Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y4: Polymer Colloids: Structure, Function and Dynamics |
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Sponsoring Units: DPOLY Chair: Eric Furst, University of Delaware Room: Ballroom A4 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y4.00001: Colloidal photonic crystals: Beyond optics, beyond spheres Invited Speaker: Monodisperse and symmetrically shaped colloidal particles tend to form ordered aggregates. When the particle size is in the hundreds of nanometres, such highly ordered structures exhibit fascinating optical properties, hence their name and fame as colloidal ``photonic crystals'' or as ``photonic bandgap material'', because they exhibit a forbidden energy band for photons, very much like semiconductor crystals are characterized by a bandgap for electrons. Photonic bandgap engineering is possible by a proper choice of the size and nature of the ``photonic atom'', and by a proper combination of different kinds of particles. The fame of monodisperse colloidal spheres as photonic atoms is largely based on the self-assembling capabilities into inherently three-dimensional photonic crystals. Colloidal photonic crystals can hence be used as an easy photonic crystal platform to demonstrate proof-of-principle for effects such as reduced local density of states for photons on their emission probability. We have induced spectral narrowing for emission from dye molecules and enhanced energy transfer between light-absorbing molecules in colloidal photonic crystals. By inserting superparamagnetic particles in the tens of nanometres range, it is possible to additionally impart magnetic properties to the photonic crystal. Tuning and enhancing Faraday rotation was possible by careful nanoscale bandgap engineering at two different nanoscales. One disadvantage of colloidal spheres for photonic crystals is the incomplete bandgap that is typical for the highly symmetrical crystal structures that are commensurable with dense packing of spheres. A number of approaches allow deviating from this paradigm towards a complete bandgap in the visible. Etching of material allows a less dense crystal, while non-spherical colloidal particles provide alternate crystal structures. Orientational ordering of such anisotropic particles in an anisotropic photonic crystal requires an additional handle on the particles, the colloidal assembly providing the positional order. Magnetism again provides this handle. Post-formation processing of crystals of positionally ordered spheres into orientationally anisotropic crystals represents another approach. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 9:12AM |
Y4.00002: Near Wall Dynamics in Colloidal Suspensions Studied by Evansescent Wave Dynamic Light Scattering Invited Speaker: The dynamics of dispersed colloidal particles is slowed down, and becomes anisotropic in the ultimate vicinity of a flat wall due to the wall drag effect. Although theoretically predicted in the early 20th century, experimental verification of this effect for Brownian particles became possible only in the late 80s. Since then a variety of experimental investigations on near wall Brownian dynamics by evanescent wave dynamic light scattering (EWDLS) has been published. In this contribution the method of EWDLS will be briefly introduced, experiments at low and high colloid concentration for hard-sphere suspensions, and the theoretical prediction for measured initial slopes of correlation functions will be discussed. On increasing the particle concentration the influence of the wall drag effect is found to diminishes gradually, until it becomes negligible at volume fractions above $\phi> 0.35$. The effect that a wall exerts on the orientational dynamics was investigated for different kinds of colloids. Experiments, simulations and a virial expansion theory show that rotational dynamics is slowed down as well. However, the effect is prominent in EWDLS only if the particles' short axis is of the order of the evanescent wave penetration depth. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:48AM |
Y4.00003: Assembly of Dimer-Based Photonic Crystals Invited Speaker: Recent advances in colloid synthesis to prepare monodisperse shape anisotropic particles provide the opportunity to address challenges related to structural diversity in ordered colloidal solids. In particular, computational simulations and mechanical models suggest that upon system densification nonspherical dimer colloids undergo disorder-order and order-order phase transitions to unconventional solid structures including, base-centered monoclinic crystals, degenerate aperiodic crystals, plastic crystal or rotator, etc. based on free energy minimization. The particle systems have notable analogy to molecular systems, where the shape of molecules and their packing density has been shown to critically influence structural phase behavior and lead to a rich variety of structures, both natural and synthetic. The materials engineering challenges have been in attaining sufficiently monodisperse (size uniformity) colloidal building blocks, as well as the lack of understanding and control of self-assembly processes for non-spherical colloids. This talk highlights our investigations of how particle shape programs the self-organization of colloidal structures. Methods including evaporation mediated assembly and confinement provide a platform to understand the formation of complex colloidal structures from non-spherical building blocks (silica-coated iron oxide, polystyrene, hollow silica shell). Optical property simulations for unconventional 2D and 3D structures with nonspherical particle bases will also be discussed. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:24AM |
Y4.00004: Directed self-assembly of small colloidal clusters Invited Speaker: We study the formation and structure of equilibrium colloidal clusters at small particle number ($N \sim 10$) using optical microscopy. Our experimental system consists of isolated groups of colloidal microspheres with short-ranged attractions. With non-specific depletion interactions, we observe that the number of configurations increases sharply with $N$. The most favorable states are those with the lowest symmetry. With specific DNA-mediated attractions, the number of states is reduced. Experiments and theoretical calculations suggest that it is possible to direct the assembly of specific structures through multiple competing DNA-mediated interactions. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 11:00AM |
Y4.00005: Convective microsphere monolayer deposition Invited Speaker: There is perhaps no simpler way of modifying surface chemistry and morphology than surface deposition of particles. Micron-sized microspheres were deposited into thin films via rapid convective deposition, similar to the `coffee ring effect' using a similar method to that studied by Prevo and Velev, Langmuir, 2003. By varying deposition rate and blade angle, the optimal operating ranges in which 2D close-packed arrays of microspheres existed were obtained. Self-assembly of colloidal particles through a balance of electrostatic and capillary forces during solvent evaporation was revealed. These interactions were explored through a model comparing the residence time of a particle in the thin film and the characteristic time of capillary-driven crystallization to describe the morphology and microstructure of deposited particles. Co-deposition of binary suspensions of micron and nanoscale particles was tailored to generate higher-quality surface coatings and a simple theory describes the immergence of instabilities that result in formation of stripes. Optical and biomedical applications that utilize the described nanoscale control over surface morphology will also be discussed. [Preview Abstract] |
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