Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W9: Self Assembly II followed by Vesicles and Micelles II |
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Sponsoring Units: DFD Chair: Remi Dreyfus, CNRS Room: D220 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W9.00001: Self-assembly of two-dimensional systems with off-center core-corona architecture Daniel Salgado, Carlos Mendoza Physical systems with core-corona architecture, such as dendritic polymers or hyper-branched star polymers which are characterized by two repulsive length scales, related to the hard and soft repulsions, respectively, show the spontaneous formation of stripe phases. Here we study, by using Monte Carlo simulations, how robust is the stripe formation process upon a shift in the center of the core with respect to the corona in a two-dimensional system of colloidal particles. We find that for sufficiently large shifts, the strip phases are replaced by a sort of plastic (or glassy) colloidal crystal consisting of a regular lattice of coronas inside of which disordered aggregates of cores coexist. The model investigated in this work could be useful for the design of colloidal plastic crystals. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W9.00002: Limit of validity of Ostwald's rule of stages in a model of solution crystallization Lester Hedges, Stephen Whitelam Many systems take ``nonclassical'' crystallization pathways, forming ordered solids via intermediates that do not share the architecture of the stable material. We possess only rules-of-thumb to explain such dynamics. Chief among them is Ostwald's rule of stages, which states that the phase that first emerges is the one closest in free energy to the parent phase. Although widely applicable, the rule breaks down in many experiments and computer simulations. It is therefore clear that the rule is without firm theoretical foundation, but it is not clear when it should apply. To this end we test Ostwald's rule of stages in a lattice model of solution crystallization. We find that rule holds in certain regions of parameter space and breaks down in others. We argue that its breakdown can be predicted using simple arguments. In addition, we find that crystallization pathways depend qualitatively on both the thermodynamic landscape prescribed by inter-particle interactions and on the relative rates of particle rotations and translations. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W9.00003: Mesophase behavior and rheology of polyhedral particles Umang Agarwal, Fernando Escobedo Translational and orientational excluded volume fields can guide assembly of particles with anisotropic shape to diverse morphologies. A roadmap elucidating correlations between phase behavior and particle shape may help devising efficient strategies for self-assembly of desired nanocrystal superlattices. To explore these complex correlations we performed detailed Monte Carlo simulations of six convex multi-faceted shapes belonging to the diverse class of space-filling polyhedrons. Simulations predict formation of various novel liquid-crystalline and plastic-crystalline phases at intermediate volume fractions. By correlating these findings with particle anisotropy and order of rotational symmetry, simple guidelines for predicting phase behavior of polyhedral particles are proposed. Moreover, detailed analysis of the structures of mesophases reveals importance of dynamical order in defining these phases and preliminary information about kinetics of these transitions is also obtained. Finally, to elucidate the effect of particle shape anisotropy on rheology, preliminary results will be reported from non equilibrium molecular dynamics simulations of the isotropic and cubatic(LC) phase of cuboidal particles. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W9.00004: Triblock Janus Spheres Qian Chen, Sung Chul Bae, Steve Granick We show that spheres that attract one another on two polar regions but repel at the middle band (``triblock Janus'') assemble into nontrivial reticulated networks. We have constructed such spheres and have visualized their aqueous assembly dynamics on the single-particle level. The building blocks are simple micron-sized colloidal spheres whose interactions (electrostatic repulsion in the middle, hydrophobic attraction at the poles) are likewise simple, but their self-assembly into this open structure contrasts with previously-known close-packed periodic arrangements of spheres. This strategy of ``convergent'' self-assembly from facilely fabricated colloidal building blocks encodes the target supracolloidal architecture not in localized attractive spots but rather in large redundantly attractive regions of the building blocks. The idea extends to designing other supracolloidal networks. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W9.00005: Novel structure formation of dipolar Janus particles (JP) in electrolytes: A molecular dynamic (MD) simulation study Mahdy Malekzadeh, Bamin Khomami There have been tremendous number of experimental studies and number of simulations in recent years trying to elucidate the underlying principals which determine structure formation of colloidal systems of JP. However most of simulations utilize relatively simple models and lack inclusion of long range columbic interactions. In this work MD simulations have been performed to understand effects of surface charge density and volume fraction (0.01-0.17) on structure formation and radial pair distribution function (RDF) of JPs of 6 nm in diameter with opposite charges on each hemisphere. Inclusion of long range columbic interaction via Ewald summation leads to formation of novel structures such as rings, chains and layered large spheres (about hundreds of nanometers) in accord to experimental observations. Moreover based on possibility of defect formation during synthesis, defects were introduced into each JP by slightly altering the uniform charge distribution on each hemisphere. Our results show in presence of small amount of defects ($<$10{\%}) no significant changes occur in RDF, however increasing defect sites up to around 20{\%} will significantly changes structure formation and combination of aforementioned structures concur to SFM and SEM images. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W9.00006: Glassy Dynamics in the Rotator Phase of Two-Dimensional Janus Crystals Jing Yan, Shan Jiang, Jonathan Whitmer, Stephen Anthony, Erik Luijten, Steve Granick Janus particles, spheres with two different sides, represent the simplest building blocks whose interparticle interaction is orientation dependent. When confined on regular lattices, they epitomize basic physical problems from the arrangement of spins in magnetic materials, to rotating molecules in plastic crystals. Here we study both in experiment and in simulation, the heterogeneous dynamics in a two-dimensional crystal of amphiphilic Janus spheres. Single particle tracking reveals that orientation along can generate phenomenology resembling conventional translational supecooled liquids and glasses. Characteristic cage break events, which requires anti- correlated rotation of particles sitting on neighboring lattices, were indentified and characterized in detail. Recent experiments aiming at selectively perturbing the system using external field, such as magnetic field, will also be discussed. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W9.00007: Chiral Control of Interfacial Tension Mark Zakhary, Thomas Gibaud, Edward Barry, Robert Meyer, Zvonimir Dogic The interfacial tension between molecular species in self-assembling systems plays a crucial role in determining the physical properties of the mesoscopic assemblages. The predominant method for controlling interfacial tension is the addition of surfactant molecules, which preferentially adsorb onto the interface and modify the interactions between the two phases. In this talk, using a model colloidal membrane composed of chiral, rod-like \textit{fd }viruses, I will present a new method for controlling interfacial tension which does not require additional surfactant components, but instead utilizes the intrinsic chirality of the constituent rods. I will demonstrate that chirality can be used to continuously tune the interfacial tension of a membrane and to drive a dramatic phase transition from two-dimensional membranes to one-dimensional twisted ribbons. Using a wide variety of microscopic techniques, this transition is characterized over all relevant length-scales, ranging from nanometers to microns. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W9.00008: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W9.00009: Dynamics of a compound vesicle in shear flow Shravan Veerapaneni, Yuan-Nan Young, Petia Vlahovska, Jerzy Blawzdziewicz The dynamics of compound vesicle (a lipid bilayer membrane enclosing a fluid with a suspended particle) in shear flow is investigated using both numerical simulations and theoretical analysis. We find that the non-linear coupling (via hydrodynamic interaction) between the inclusion motion and the confining membrane deformation gives rise to new features in the vesicle dynamics. Transition from tank--treading to tumbling can occur even in the absence of any viscosity mismatch. An initially non-concentric inclusion induces transient vesicle waltzing. A swinging-like vesicle motion is observed if the enclosed particle is an ellipsoid. The rheology of a suspension of compound vesicles is also strongly affected by the inclusion confinement. Our results highlight the complex effects of internal cellular structures on cell dynamics in external flow. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W9.00010: Exploring Structure, Shape, and Dynamics of Elastin-like Polypeptide Nanoparticles Kiril A. Streletzky, Kaitlin Vandemark, Ali Ghoorchian, Nolan Holland Environmentally responsive nanoparticles synthesized from elastin-like polypeptides (ELP) present a promising system for applications as biosensors, drug delivery vehicles, and viscosity modifiers. These nanoparticles undergo a transition from a soluble state at room temperature to micellar aggregates above the transition. The size, shape, and dynamics of micelles above the transition as well as effects of the solvent salt concentration and pH on the transition are important to understand from a fundamental science point of view as well as for potential applications. The system has been characterized with high resolution multiangle Dynamic and Static Light Scattering Spectroscopies. It was confirmed that the system undergoes a transition from mixture of ELP extended trimers and their non-spherical formations to a solution of micelles. It was discovered that micellar size and structure are very sensitive to solution's pH. The micelles were generally found to exhibit properties of the hyperbranched spheres below pH of 10 and above pH of 10.3 with their shape becoming significantly elongated in the pH window of 10 to 10.3. It was also found that the size of micelles strongly depends on salt concentration displaying at least two size regimes (20-45nm at 0-20mM and 100-150nm at 25-40mM) with different salt concentration dependences. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W9.00011: The Lipid domain Phase diagram in a Dipalmitoyl-PC/Docosahaexnoic Acid-PE/Cholesterol System Chai Lor, Linda Hirst Lipid domains in bilayer membrane and polyunsaturated fatty acids (PUFAs) are thought to play an important role in cellular activities. In particular, lipids containing docosahaexnoic acid are an interesting class of PUFAs due to their health benefits. In this project, we perform oxidation measurements of DHA-PE to determine the rate of oxidation in combination with antioxidants. A ternary diagram of DPPC/DHA-PE/cholesterol is mapped out to identify phase separation phenomena using atomic force microscope (AFM). Fluorescence microscopy is also used to image lipid domains in a flat bilayer with fluorescent labels. As expected, we observe the phase, shape, and size of lipid domains changes with varying composition. Moreover, we find that the roughness of the domains changes possibly due to overpacking of cholesterol in domains. This model study provides further understanding of the role of cholesterol in the bilayer membrane leading towards a better understanding of cell membranes. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W9.00012: Interfacial Microrheology with a Magnetic Needle Viscometer and Two-Particle Correlated Motion James Sebel, Kenneth W. Desmond, Eric R. Weeks We measure the viscoelastic moduli of thin films using two different methods. First, we use a magnetic needle viscometer. Our apparatus employs Helmholtz coils to control the position and orientation of the needle in the film. By driving the needle we can produce a response in the film which allows us to probe the bulk viscoelastic properties of the film. Second, we use two particle microrheology to probe the local properties of the film. Tracking the correlated motion between two particles as they undergo Brownian motion probes the local viscoelastic properties of any heterogeneous domains. Examining the correlations between pairs of particles with large separations helps us infer information about the bulk properties. Coupling this technique with the magnetic needle viscometer provides information on the effect local viscoelastic properties have on the bulk properties. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W9.00013: Ion-Specific Induced Charges at Aqueous Soft Interfaces Wenjie Wang, Alex Travesset, David Vaknin Surface-sensitive X-ray scattering and spectroscopic techniques are employed to monitor ion binding specifically to Langmuir monolayers of densely packed carboxyl or phosphate groups. By systematically varying pH of Fe$^{3+}$, Fe$^{2+}$ and La$^{3+}$ solutions, we show that the critical surface pressure at the tilted (L2) to untilted (LS) transition is ionic specific and pH dependent. While the maximum density of surface bound La$^{3+}$ per carboxylic group is $\sim$ 0.3, the amount necessary to neutralize the fully charged surface, for Fe$^{3+}$ it is nearly 0.6. Furthermore, the binding of Fe$^{3+}$ is accompanied with a significant accumulation of Cl$^-$ co-ions implying interfacial charge inversion. Similar experiments with charged phosphate groups at the interface show that the bindings of Fe$^{2+}$ and La$^{3+}$ are electrostatically driven. Our results have implications on biomineralization processes and ionic functions at cell membranes. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W9.00014: Structural characterization of a multiple stacked supported bilayer system Curt Decaro, Justin Berry, Laurence Lurio, Yicong Ma, Gang Chen, Sunil Sinha, Lobat Tayebi, Atul Parikh Supported Lipid Bilayers are a popular model system for cell membranes since their defined orientation allow characterization with probes such as AFM, x-ray and neutron scattering. A significant concern, however, is that strong interactions with the substrate can suppress dynamics within the bilayer. One method that has been successful at overcoming this limitation is to cushion the supported bilayer on a softer material. In the present work, we have stacked up to five successive bilayers of DPPE on top of each other, in effect using the lower bilayers as cushions. X-ray reflectivity shows that each stack preserves the orientation of the first, and that each bilayer exhibits full coverage of the one below. The roughness of each bilayer is found to increases with distance from the substrate as would be expected if thermal fluctuations are increasing with distance from the substrate. We also find that upon heating from the gel to the fluid state that an unbinding transition is observed. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W9.00015: Shape and Size of highly concentrated micelles in CTAB/NaSal solutions by small angle neutron scattering (SANS) Hu Cao, Helmult Kaiser, Narayan Das, Paul Sokol, Joseph Gladden Highly concentrated micelles CTAB/NaSal with a fixed salt/surfactant ratio of 0.6 have been studied by small angle neutron scattering (SANS) as a function of temperature and concentrations. A modeling analysis with a combination of ellipsoid, Gaussian size distribution and Hard Sphere Model (HSM) on SANS data suggests that these micelle solutions have an ellipsoidal structure, which is independent on the concentrations and temperature. However, the micelle size decreases monotonically as increasing the temperature or concentration. Besides, it was found that the number density of particles increases as increasing the temperature, while the total volume keeps unchanged. These observations indicate that large micelles at low temperature begin to break to form small ones as increasing the temperature and these broken surfactant molecules aggregate again under the effect of strongly binding counterions to form more micelles. [Preview Abstract] |
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