nanoparticles having hydrocarbon as capping ligands, and a new strategy developed at Ames lab consisting of crystallization of nanoparticle neutral (uncharged) polymer brushes by induced electrostatic phase separation. [Preview Abstract] |
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