Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E37: Self and Directed Assembly (Equilibrium and Non-Equilibrium) |
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Sponsoring Units: GSOFT DPOLY Chair: Kazem Edmond, Oxford University Room: 340 |
Tuesday, March 15, 2016 8:00AM - 8:12AM |
E37.00001: Phase Behavior of Thermodynamically Small Clusters of Colloidal Particles Raghuram Thyagarajan, Dimitrios Maroudas, David Ford The self-assembly of finite clusters of colloidal particles into crystalline objects is a topic of technological interest, as a route to produce photonic crystals and other metamaterials. Such assembly problems also are fundamentally interesting because they involve thermodynamically small systems, with number of particles between 10 and 1000 that is far below the bulk limit. In contrast to bulk systems, these colloidal assemblies exhibit phase coexistence over a finite range of physical conditions. Here, we report the results of a computational study of phase behavior of small colloidal clusters interacting via the Asakura-Oosawa depletion pair potential. We conducted Monte Carlo simulations for various levels of the osmotic pressure that controls the strength of the interparticle interactions, using potential energy histograms to identify distinct phases. Over a narrow but finite range of the osmotic pressure, we find bimodal distributions in the potential energy space that are indicative of coexistence between fluid-like and crystalline configurations. We also report systematic quantitative comparisons of the phase behavior observed here with results from a Fokker-Planck order-parameter approach. [Preview Abstract] |
Tuesday, March 15, 2016 8:12AM - 8:24AM |
E37.00002: Two-dimensional self-assembly of DNA-functionalized gold nanoparticles Wenjie Wang, Honghu Zhang, Noah Hagen, Ivan Kuzmenko, Mufit Akinc, Alex Travesset, Surya Mallapragada, David Vaknin 2D superlattices of nanoparticles (NPs) are promising candidates for nano-devices. It is still challenging to develop a simple yet efficient protocol to assemble NPs in a controlled manner. Here, we report on formation of 2D Gibbs monolayers of single-stranded DNA-coated gold nanoparticles (ssDNA-AuNPs) at the air-water interface by manipulation of salts contents. MgCl2 and CaCl2 in solutions facilitate the accumulation of the non-complementary ssDNA-AuNPs on aqueous surfaces. Grazing-incidence small-angle X-ray scattering (GISAXS) and X-ray reflectivity show that the surface AuNPs assembly forms a mono-particle layer and undergoes a transformation from short-range to long-range (hexagonal) order above a threshold of [MgCl2] or [CaCl2]. For solutions that include two kinds of ssDNA-AuNPs with complementary base-pairing, the surface AuNPs form a thicker film and only in-plane short-range order is observed. By using other salts (NaCl or LaCl3) at concentrations of similar ionic strength to those of MgCl2 or CaCl2, we find that surface adsorbed NPs lack any orders. X-ray fluorescence measurements provide direct evidence of surface enrichment of AuNPs and divalent ions (Ca2$+)$. [Preview Abstract] |
Tuesday, March 15, 2016 8:24AM - 8:36AM |
E37.00003: Novel liquid crystal phase transition of linear defects in an epitaxial layer of DNA-nanoparticle superlattices Saijie Pan, Niels Boon, Monica Olvera de la Cruz We use Monte Carlo simulations and mean-field theory to study a lattice model system in which DNA-coated nanoparticles form an epitaxial layer onto a patterned bcc (100) template. If nanoparticles only attach to the so-called ``center'' sites, each of which is the center of a unit cell in the square lattice template, it would result in a perfect bcc epitaxial layer. However, defects arise due to attachment to ``edge'' sites and ``corner'' sites. In simulation, we show that edge-binding defects prefer to form linear clusters in horizontal and vertical directions. These linear defects can undergo a second-order isotropic-nematic phase transition in some regimes. A mean-field approach is introduced to provide theoretical descriptions for the system in each of the phases and predict the critical phase transition conditions. Striking agreement is observed between the theory and simulation. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E37.00004: Transformations and Reconstructions of DNA-directed colloidal crystals John Crocker, Yifan Wang, Ian Jenkins, Talid Sinno DNA is a versatile tool for directing the equilibrium self-assembly of nanoscopic and microscopic objects, but also for subsequently transforming them into new structures. In experiment, at high densities of long grafted DNA strands, and temperatures where the binding is reversible, these systems readily form colloidal crystals and colloidal clusters having a range of symmetries. For interactions that favor alloying between two differently-sized colloidal species, our experimental observations compare favorably to a simulation framework that predicts the equilibrium phase behavior, growth kinetics and solid-solid transitions. Overall, we find that this system recapitulates both ionic crystals and noble-metal alloys. We will discuss the crystallography of the alloy structures formed as well as the interesting Martensitic-type transformations and super-lattice reconstructions they undergo. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E37.00005: Bioinspired Composites with Spatial and Orientational Control of Reinforcement Ahmet Demiroers, Andre Studart Living organisms combine soft and hard components to fabricate composite materials with out-standing mechanical properties. The optimum design and assembly of the anisotropic components reinforce the material in specific directions against multidirectional external loads. Although nature does it quite readily, it is still a challenge for material scientists to control the orientation and position of the colloidal components in a matrix. Here, we use external electric and magnetic fields to achieve positional and orientational control over colloid-polymer composites to fabricate mechanically robust materials to capture some of the essential features of natural systems. We first investigated the assembly of spherical micron-sized colloids using dielectrophoresis, as these particles provided an easily accessible and instructive length scale for performing initial experiments. We used dielectrophoresis for spatial control of reinforcing anisotropic components and magnetic fields to provide control over the orientation of these reinforcing constituents. The obtained composites with different orientational and spatial reinforcement showed enhanced mechanical properties, such as wear resistance, which exhibits similarities to tooth enamel. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E37.00006: Harmonic and Anharmonic Free Energies in Superlattices of Soft Particle Systems Alex Travesset, Carles Calero, Chris Knorowski Many problems in self and directed assembly rely on the rigorous calculation of free energies. In systems of nanoparticles with capping ligands, for example, superlattices are found in closely competing structures, such as hcp/fcc, cubic/hexagonal diamond or those isomorphic between MgCu$_2$ and MgZn$_2$. With this motivation, we investigate a general method to calculate free energy of crystalline solids by considering the harmonic approximation and quasistatically switching the anharmonic contribution. The advantage of the method is that the harmonic approximation provides an already very accurate estimate of the free energy, and therefore the anharmonic term is numerically very small and can be determined to very high accuracy. We further show that the anharmonic contribution to the free energy satisfies a number of exact inequalities that place con- strains on its magnitude and allows approximate but fast and accurate estimates. We apply it to Lennard Jones sytems where we demonstrate that hcp is the equilibrium phase at low temperature and pressure and obtain the coexistence curve with the fcc phase, which exhibits reentrant behavior and binary systems that model nanoparticle superlattices with hydrocarbon capping ligand. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E37.00007: Designing self-assembling 3D structures of microcapsules Like Li, Henry Shum, Oleg Shklyaev, Victor Yashin, Anna Balazs Self-assembly of complex, three-dimensional structures is commonly achieved by biological cells but difficult to realize in synthetic systems with micron-scale or larger components. Some previous modeling studies have considered only the planar self-assembly of microcapsules on a substrate. In this work, nanoparticles released from the capsules bind to the substrate and to the shells of nearby capsules. The non-uniform nanoparticle deposition on a capsule's surface leads to adhesion gradients, which drive the capsules to effectively ``climb'' on top of one another and self-organize in the vertical direction. We determine conditions that favor this structural organization. In particular, we study how the vertical structuring depends on the background fluid flow, the topography of the microcapsules and the underlying surface, the capsule-capsule interaction and that between the capsules and the substrate. The findings can provide design rules for the autonomous creation of novel nanocomposites, where the layers are formed from nanoparticle-containing and nanoparticle-decorated microcapsules. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E37.00008: Stripes or Checkerboards: Distinguishing Patterns of Self-Assembled Water Drops to Chiral Structures Laura Adams, Sam Ocko, David Weitz A robust route for the biased production of single-handed chiral structures has been found in generating non-spherical, multi-component double emulsions, drops within drops, using glass microfluidic devices. Driving the minimization of surface energy are capillary forces that cause linear chains of encapsulated water drops to self- assemble into three-dimensional configurations with a well-defined preference to one type of handedness; thus, breaking left -right symmetry. In two dimensions, the encapsulated drops form patterns of stripes or checkerboards that are captured with a high speed camera. We quantify the dynamics of the evolving structures by measuring the second moment of the mass distribution and their growth and evolution rates. These new self-assembled soft structures are highly stable and open the door for a wide range of exotic configurations. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E37.00009: Bias-free simulation of diffusion-limited aggregation on a square lattice Yen Lee Loh We identify sources of systematic error in traditional simulations of the Witten-Sander model of diffusion-limited aggregation (DLA) on a square lattice. Based on semi-analytic solutions of the walk-to-line and walk-to-square first-passage problems, we develop an algorithm that reduces the simulation bias to below $10^{-12}$. We grow clusters of $10^8$ particles on $65536\times 65536$ lattices. We verify that lattice DLA clusters inevitably grow into anisotropic shapes, dictated by the anisotropy of the aggregation process. We verify that the fractal dimension evolves from the continuum DLA value, $D=1.71$, for small disk-shaped clusters, towards Kesten's bound of $D=3/2$ for highly anisotropic clusters with long protruding arms. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E37.00010: The role of symmetry for the orientational ordering of hard regular polygons Wenbo Shen, Michael Engel, Joshua A. Anderson, James A. Antonaglia, Sharon C. Glotzer Understanding the relationship between particle shape and structure is critical for targeted self-assembly. Hard particles, whose phase behavior is governed by geometry alone, spontaneously order when compressed to high enough packing density. Different routes of ordering have been suggested: a direct transition from fluid to crystal as well as the appearance of an intermediate liquid crystalline or a rotator phase. Here, we investigate a family of hard shapes in two dimensions that interpolate from highly anisotropy to highly circular. For this purpose, we determine the phase behavior of hard regular polygons from triangles to dodecagons at densities comprising the development of orientational order. In particular, we focus on the role of particle symmetry on rotational motion and the appearance of rotator phases. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E37.00011: Shape Allophiles Improve Entropic Assembly Eric Harper, Ryan Marson, Joshua Anderson, Greg van Anders, Sharon Glotzer We investigate a class of ``shape allophiles'' that fit together like puzzle pieces as a method to access and stabilize desired structures by controlling directional entropic forces. Squares are cut into rectangular halves, which are shaped in an allophilic manner with the goal of re-assembling the squares while self-assembling the square lattice. We examine the assembly characteristics of this system via the potential of mean force and torque, and the fraction of particles that entropically bind. We generalize our findings and apply them to self-assemble triangles into a square lattice via allophilic shaping. Through these studies we show how shape allophiles can be useful in assembling and stabilizing desired phases with appropriate allophilic design. [1] Harper, et. al., Soft Matter, 2015, 11, 7250-7256. DOI: 10.1039/C5SM90160J. This work was featured on the cover of Soft Matter 07 October, 2015. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E37.00012: Molecular Dynamics Investigation of the Products of Alkoxysilane Condensation: Bulk Gels and Surface Coatings Roland Faller, Joshua Deetz We characterize silica gels and organo-silicon surface coatings using reactive molecular dynamics simulations. To model the chemical reactions, we use a reactive force field (ReaxFF) which we have optimized in a novel parallelized semi-automatic way to model hydrolysis and condensation reactions. The morphologies of silica gels obtained from tetra- and tri-alkoxysilanes are determined by allowing the system to condense while simultaneously removing water and replacing it with precursor solution. It is found that the gels obtained from trialkoxysilanes are more loosely bonded, and that the chemistry of the headgroup is important to the gel morphology. We furthermore simulated the chemisorption of alkoxysilanes with organic headgroups to hydroxylated silica surfaces. We observe a competition between alkoxysilanes condensing with themselves or with the silica surface. [Preview Abstract] |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E37.00013: Multiscale Self-Assembly of Quantum-Dots into an Anisotropic Three-Dimensional Random Network Serim Ilday, Fatih Ilday, Rene Hübner, Ty Prosa, Isabelle Martin, Gizem Nogay, Ismail Kabacelik, Zoltan Mics, Mischa Bonn, Dmitry Turchinovich, Hande Üstünel, Daniele Toffoli, David Friedrich, Bernd Schmidt, Karl-Heinz Heinig, Rasit Turan Multiscale self-assembly is ubiquitous in nature but its deliberate use to synthesise multifunctional materials remains rare, partly due to the notoriously difficult problem of controlling topology from atomic to macroscopic scales to obtain properties by design. Here, we demonstrate an anisotropic random network of silicon quantum-dots that hierarchically self-assembles from the atomic to the microscopic scales: First, quantum-dots form, to subsequently interconnect without inflating their diameters to form a random network. This network then grows in a preferential direction to form undulated and branching nanowire-like structures. This specific topology allows simultaneous good electrical conduction and a tuneable bandgap. These scale-dependent features were previously thought to be mutually exclusive. Furthermore, we show that the topology is designed and self-assembled following an inherently modular, material-independent methodology, so that the approach is applicable to achieve programmable properties in other materials. [Preview Abstract] |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E37.00014: Experimental Investigations of Ionic Self-Assembly of Silica Nanoparticles Gillenhaal Beck, Sabin Nshimyumukiza, Mohammad Abudayyeh, Rebecca Melkerson, Estevan Hall-Mejia, Irina Mazilu, Dan Mazilu We present a novel experimental method for determining the rate at which anionic silica nanoparticles in a colloidal suspension are adsorbed to a cationic polymer on a glass substrate. This method allows us to study particle self-assembly at time scales under one tenth of a second, two orders of magnitude smaller than previously reported in literature. We compare our experimental findings with a class of stochastic models for cooperative sequential adsorption of particles. [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E37.00015: Programmable concatenation of conductively linked gold nanorods using molecular assembly and femtosecond irradiation Jake Fontana, Steve Flom, Jawad Naciri, Banahalli Ratna The ability to tune the resonant frequency in plasmonic nanostructures is fundamental to developing novel optical properties and ensuing materials. Recent theoretical insights show that the plasmon resonance can be exquisitely controlled through the conductive concatenation of plasmonic nanoparticles. Furthermore these charge transfer systems may mimic complex and hard to build nanostructures[1]. Here we experimentally demonstrate a directed molecular assembly approach to controllably concatenate gold nanorods end to end into discrete linear structures, bridged with gold nanojunctions, using femtosecond laser light. By utilizing high throughput and nanometer resolution this approach offers a pragmatic assembly strategy for charge transfer plasmonic systems. [1] J. Fontana and B. R. Ratna, Applied Physics Letters \textbf{105} (2014) [Preview Abstract] |
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