Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y18: Self- and Directed Assembly |
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Sponsoring Units: GSNP DCMP Chair: Tom Truskett, University of Texas at Austin Room: 403 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y18.00001: Directing Colloidal Structure Using a Quench-Disordered Large Mesh Polymer Gel Ryan Jadrich, Kenneth Schweizer The use of a quench disordered template, such as a large mesh gel composed of long rigid rod polymers, may provide a powerful tool to mediate inter-particle interactions, structure, thermodynamics and properties of colloidal/nanoparticle suspensions. We employ the Replicated Reference Interaction Site Model integral equation approach to study a model system composed of a quenched rod or fiber gel immersed in a spherical colloid fluid. The theory predicts a sharp wetting-like transition with increasing colloid-fiber attractions accompanied by strong thermodynamic and colloid packing changes. By increasing the colloid-colloid attractions at constant colloid-fiber interactions, greatly enhanced adsorption onto the gel network and a well-defined state of maximum adsorption is predicted. This phenomenon suggests a strategy for avoiding macrophase separation and instead achieving a sharp, cusp-like transition driven by large density fluctuations of order the long rod length. The possibility of exploiting these phenomena to create responsive and functional colloidal assemblies that can be switched between electrically conductive and non-conductive states is explored. The approach can be generalized to nonspherical colloids, both chemically homogeneous and Janus. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y18.00002: Patterned Substrates to Direct Self-Assembly of Particle Monolayers Mark Ferraro, Thomas Truskett, Roger Bonnecaze As current lithographic techniques approach practical engineering limits for resolution, directed self-assembly of nanoparticles becomes an attractive scalable nanomanufacturing process for creating ordered arrays of particles at a variety of length scales that could be used both as patterning agents and functional materials. However the roles of interparticle forces and external fields on directed self-assembly of particles is not well understood. In this presentation density functional theory (DFT) and Monte Carlo (MC) simulations are used to explore the use of larger scale patterned substrates to drive smaller scale directed self-assembly of particle monolayers. Square patterned substrates with varying energy barriers at length scales N-fold the final desired particle pitch are considered (N\textgreater 1). Ranges of N, bulk density and patterned substrate field strength are identified that disrupt the entropically favored hexagonally close-packed lattice and promote square lattice formation for hard-spheres. Monte Carlo simulations are then employed to verify the predictions from DFT and to further analyze the self-assembly process. These DFT and MC results are used to discuss and define the energetically and kinetically accessible spaces for non-hexagonal lattice formation. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y18.00003: Shape-sensitive crystallization in colloidal superball fluids Laura Rossi, Stefano Sacanna, Vishal Soni, Paul Chaikin, David Pine, Albert Philipse, William Irvine Uniform colloidal silica superballs crystallize into a variety of ordered phases when depletant objects induce attraction between the colloids. The differences in these entropy-driven self-assembled structures are driven by minute deviations of the particle shape and are uniquely determined by an interplay between the size of depletants and superballs. Tuning this ratio allows to smooth the deviation in particle shape, allowing the observation of different depletion-stabilized crystalline structures for the same superball fluid. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y18.00004: Elasto-capillary interactions between solid spheres at smectic thin films Mohamed A. Gharbi, Randall D. Kamien, Shu Yang, Kathleen J. Stebe Colloidal particles organize spontaneously at fluid interfaces owing to a variety of interactions to form well organized structures that can be exploited to synthesize advanced materials. While the physics of colloidal assembly at isotropic interfaces is well understood, the mechanisms that govern interactions between particles at liquid crystal interfaces are not yet clearly established. In particular, smectic liquid crystal films offer important degrees of freedom that can be used to direct particles into new structures. In this work, we report the behavior of solid micrometric beads with homeotropic anchoring confined at interfaces of thin smectic films. We study the interactions and self-assembly of these particles in both supported and free standing films. When particles are captured in thin membranes, they induce distortions of the smectic interface to satisfy wetting properties at particle boundaries, leading to capillary interactions. These forces compete with elastic ones induced by the distortion of the smectic layers. The resulting potential drives assembly of the spheres into new different structures in a self-assembly process. Recent progress in understanding the mechanism of particle self-organization is presented. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y18.00005: Size-dependent scaling and ordering in nanoparticle island self-assembly Jacques Amar, Chakra Joshi, Yunsic Shim, Terry Bigioni While there are a number of similarities between nanoparticle (NP) island self-assembly at a liquid-air interface during drop-drying and epitaxial growth there are also some important differences. Here we present experimental results for the dependence of the island density, island-size distribution, and capture-zone distribution on coverage, deposition flux, and NP diameter which we then compare with epitaxial growth models. Our results indicate that, due to the increase in the strength of the short-range attraction between NPs with increasing NP size, the critical island-size decreases with increasing NP size. However, we also find deviations from epitaxial growth models for small NPs which indicate that additional effects may play a role. We also present results for the ordering of large NP islands which indicate the existence of long-range repulsive interactions. One possible mechanism for such an interaction is the existence of a small net dipole moment on each NP which occurs as a result of an asymmetry in the distribution of attached thiols. Consistent with this mechanism, we find good agreement between experimental results for the nearest-neighbor distribution between islands and simulations which include dipole repulsion. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y18.00006: Kinetic Monte Carlo simulations of nanoparticle island growth Bradley Hubartt, Jacques Amar In recent studies of the self-assembly of dodecanethiol-coated Au nanoparticle (NP) islands dissolved in toluene, the island-size distribution (ISD) was found to be quite sharp - despite the fact that island diffusion and coalescence are expected to lead to a broad ISD - while the island density was found to be anomalously low. In order to understand this, we have first used molecular dynamics (MD) simulations of a simplified model of islands adsorbed at an interface in order to study the dependence of the island diffusion coefficient on island-size. In order to understand island stability, we have also carried out additional MD simulations using an effective potential which takes into account the van der Waals and ligand-ligand interactions, which indicate that the rate of dimer break-up is surprisingly large. By including these results in kinetic Monte Carlo simulations we have obtained reasonable agreement with experiment. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y18.00007: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y18.00008: New Experimental Determination of Topology for Two-Dimensional Nanostructured Polymers Peter Beaucage, Gregory Beaucage In recent years, considerable interest in the unique structure and properties of graphene has expanded to include a wide variety of other inorganic and organic two-dimensional materials including molybdenum disulfide, tungsten oxide, polyfullerene networks, synthetic two-dimensional polymers, and biological membranes, among others. We have previously developed a fractal scaling model to describe the structure of a crumpled two-dimensional sheet in solution and applied it to characterization of crumpling behavior of graphene in solution, with the capability to statistically quantify the mole fraction degree of crumpling and lead to the effective projected area of the structure, which is useful for the Helfrich dynamic models. Recent efforts have focused on applying this model to characterization of a wide variety of polymeric materials, ultimately moving toward \textit{in situ} methods for elucidation of structure-processing-property relationships in 2D polymers and other materials. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y18.00009: Optical magnetic shift of a directed assembly of CdSe/ZnS quantum dots and Fe$_{3}$O$_{4}$ nanopaticles in soft matter at low magnetic fields Jose Amaral, A.L. Rodarte, J. Wan, M.T. Quint, M. Scheibner, S. Ghosh We are investigating the ensemble behavior of magnetic nanoparticles (MNPs) and CdSe/ZnS quantum dots (QDs) when dispersed in an electro-optically active liquid crystalline (LC) matrix. The directed assembly of NPs in the matrix is driven by the temperature-induced transition of the LC from the isotropic to the nematic phase as the NPs are mostly expelled into the isotropic regions, finally ending up clustered around LC defect points when the transition is complete. Using high-resolution scanning magneto-optical Kerr effect (MOKE), we characterize the spatial distribution and magnetic behavior of Fe$_{3}$O$_{4}$ MNPs in a room temperature nematic LC, 5CB. Our results show a two-fold intensity increase of QD photoluminescence (PL) intensity with applied fields lower than 200 G. We speculate this increase is due to a reorientation of LC molecules at the edge of the NP clusters causing QDs to coalesce toward the center of the cluster. This work was funded by NSF. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y18.00010: Heterotypic Self-Assembly of Type-I and Type-III Collagens Esma Eryilmaz, Winfried Teizer, Wonmuk Hwang Collagen fibrils, the main constituents of the extracellular matrix, are ``biological alloys'' that contain many additive molecules for fine-tuning the dynamical and biological properties. A representative example is the type-I collagen fibril, the most abundant among the 28 collagen types, which also contains type-III collagen. We perform atomic force microscopy (AFM) to elucidate the co-assembly of these two important members into heterotypic fibrils on mica surfaces. Time-lapse AFM imaging of samples at different ratios of type-I and type-III collagen molecules revealed that type-III assembles and nucleates fibrils slower than type-I. Furthermore, in the type-I/III mixture, nucleation appeared to be enhanced, resulting in formation of more fibrils compared to cases with either type-I or III only. We discuss possible mechanisms for the enhanced fibril nucleation in the co-assembly process of the two molecules that differ slightly in physical properties. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y18.00011: Emergence of limit-periodic structure without matching rules Catherine Marcoux, Travis Byington, Joshua Socolar We study the emergence of nonperiodic order in a tiling model based on a certain 2D hexagonal prototile with nearest-neighbor interactions. The model is closely related to the Taylor-Socolar tiling model~[1], but with a simpler Hamiltonian with a degenerate class of ground states that includes both periodic and limit-periodic structures. We present the results of a lattice Monte Carlo simulation of the orientational degrees of freedom of a system of the prototiles. We find that the limit-periodic structure emerges from a sufficiently slow quench through the same infinite sequence of second-order phase transitions observed in the full Taylor-Socolar model. A related 3D model with a simple cubic prototile exhibits similar behavior, but with first-order transitions and a more complex set of limit-periodic ground states.\\[4pt] [1] T. W. Byington and J. E. S. Socolar, {\it Phys. Rev. Lett.} {\bf108}, 045701 (2012). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y18.00012: Why (almost) all bundles are chiral Zachary V. Kost-Smith, Robert A. Blackwell, Matthew A. Glaser We examine the self assembly of bundles of achiral hard rods with distributed, short-range attractive interactions. We show that in the majority of cases the equilibrium state of the bundle is chiral, with a double twist structure. We use biased Monte Carlo techniques and cell theory to compute the free energy as a function of an appropriately defined twist order parameter, and show that the formation of spontaneously chiral bundles is driven by maximization of orientational entropy. The finite curvature of the bundle boundary permits {\em orientational escape}, in which the circumferential angular range of motion of the rods is maximized for some finite average tilt. We map out the phase diagram of bundles in terms of the density, the ratio of rod length to bundle radius, $L/R$, and rod aspect ratio, $L/D$, and find transitions between untwisted, weakly twisted, and strongly twisted states. This work helps explain the common observation of twisted macroscopic bundles, and may provide insight into observations of twist in self-assembled membranes of colloidal rods.\footnote{Reconfigurable self-assembly through chiral control of interfacial tension, Nature, 481:348-351, Jan 2012} [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y18.00013: Critical adsorption and colloidal interaction in multi-component liquids Sharmine Alam, Ashis Mukhopadhyay We studied critical adsorption on colloidal nanoparticles in binary liquid mixture of 2,6 lutidine $+$ water by using fluorescence correlation spectroscopy (FCS). Our results indicated that the adsorbed film thickness is of the order of correlation length associated with concentration fluctuations. The excess adsorption per unit area increases following a power law in reduced temperature with an exponent of -1, which is the mean-field value for the bulk susceptibility exponent. The measurements at higher particle volume fractions, where phenomena such as the particle-particle interaction, self-assembly, ternary phase separation become important will be presented. [Preview Abstract] |
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