Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X19: Focus Session: Polymer Colloids: Particle Interactions and Assembly |
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Sponsoring Units: DPOLY Chair: Tony Dinsmore, University of Massachusetts, Amherst Room: B118-B119 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X19.00001: Directing the Self-Assembly of Colloidal Particles Natalie Arkus, Vinothan Manoharan, Michael Brenner A mechanism for directing the self-assembly of spherical colloidal particles into any desired structure is presented. The mechanism is derived from an exact understanding of what structures the particles can form in thermodynamic equilibrium. This method can, in principle, be used to create materials with a specified structural arrangement. We discuss potential applications towards deriving an exact relationship for how the microscopic structural arrangement of a material determines its macroscopic properties. [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X19.00002: Anisotropic self-assembly of colloidal particles in polymer-colloid composites: A simulation study Monojoy Goswami, Bobby Sumpter The self-assembly of colloidal particles has potential applications in optical fibers, sensors and photovoltaic cells. In this work we have carried out stochastic molecular dynamics simulations of colloid-polymer composites in order to investigate the fundamental self-assembly processes of the particles, in an effort to design more optimal materials for the applications stated above. Results were obtained for spherical colloidal particles of different screening lengths dispersed in a polymer matrix at melt density. By tuning the screening length and interaction strengths between the colloid and polymer, self-assembly into structures that generate anisotropy in the composite material is demonstrated. This phenomenon in colloid-polymer mixtures is analogous to the previously observed self-assembly of grafted nanoparticles in polymer nanocomposites. Our results show a potentially easier way of producing anisotropic self-assembly in polymer-nanocomposites based on colloidal particles as fillers. We also discuss the dynamics of the polymer chains and colloidal particles for different screening lengths and polymer-filler interaction strengths. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X19.00003: Patchy particles by self assembly Dennis Discher, David Christian Patchy particles offer novel means for colloidal assembly or clustering of functional groups but are generally made by batch processes rather than self-assembly. Selective binding of multivalent ligands within a mixture of polyvalent amphiphiles provides, in principle, a mechanism for driving domain formation in self-assemblies. Divalent cations are shown here to crossbridge polyanionic amphiphiles, which thereby demix from neutral amphiphiles and form spots or rafts within vesicles as well as stripes within cylindrical micelles. Calcium- and copper-crossbridged domains of synthetic block copolymers or natural lipid (phosphatidylinositol-4,5-bisphosphate) possess tunable sizes, shapes and/or spacings that can last for years. Lateral segregation in these `responsive Janus assemblies' couples weakly to curvature and proves to be restricted within phase diagrams to narrow regimes of pH and cation concentration that are centred near the characteristic binding constants for polyacid interactions. Remixing at high pH is surprising, but a theory for strong lateral segregation shows that counterion entropy dominates electrostatic crossbridges, thus illustrating the insights gained into ligand-induced pattern formation within self-assemblies. REFERENCES - [1] D.A. Christian, A. Tian, W.G. Ellenbroek, I. Levental, P.A. Janmey, A.J. Liu, T. Baumgart, D.E. Discher. Spotted vesicles, striped micelles, and Janus assemblies induced by ligand binding. Nature Materials 8: 843--849 (2009). [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X19.00004: Tunable helical micelles via kinetic assembly of charged block copolymer Sheng Zhong, Ke Zhang, Karen Wooley, Darrin Pochan Helical cylinder micelles are made from the assembly of poly(acrylic acid)-\textit{block}-poly(methyl acrylate)-\textit{block}-polystyrene with organic multiamines in a THF/H$_{2}$O mixtures. Single- and double-stranded helices and left- and right-handed helices are found. Cryo-TEM study shows that the kinetic pathway for formation of helical cylinders follows a complex nanostructure evolution which involves the stacking of bended cylinders at early stages and the subsequent interconnection of these bended cylinders. Spherical micelles bud off of the interconnected nanostructure as the final step towards a defect-free helix. The stable pitch distance of the helices, which is due to unevenly distributed amine molecules in the corona, can be efficiently tuned. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X19.00005: Modeling the Structures Arising From Defect Bonding Among Colloidal Particles in a Cholesteric Liquid Crystal Frances Mackay, Colin Denniston We use a lattice-Boltzmann algorithm to investigate the possible structures formed by interactions among spherical colloidal particles emersed in a cholesteric liquid crystal. Strong tangential anchoring is assumed at the particle surfaces, which for a given sphere, results in the formation of a pair of +1/2 defect lines, which originate on the sphere, spiral around it at some distance, and then terminate on it. When the helical pitch of the cholesteric is comparable in size to the particle diameter, these defects have the appearance of two handles emerging from the sphere. With more than one particle present, the handles from different spheres tend to join together, forming a type of bond between them. Both single and double bonding among particles is found to be possible. We present results showing chains and other types of structures produced with this type of bonding. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 3:42PM |
X19.00006: Measurement of Colloidal Interactions Using Holographic Microscopy and Multi-particle Scattering Theory Kristopher Eric Martin, Jerome Fung, Vinothan Manoharan Holographic microscopy provides the ability to record particle information in three dimensions with rapid time resolution. Single-particle scattering theory has been used to interpret holographic images of single colloids and provide highest available resolution for imaging single colloids in three dimensions. This method, however, has yet to be employed to interpret images of multiple colloids. We demonstrate the implementation of a multiple-particle generalization of Lorenz-Mie scattering solution to interpret holographic images of clusters of spherical colloids. Highly precise theoretical holograms of multiple spherical colloids are calculated using the multiple-particle scattering theory, and recorded holographic images of colloidal clusters are fit to those of the theoretical method. The parameters of the fitting routine are used to characterize colloids' sizes, indices of refraction and separation radii, amongst other properties. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 4:18PM |
X19.00007: Self-Replication Without Life (yet) Invited Speaker: We want to make a non-``biological'' system which can self-replicate. The idea is to design particles with specific and reversible and irreversible interactions, introduce seed motifs, and cycle the system in such a way that a copy is made. Repeating the cycle would double the number of offspring in each generation leading to exponential growth. Using the chemistry of DNA either on colloids or on DNA Tiles makes the specific recognition part easy. In the case of DNA Tiles we have in fact replicated the seed at least to the third generation. The DNA linkers can also be self-protected so that particles don't interact unless they are held together for sufficient time - a nano-contact glue. We have also designed and produced colloidal particles that use novel geometries to get specific and reversible physical interactions. **With M. Leunissen, R. Dreyfus, R. Sha, T. Wang, S. Sacanna, D. Pine and N. Seeman [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X19.00008: Dynamics of nanoparticles in an Optical Trap studied by Fluorescence Correlation Spectroscopy Yi Hu, H.D. Ou-Yang This paper reports the results of an experiment that combined fluorescence correlation spectroscopy (FCS) and optical trapping to study the dynamic behavior of nanoparticles in a potential well. Using FCS, we were able to measure the enhanced nanoparticle concentration as a function of optical trapping intensity. Quantitative analysis of this increased density using a balance of inward radiation and outward osmotic pressures permitted the determination of the trapping energy per individual nanoparticle. The values for the trapping energy agreed with those found from a diffusion analysis of the optical trapping-elongated residence times of particles in the focal region. With these self-consistently determined trapping energies, we were able to rescale the trapping power into radiation pressure and calculate the osmotic compressibilities for different sized nanoparticle systems. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X19.00009: Critical Depletion Roberto Piazza, Stefano Buzzaccaro, Alberto Parola, Jader Colombo Most experimental studies of the effects brought in a colloidal suspensions by the presence of depletion forces have so far been performed on systems where the depletion agent can be regarded as ideal or weakly interacting. Here, by investigating the depletion effects brought in by surfactants that show a liquid-liquid phase separation with water, we shall conversely deal with a situation where long-range spatial correlations are of primary importance in setting the phase behavior of the colloidal fluid. In particular, we shall show that, in the proximity of the critical demixing point, depletion effects merge continuously into critical Casimir effects, displaying distinctive scaling properties. Our results suggests therefore an unified view of these two apparently unrelated phenomena [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X19.00010: Depletion-Enhanced Nanoparticle Compressibility Joseph Junio, H.D. Ou-Yang Binary mixtures of colloids have been widely studied for their ability to spontaneously phase separate under certain size and mixing proportions. This separation is theorized to be due to induced entropic depletion attractions, where smaller particles gather and crowd out the larger ones. A consequence of this induced attraction is an enhanced osmotic compressibility. With an optical bottle, we were able to use the gradient force from a focused laser to locally concentrate nanoparticles in the presence of small polymers, and quantitatively analyze the increased density as a function of laser power, and calculate the osmotic compressibility. Measurements of the compressibility of the binary suspension as a function of added polymer were conducted to determine the strength of the induced depletion attraction and its effects on phase separation. Theoretical calculations of the expected compressibility for suitably defined binary systems are compared to experimental results. [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X19.00011: Structural transitions in condensed colloidal virus phases Nathan Schmidt, Steve Barr, Andrew Udit, Leonardo Gutierrez, Thanh Nguyen, M. G. Finn, Erik Luijten, Gerard Wong Analogous to monatomic systems colloidal phase behavior is entirely determined by the interaction potential between particles. This potential can be tuned using solutes such as multivalent salts and polymers with varying affinity for the colloids to create a hierarchy of attractions. Bacteriophage viruses are a naturally occurring type of colloidal particle with characteristics difficult to achieve by laboratory synthesis. They are monodisperse, nanometers in size, and have heterogeneous surface charge distributions. We use the MS2 and Qbeta bacteriophages (diameters 27-28nm) to understand the interplay between different attraction mechanisms on nanometer-sized colloids. Small Angle X-ray Scattering (SAXS) is used to characterize the inter-particle interaction between colloidal viruses using several polymer species and different salt types. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X19.00012: Entropically Driven Exfoliation of Clay in a Clay/Polymer Nanocomposite Horst Winter, Katie Lania, Fei Li, Xiaoliang Wang Dynamic mechanical spectroscopy was used to monitor the maturing of structure in a clay/polymer nanocomposite. The rheological observations show that the classical mechanism of exfoliation by diffusion has to be modified. A new exfoliation mechanism is proposed as follows: when end-functionalized (``sticky'') macromolecules anchor at the surface of clay particles, the thermal motion of the polymer is restricted due to the presence of the solid surface. This gives rise to an entropic pulling force on the clay surface which peels clay sheets away from the clay particles (stacks of clay sheets). The clay, organically modified with macro-counterions, is only weakly connected internally so that the macromolecular entropic outer forces can overcome the internal cohesion. The entropic force increases with temperature due to the increased thermal motion, but then decreases again when macromolecules begin to detach. Polymer molecules and clay connect into a sample-spanning network with increasing modulus and decreasing relaxation time as the clay exfoliation proceeds and more and more clay surface becomes accessible to the polymer. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X19.00013: Phase behavior of repulsive polymer-tethered colloids Behnaz Bozorgui, Maya Sen, William L. Miller, Josep C. Pamies, Angelo Cacciuto We report molecular dynamics simulations of a system of repulsive, polymer-tethered colloidal particles. We use an explicit polymer model to explore how the length and the behavior of the polymer (ideal or self-avoiding) affect the ability of the particles to organize into ordered structures when the system is compressed to moderate volume fractions. We find a variety of different phases whose origin can be explained in terms of the configurational entropy of polymers and colloids. Finally, we discuss and compare our results to those obtained for similar systems using simplified coarse-grained polymer models, and set the limits of their applicability. [Preview Abstract] |
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