Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S50: Self- and Directed Assembly |
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Sponsoring Units: GSOFT Chair: Eric Corwin, University of Oregon Room: 218 |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S50.00001: Self-Assemblies of novel molecules, VECAR Bijay Shrestha, Hye-Young Kim, Soojin Lee, Brian Novak, Dorel Moldovan VECAR is a newly synthesized molecule [1], which is an amphiphilic antioxidant molecule that consists of two molecular groups, vitamin-E and Carnosine, linked by a hydrocarbon chain. The hydrocarbon chain is hydrophobic and both vitamin-E and Carnosine ends are hydrophilic. In the synthesis process, the length of the hydrophobic chain of VECAR molecules can vary from the shortest (n$=$0) to the longest (n$=$18), where n indicates the number of carbon atoms in the chain. We conducted MD simulation studies of self-assembly of VECAR molecules in water using GROMACS on LONI HPC resources. Our study shows that there is a strong correlation between the shape and atomistic structure of the self-assembled nano-structures (SANs) and the chain-length (n) of VECAR molecules. We will report the results of data analyses including the atomistic structure of each SANs and the dynamic and energetic mechanisms of their formation as function of time. In summary, both VECAR molecules of chain-length n$=$18 and 9 form worm-like micelles, which may be used as a drug delivery system. \\[4pt] [1] C. E. Astete, D. S. Meador, D. Spivak, C. Sabliov, Synthetic Commun. 43, 1299 (2013). [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S50.00002: Self-assembly of Colloids in a Suspended Droplet: In-situ small-angle X-ray scattering Jitendra Bahadur, Debasis Sen, S. Mazumder, G. Santoro, S. Yu, S.V. Roth, Y.B. Melnichenko An in-situ scanning SAXS experiments have been performed to probe the drying of a single suspended colloidal droplet. It has been demonstrated that shell formation can be predicted just by measuring the temporal evolution of the spatial transmission profile. The shrinkage of the droplet stops after formation of a shell. The shell thickness and droplet radius have been estimated by fitting the transmission profiles using analytical expressions. It is revealed that the evolution of volume fraction of colloids is linear during the initial stage of drying and follows a sigmoidal growth behavior at later stages. Further, the interaction between colloidal particles at different drying stages has been investigated. We provide experimental proofs of a transition from repulsive interaction between colloids into a capillary driven short range attraction for shell formation during drying. The present work demonstrates that in-situ SAXS is a very valuable technique for monitoring the dynamic processes of colloidal self-assembly and provides the opportunity to probe the drying of complex fluids without the interference from a substrate. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S50.00003: Self-assembly of graded refractive index in squids: a patchy colloid explanation Jing Cai, Paul Heiney, Alison Sweeney Squids have a spherical eye lens that achieves both acute and highly sensitive vision underwater. The spherical shape necessitates a graded refractive index (GRIN) to form sharp images. This index variation comes from a gradient in protein packing fraction ranging from approximately 0.05 to 1.0. This presents a materials conundrum: optical transparency requires that the protein density fluctuation at length scales $>$ 100 nm is minimized throughout the lens, something that is difficult to achieve with simple spherical particles. Here we show that squids have accomplished this by evolving a suite of proteins that can act as patchy colloids with specific low valence (M=2 or M=3). We conducted small x-ray scattering (SAXS) at different radial positions of the lens, and performed a Monte Carlo simulation to estimate structures consistent with the SAXS result. This analysis suggests that lens proteins may form a density gradient gel structure, with density mediated by a tightly controlled protein coordination number in each region. Patchy colloid theory may therefore explain both the GRIN and the transparency evolved in the lens. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S50.00004: Self-assembly of tetrahedral plasmonic nanoclusters for optical metafluids Nicholas Schade, Vinothan Manoharan We direct the assembly of clusters of gold nanospheres that behave as nanoscale electromagnetic resonators. We use spherical gold nanoparticles that are exceptionally smooth, monocrystalline, and monodisperse. These particles exhibit highly reproducible scattering spectra compared with gold colloids that are available commercially. We mix these positively charged particles with negatively charged dielectric particles. The gold particles stick to the dielectric particles permanently and randomly in a process that can be modeled mathematically as ``random parking,'' a type of non-equilibrium self-assembly. By controlling the particles' sizes, stoichiometry, and interactions, we maximize the yield of tetrahedral clusters, the ideal structures for isotropic metamaterials. We measure the optical properties of these structures with dark-field spectroscopy to characterize their suitability as building blocks for a bulk, isotropic, optical metafluid. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S50.00005: Drying of Discotic Suspensions Adithyaram Narayan, Zhengdong Cheng We study the evaporation driven self-assembly of exfoliated ?-Zirconium Phosphate nanoplatelets that formed continuous films at various low concentrations. These self-assembly mechanism is different from the well-known coffee-ring effect for low aspect ratio particles. The film formation can be tuned by solvent properties, temperature and concentration of the nanoplates. By the virtue of very large surface area, these platelets can be used as flame retardant coatings after proper functionalization. These work point to a simple procedure to create uniform films by high aspect ratio nanoplates with potentially diverse applications. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S50.00006: Self-assembly mechanism for limit-periodic structure Catherine Marcoux, Joshua Socolar Limit-periodic (LP) structures, which are the union of an infinite set of periodic lattices with ever increasing lattice constants, present a challenge for self-assembly protocols. We consider the possibility of forming a LP phase in a slow quench of a collection of colloidal particles designed to mimic the Taylor-Socolar monotile system.\footnote{J.\ E.\ S.\ Socolar and J.\ M.\ Taylor, {\it J.\ Comb.\ Theory A} {\bf 118}: 2207 (2011).} A toy model with discrete tile orientations and mismatch energies yields the LP state through an infinite sequence of phase transitions.\footnote{C.\ Marcoux, T.\ W.\ Byington, Z.\ Qian, P.\ Charbonneau, and J.\ E.\ S.\ Socolar, {\it Phys. Rev. E} {\bf 90}, 012136 (2014).} Here we present the results of Monte Carlo simulations of slow quenches of identical hard disks with embedded magnetic dipoles, allowing for continuous rotations of the close-packed disks. Surprisingly, an extremely slow quench still results in the spontaneous emergence of the LP state even when the system has a periodic ground state. The series of phase transitions preempts the formation of the periodic phase, leading to low energy states separated from the ground state by insurmountable free energy barriers. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S50.00007: Non-additive simple potentials for pre-programmed self-assembly Carlos Mendoza A major goal in nanoscience and nanotechnology is the self-assembly of any desired complex structure with a system of particles interacting through simple potentials. To achieve this objective, intense experimental and theoretical efforts are currently concentrated in the development of the so called ``patchy'' particles. Here we follow a completely different approach and introduce a very accessible model to produce a large variety of pre-programmed two-dimensional (2D) complex structures. Our model consists of a binary mixture of particles that interact through isotropic interactions that is able to self-assemble into targeted lattices by the appropriate choice of a small number of geometrical parameters and interaction strengths. We study the system using Monte Carlo computer simulations and, despite its simplicity, we are able to self assemble potentially useful structures such as chains, stripes, Kagom\'{e}, twisted Kagom\'{e}, honeycomb, square, Archimedean and quasicrystalline tilings. Our model is designed such that it may be implemented using discotic particles or, alternatively, using exclusively spherical particles interacting isotropically. Thus, it represents a promising strategy for bottom-up nano-fabrication. Reference: Daniel Salgado-Blanco and Carlos I. Mendoza, ``Non-additive simple potentials for pre-programmed self-assembly'', arXiv:1409.2916 [cond-mat.soft] [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S50.00008: Heterogeneous crystal nucleation on curved surfaces observed by real-space imaging Urs Gasser, Andrea Scotti, Florian Ziese, Georg Maret Crystal nucleation in a supercooled liquid, i.e. the formation of critical crystal nuclei, is not well understood for both homogenous and heterogeneous nucleation. The structural transformation from the liquid to crystal precursors and finally to the structure of bulk crystal and its connection with the free energy barrier for nucleation are not well understood. The large differences between measured and calculated nucleation density rates obtained for many materials reflect this lack of understanding. We use confocal microscopy with single-particle resolution to observe heterogeneous nucleation of colloidal crystals on curved seed surfaces. The radius of curvature ranges from 4 to 40 particle diameters, allowing to observe the transition from the strong suppression of heterogeneous nucleation at small radii of curvature - an effect not captured by classical nucleation theory - to fast heterogeneous nucleation as expected from classical nucleation theory. We determine the critical nucleus size, estimate the surface tension of crystal precursors and critical nuclei, characterize their structure, and compare with expectations from classical nucleation theory. While the smallest crystal precursors are found to be almost unaffected by the curvature, the effect is significant for nucl [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S50.00009: Computer Simulation Study of the Nucleation of Rotator Phases in Hard Polyhedral Particles Vikram Thapar, Fernando Escobedo The nucleation kinetics of the rotator phase in hard cuboctahedra, truncated octahedra, and rhombic dodecahedra is simulated via a combination of forward flux sampling and umbrella sampling. We compute the degrees of supersaturation at their corresponding pressures by improving upon the interfacial method used to estimate the liquid-rotator coexistence pressure. The nucleation rates are obtained by calculating the mean first passage time from liquid to rotator phase using forward flux sampling, and the free-energy barriers are estimated using umbrella sampling. For comparable degrees of supersaturation, the polyhedra are found to have significantly lower free-energy barriers and faster nucleation rates than hard spheres. This difference primarily stems from localized orientational ordering, which steers polyhedral particles to pack more efficiently. Orientational order hence fosters here the growth of orientationally disordered nuclei. The results are compared to preliminary data for the disorder-to-order transition for other polyhedral systems including systems pinned on a 2D interface. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S50.00010: Interplay of isotropic and directional interactions and its role in phase behavior Debra Audus, Francis Starr, Jack Douglas Patchy particles, which interact through non-isotropic interactions have been studied extensively both computationally and theoretically in part because they are minimal models of protein solutions. Although proteins are inherently complicated molecules with complex shapes and interactions, when in solution, they associate and phase separate like patchy particles. However, patchy particles considered computationally are often composed of hard spheres with short-ranged attractive spots decorating the surface. Such a parameterization ignores the isotropic attractive interactions, which can potentially play an important role in phase behavior. To gain insight into this problem, we investigate patchy particles with isotropic interactions that range from purely repulsive to weakly attractive and explore how the interplay between isotropic interactions and directional interactions due to the spots affects both the phase coexistence and association in these systems. We find that for our model that even when the strength of isotropic interactions is weaker than the strength of directional interactions, the isotropic interactions can still dominate. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S50.00011: Engineering Crystals Through Shape Greg van Anders, Daphne Klotsa, Sharon Glotzer Advances in synthesis techniques have produced colloids and nanoparticles in a diverse array of shapes that can be assembled into bulk crystals. That bulk structure is strongly affected by particle shape in idealized systems is widely established in the literature. However, this literature leaves open three key questions: (i) We know that shape affects structure, but how? (ii) Does shape matter in experimental systems where other interactions are present? (iii) How do we tailor particle shape for a target structure? In this talk we discuss recent work aimed at answering these questions. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S50.00012: The size of the boat matters: Scale dependence in macroscopic chains thermalized by the motion of a laboratory-scale ocean Kyle Welch, Clayton Kilmer, Eric Corwin We use a bath of chaotic surface waves in water to mechanically and macroscopically mimic the thermal behavior of various microscopic systems. The chaotic waves provide isotropic and random agitation to which a temperature can be ascribed. This allows us to passively explore the degrees of freedom of a system, in analogy to thermal motion. We report on a study of 2D macroscopic chains thermalized in this fashion. We show that the behavior of short chains is fundamentally different than the behavior of long chains in both winding angle and end-to-end distance. Furthermore, we find that short chains show anomalous compressional stiffness that rapidly softens as chain length increases. We present simulational work exploring this transition from short to long, treating the chains as self-avoiding polymers. We further apply our techniques to explorations of the evolution of a system of many interacting buoyant particles, focusing on transitions from ordered to disordered states. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S50.00013: Directing Translational and Orientational Order of Rectangular Particle Monolayers Mark Ferraro, Thomas Truskett, Roger Bonnecaze Recent advances have shown that the tunability of nanoparticle interactions can lead to a large number of thermodynamically accessible structures. The role of an external field in the assembly of particulate systems, however, is still incompletely understood. The use of larger scale patterned substrates to drive smaller scale assembly of particle monolayers can potentially expand the set of achievable lattices, and could be used in nanopatterning processes or in the manufacture of functional materials. In this presentation, grand canonical Monte Carlo (GCMC) simulations are used to assess the suitability of graphoepitaxial assembly for particle monolayers. Our prior work has shown that topographically or chemically patterned substrates can sufficiently organize hard-spheres, but many motivating applications can utilize anisotropic particle shapes (e.g. rectangular particles for bit-patterned media). Here, we describe our recent GCMC results for structures formed by rectangular particles in the presence of sparse enthalpic barriers. We examine systems of varying chemical potential, template geometry, and particle aspect ratio. Templates are evaluated by their ability to induce orientational and translational order, while maximizing pattern multiplication effects. [Preview Abstract] |
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