Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q12: Self-Assembly: Equilibrium and Non-Equilibrium |
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Sponsoring Units: DFD DCMP Chair: Jacques Amar, University of Toledo Room: B110-B111 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q12.00001: Two-parameter sequential adsorption model of microfiber clustering Jayson Paulose, David Nelson, Joanna Aizenberg Capillary-mediated self-assembly and self-organization are useful techniques for constructing ordered superstructures from nanoscale and microscale building blocks. Square arrays of microfibers attached to a substrate have been shown to form highly ordered patterns of 2x2 fiber clusters (tetramers) under the influence of capillary forces at the surface of an evaporating liquid layer. We model this pattern formation as an irreversible sequential adsorption process on a square lattice, in which tetramers form sequentially on an initially empty lattice and locally enhance the formation of nearby tetramers, giving rise to ordering over several lattice lengths. Two parameters regulate the enhancement in tetramer formation at third- and fourth-near neighbor positions. The model is studied using computer simulations and compared to a particular realization of a self-organization experiment. We show that the model quantitatively reproduces many features of the observed patterns when the two parameters are chosen by a least-squares fit to a single experimental quantity. [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q12.00002: Directed Self-assembly of Gold Nanoparticles using Chemically Patterned Templates Robert Nidetz, Jinsang Kim Templated assemblies of metallic nanoparticles are useful for sensors, medical diagnostics, catalysts, and optical devices. We controlled the loading density of gold nanoparticles on a chemical template by manipulating the dimensions of the chemical template. Electron beam lithography was used to fabricate the template on Si wafers, which was chemically patterned with functional silanes that form self-assembled monolayers. The lithographically patterned regions were given a positive charge via aminopropyldimethylethoxysilane, while the remainder of the substrate was made hydrophilic via dodecyltrimethoxysilane. The charge-charge interaction between the negatively charged gold nanoparticles and the positively charged chemical template cause the nanoparticles to self-assemble onto the template. By altering the diameter of the chemical template, it was possible to control the number of 40 nm diameter gold particles that self-assemble. We are currently expanding the directed assembly strategy to nanorods assembly. The developed templated assembly method will find various applications of nano-size objects. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q12.00003: Isomers of Janus colloidal clusters Qian Chen, Steve Granick Colloidal clusters formed by the reversible assembly of amphiphilic Janus spheres show time-dependent isomerization between different isomeric shapes. Here, the rate constant is obtained experimentally by decoupling isomerization from other interconversions between clusters of different mass. The equilibrium constant is found to be controlled by ionic strength, volume fraction and Janus balance. This isomerization between clusters of the same mass but different shape has bearing on understanding nucleation that precedes subsequent growth. [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:03PM |
Q12.00004: Design Rules for Ordered SAMs on Patchy Nanoparticles Aaron Santos, Sharon Glotzer Recently, the self-assembly of ordered stripe-like domains of ligands on the surface of spherical nanoparticles was reported. Molecular ligands tethered to nanoparticle surfaces play an important role in the self-assembly of many systems. These tethers contribute significantly to the free energy of the system because of their large conformational entropy. In general, the stable state can be determined by minimizing the free energy of the system. We have developed a coarse-grained model to rapidly simulate the phase separation of ligands on a surface. The model uses mean field and two-body approximations to compute the conformational entropy of tethers. Using these approximations, one can quickly compute the phase diagrams using a simple Metropolis Monte Carlo simulation. In contrast to traditional coarse-grained simulation methods, which can take hundreds to thousands of hours of CPU time, our coarse-grained model can generally find free energy minimum states in under a few hours. We use this model to study the self-assembly of monolayers on nanoparticle surfaces into a variety of patterns and predict design rules for assembling patchy particles. This work has profound implications for the design and synthesis of ordered patchy particles. [Preview Abstract] |
Wednesday, March 17, 2010 12:03PM - 12:15PM |
Q12.00005: Free Energy Landscape of Colloidal Clusters at Small N Guangnan Meng, Natalie Arkus, Michael Brenner, Vinothan Manoharan We confine small number of micron-sized colloidal particles within micro-wells, and we directly measure the structures and free energies of colloidal clusters, in which the particles act as hard spheres with short-range attractions. We find that highly symmetric clusters are strongly suppressed by rotational entropy and the most stable clusters have anharmonic vibrational modes or extra bonds. The experimental results can be explained well by the classical statistical mechanics and sphere packing theories. [Preview Abstract] |
Wednesday, March 17, 2010 12:15PM - 12:27PM |
Q12.00006: Epitaxial growth of colloidal nanoparticles Y. Kryukov, C. Joshi, T.P. Bigioni, J.G. Amar In recent drop-drying experiments involving gold nanoparticles suspended in an evaporating droplet, the formation of large highly-ordered nanoparticle monolayers growing at the liquid air-interface has been observed. While this process appears to be analogous to epitaxial growth, there are also some important differences. For example, in analogy to epitaxial growth on a solid substrate, our experimental results indicate the existence of a sharp island-size distribution as well as a well-defined critical island-size. However, in contrast to epitaxial growth, we find a power-law decay of the island density with coverage due to the coalescence of large diffusing clusters. In order to better understand these results, we have carried out kinetic Monte Carlo simulations of a realistic model which takes into account (i) ``deposition'' of nanoparticles at the liquid-air interface (ii) two-dimensional diffusion of nanoparticles, and (iii) cluster diffusion and coalescence, as well as the effects of monomer detachment and van der Waals attraction. A comparison between our simulation results and experimental results will be presented. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q12.00007: Self assembly of magnetically interacting cubes by a turbulent Fluid Flow Madhav Mani, Filip Ilievski, Michael Brenner, George Whitesides We self-assemble macroscopic objects using a turbulent flow field and find agreement with a statistical theory. Here we choose to mimic a simple process, linear aggregation, that takes place during several cellular polymerization processes via magnetic interactions between centimeter-sized cubes. The cubes are suspended in a density matched fluid which is driven into a turbulent state, modeled as a gaussian white noise source, with statistics that are well described by energy cascade models for turbulence. Employing the steady-state approximation in the Fokker-Planck equation for the system, dervied directly from the equation of motion of the particles, we can derive the stationary distribution. We find good agreement between the experimental data and the predicted yield-curve taking particular care to evaluate the statistical error due to the finite sampling of the distribution. [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q12.00008: Phase separation induced by ladder-like polymer-polymer complexation Issei Nakamura, An-Chang Shi Complexation between donor and acceptor polymers in solvent through specific binding interactions such as hydrogen bonding is studied using a self-consistent field theory. In the model, two donor and acceptor polymers are capable of forming a ladder-like duplex structure. The duplex formation discontinuously occurs with an abrupt variation in entropy, resulting in a first-order transition. Moreover, solvent-induced complexation is discussed. In this case, the duplex polymer is stabilized by solvent-polymer interactions rather than the specific binding interactions. Various types of unconventional coexistence curves are derived from the model. A phase separation with decreasing the $\chi$-parameter between duplex polymer and solvent can be induced, leading to a lower critical solution temperature (LCST) behavior. Critical points at which two, three, and four phases coexist are also obtained. Under certain conditions a homogeneous phase becomes unstable when the polymer chain length is decreased, in contrast to the standard Flory-Huggins theory. [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q12.00009: Liquid-Vapor Like Phase Transition in DNA-Coated Colloids Francisco Martinez-Veracoechea, Daan Frenkel Colloidal particles coated with DNA-molecules can be designed to bind with high specificity. The result is an unprecedented flexibility for the design of self-assembling systems. Specific interactions can be used to mimic chemical systems at the colloidal level, virtually opening a window to the re-invention of chemistry at a totally new length-scale. Molecular simulations are expected to play a decisive role in the understanding of these materials. In the present work, we use advanced Monte Carlo simulations to study binary systems of DNA-coated colloids, wherein a given type of DNA can only bind to DNA of the opposite type. The system is represented using a coarse-grained model where the DNA interacts through soft-repulsive potentials while the colloids present hard-core interactions only. Binding between DNA is modeled as a harmonic spring between the center of masses of the DNA-chains. The systems are observed to undergo a liquid-vapor like transition for systems where the number of DNA chains per colloid is equal or greater than three. The phase transition is shown to be driven by the entropy gain in bond re-arrangements when the dimers observed in dilute phase form the percolating network typical of the dense phase. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q12.00010: Self-limited self-assembly of chiral filaments Michael Hagan, Yasheng Yang, Robert Meyer The assembly of filamentous bundles with controlled diameters is common in biological systems and desirable for the development of nanomaterials. We discuss dynamical simulations and free energy calculations on patchy spheres with chiral pair interactions that spontaneously assemble into filamentous bundles. The chirality frustrates long-range crystal order by introducing twist between interacting subunits. For some ranges of system parameters this constraint leads to bundles with a finite diameter as the equilibrium state, and in other cases frustration is relieved by the formation of defects. While some self-limited structures can be modeled as twisted filaments arranged with local hexagonal symmetry, other structures are surprising in their complexity. We discuss the relation between model structures and finite bundles in biological or biomaterials systems, and implications for the design of nanostructured materials with controlled sizes. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q12.00011: Optimal arrangement of lamellar and triangular lattices confined to cylindrical fibers Kevin Kohlstedt, Graziano Vernizzi, Francisco Solis, Monica Olvera de la Cruz The optimal packing of ionic lamellar and triangular lattices on the surface of a nanofiber is computed to determine the effects of the surface curvature. In ionic triangular lattices, electrostatic interactions prefer chiral arrangements only for special families of lattices that depend on the fiber diameter. However, there are families of triangular lattices that energetically promote achiral configurations. We also consider the behavior of short-range elastic forces, represented by interconnected springs between neighboring components. In this case a different family of achiral lattices is preferred. We also determine the optimal packing of lamellar lattices of cationic-anionic components. In lamellar packing of cylinders a chiral angle emerges that depends on the cylinder radius and the different dielectric constants of the interior and exterior of the cylinder. We discuss the effect of salt concentration inside and outside the cylinders on the chiral angle, and the implication of our results in ion channels structures. [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q12.00012: Self-Assembled Membranes and Chirality Driven Transitions from 2D Surfaces into 1D Ribbons Edward Barry, Zvonimir Dogic, Robert Meyer, Nadir Kaplan, Thomas Gibaud, Mark Zakhary, Hao Tu, Robert Pelcovits, Rudolf Oldenbourg We briefly outline conditions under which homogeneous non-amphiphilic colloidal rods self-assemble into two-dimensional fluid-like surfaces or membranes. Stabilized by entropic forces, these membranes have properties that are identical to lipid bilayers. ~We then focus on experiments in which the chirality of the constituent particles induces the transition from achiral 2D membranes into 1D twisted ribbons. The model system developed is unique, both in our ability to tune chiral interactions between constituent particles and directly visualize the structure and fluctuations of the final assemblage on all relevant lengthscales. These features allow us to test the theoretical model describing the transition from chiral ribbons to achiral membranes in great detail. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q12.00013: Organic crystal needle formation via solvent-annealing: coarsening, wetting, and crystallization in a thin liquid film Tony S. Yu, A. E. Hosoi Single-crystal, organic semiconductors are attractive because of their highly-nonlinear optical properties and, relative to their amorphous counterparts, they exhibit higher charge mobilities and improved morphological stability. But for such materials to be practical, researchers must develop methods for controlled growth of organic single-crystals. Here we investigate the growth of high-aspect-ratio ``needles'' of single-crystal Alq$_3$---tris(8-hydroxyquinoline) aluminum---from amorphous Alq$_3$ films. During deposition, thin films of amorphous Alq$_3$ are locked in an energetically-unfavorable state. When exposed to solvent, these films evolve into three distinct states: Alq$_3$-rich droplet regions, solvent-rich wetting regions, and Alq$_3$ needles. To understand the experimental results, we model the dynamics of a thin fluid film driven by capillary pressure and intermolecular forces. The resulting flow, coupled with diffusion, transports Alq$_3$ to a growing needle, which is represented by a moving, absorbing interface bounding the fluid. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q12.00014: A Monte Carlo Free Energy Approach to Microphase study and Its Application on the ANNNI Model Kai Zhang, Patrick Charbonneau Microphases are complex and interesting patterns that self-assemble in systems with competing short-range ordering and long-range frustrating interactions. They form in certain colloidal suspensions and in diblock copolymers, but controlling the morphology is notoriously difficult. Knowing the equilibrium behavior of model systems would help understand how to tune the modulated phases. But even for these systems, reliable results for the microphase regime are notoriously difficult to obtain, because of the presence of long-lived metastable states. We develop a Monte Carlo simulation method based on thermodynamic integration that resolves this problem. With our method, we determine with high accuracy the phase behavior of the canonical three-dimensional axial next nearest-neighbor Ising (ANNNI) model, which is one of the simplest models to form lamellas. The XY nature of the modulated-disorder phase transition is confirmed and critical exponents are obtained. We discuss how to generalize our simulation approach to particle-based microphase-forming model. [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q12.00015: From gas-liquid to liquid crystalline phase behavior via anisotropic attraction: A computer simulation study in three and two dimensions Reinhard Hentschke, Wen-Ze Ouyang The partial phase behavior of a continuum molecular model for self-assembling semiflexible equilibrium polymers is studied via Monte Carlo and Molecular Dynamics simulation. We investigate the transfer from ordinary gas-liquid coexistence to the appearance of liquid crystallinity driven by excluded volume interaction between rod-like aggregates. The transfer between the two types of phase behavior is governed by a tunable anisotropic attractive interaction between monomer particles. The relation to dipolar fluid models, which are also known to form reversible chains, is discussed. In two dimensions, depending on the strength of the anisotropy, we find the formation of reversible networks as well as stiff rod-like aggregates. The phase transition observed in the presence of the network structures is compared to predictions of the Tlusty-Safran defect model. [Preview Abstract] |
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