Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Z18: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics |
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Sponsoring Units: DPOLY Chair: Vivek Sharma Room: B117 |
Friday, March 19, 2010 11:15AM - 11:27AM |
Z18.00001: Computational modeling of red blood cells: A symplectic integration algorithm Ulf D. Schiller, Anthony J. C. Ladd Red blood cells can undergo shape transformations that impact the rheological properties of blood. Computational models have to account for the deformability and red blood cells are often modeled as elastically deformable objects. We present a symplectic integration algorithm for deformable objects. The surface is represented by a set of marker points obtained by surface triangulation, along with a set of fiber vectors that describe the orientation of the material plane. The various elastic energies are formulated in terms of these variables and the equations of motion are obtained by exact differentiation of a discretized Hamiltonian. The integration algorithm preserves the Hamiltonian structure and leads to highly accurate energy conservation, hence he method is expected to be more stable than conventional finite element methods. We apply the algorithm to simulate the shape dynamics of red blood cells. [Preview Abstract] |
Friday, March 19, 2010 11:27AM - 11:39AM |
Z18.00002: Testing Theory Against Experiment and Simulation for Chain Fluids: Can Lattice Compete with Continuum? Ronald White, Jane Lipson We present new results from an in-depth four-way comparison which contrasts the performance of analogous lattice and continuum integral equation theories, with both being held accountable to recently obtained Monte Carlo simulation results and real experimental data. The success of the modeling methods is compared in terms of both experimentally accessible physical properties (e.g., \textit{PVT} surfaces and coexistence boundaries), as well as the more fundamental underlying quantities, such as free energies, and model internal energies. Without fitting to any mixture data, we find good to excellent predictive ability for mixing behavior, even for the simplest lattice-based approach. Our results lead us to propose the most crucial elements required in constructing simple, yet effective, theories. [Preview Abstract] |
Friday, March 19, 2010 11:39AM - 11:51AM |
Z18.00003: Efficient calculation of electrostatic interactions within dynamic dielectric environments Kipton Barros, Erik Luijten Many biological and soft-matter systems exhibit self-assembly driven by electrostatic interactions. The resulting structures are often dense aggregates, and consequently the electrostatic interactions are significantly modified by dielectric inhomogeneities. This effect is typically ignored in computer simulations, as it greatly complicates the evaluation of energies and forces. Pioneering simulations have demonstrated that fixed dielectric inhomogeneities play a crucial role in biological processes such as water permeation through nanopores [Allen \emph{et al.}, J. Chem.\ Phys.\ {\bf 119}, 3905 (2003)], but the effects of \emph{dynamic} inhomogeneities remain relatively unexplored. We introduce a new method for the efficient calculation of electrostatic interactions within dynamic dielectric environments, and show preliminary simulation results for aqueous suspensions of synthetic colloids. [Preview Abstract] |
Friday, March 19, 2010 11:51AM - 12:03PM |
Z18.00004: Theory of spinodal decomposition assisted polymer crystallization in a binary polyolefin mixture Mithun Mitra, Murugappan Muthukumar Recent experiments by Han et. al. have observed a new kind of coupling process for polymer crystallization in a binary polyolefin blend system, which originates from the fluctuation growth of a two-component phase separating system in the unstable spinodal region. A strong coupling was observed between the concentration fluctuation liquid-liquid phase separation and the nucleation of crystallization, which resulted in significant changes in the crystallization kinetics. In this paper, we propose a possible mechanism which can explain these experimental observations. The spinodal decomposition in the unstable region causes the spontaneous growth of domains of the two constituent polyolefins. It is proposed that these domains then present an interface on which heterogeneous nucleation of the crystallizable component can take place with a much reduced energy barrier. Combining the theories of heterogeneous nucleation and spinodal decomposition kinetics, we present an anaytic calculation of the nucleation rate as a function of the spinodal decomposition time. The analytic formula is found to correspond well with the experimental results for the late stage of spinodal decomposition kinetics. More detailed experiments are required to verify our prediction for the nucleation rate for early times. [Preview Abstract] |
Friday, March 19, 2010 12:03PM - 12:15PM |
Z18.00005: Field theoretic simulations in the Gibbs ensemble Robert Riggleman, Glenn Fredrickson Measuring the properties of phases in equilibrium and the calculation of phase diagrams is one of the most common applications of field theoretic simulations. To obtain properties of both phases from one simulation (such as the concentration of a species in each phase), it is necessary to perform simulations large enough so that the interface between the two phases does not affect the estimate of the bulk properties, which is computationally very demanding. Alternatively, one can perform a sweep through parameter space searching for multiple state points that meet the criteria for equilibrium, which is again computationally expensive. In this talk, I will describe how we have adapted the Gibbs ensemble to field theoretic simulations, where two simulation boxes are kept in chemical and mechanical equilibrium by allowing the boxes to exchange both particles and volume. By maintaining a constant total number of particles and total volume, such a simulation can efficiently simulate two bulk phases in equilibrium in the canonical ensemble, allowing a reliable estimate of the properties of the two phases. Our method will be demonstrated in both the mean-field limit and in simulations that fully sample the fluctuations of the field theory. [Preview Abstract] |
Friday, March 19, 2010 12:15PM - 12:27PM |
Z18.00006: Highly constrained polymer dynamics with an enhanced bond-fluctuation model Frank Bentrem, Colin McFaul We introduce a generalization to the bond-fluctuation model for simulating polymer dynamics in a highly constrained environment. The technique is applied to the quenching of self-attracting polymer chains which demonstrates a three-fold collapse. Both the extent and dynamics of the collapse are greatly enhanced by using the generalized bond-fluctuation model where the bond length $l = \sqrt{8}$ (in units of the lattice spacing) is explicitly utilized. We also show that lattice effects in dense melts ($\phi > 0.5$) are alleviated with this enhancement. Efficiency is maintained by implementing a simple check to prevent phantom chain dynamics. [Preview Abstract] |
Friday, March 19, 2010 12:27PM - 12:39PM |
Z18.00007: Molecular Dynamics Simulations of Polymer Surfaces Thomas Clancy, Sarah Frankland Due to the increased use of polymer based materials in aerospace adhesive applications, the issues of molecular structure effects and contamination at interfaces have become critical. Computational modeling is being developed to study these systems, with the dual goals of elucidating mechanisms of degradation and developing insights into key structure-property relationships both of which contribute to enabling the prediction of adhesive bond failure. Atomistically detailed models of polymer surfaces, interfaces and bulk structures are constructed and analyzed for this purpose. These models are built with a controlled distribution and content of water molecules to assess the effect of moisture ingress on relevant properties of interest. The models are analyzed and compared to study the effect of moisture on surface and interfacial properties which may influence adhesive bonding characteristics. [Preview Abstract] |
Friday, March 19, 2010 12:39PM - 12:51PM |
Z18.00008: HOOMD-blue, general-purpose many-body dynamics on the GPU Joshua Anderson, Aaron Keys, Carolyn Phillips, Trung Dac Nguyen, Sharon Glotzer We present HOOMD-blue, a new, open source code for performing molecular dynamics and related many-body dynamics simulations on graphics processing units (GPUs). All calculations are fully implemented on the GPU, enabling large performance speedups over traditional CPUs. On typical benchmarks, HOOMD-blue is about 60 times faster on a current generation GPU compared to running on a single CPU core. Next generation chips are due for release in early 2010 and are expected to nearly double performance. Efficient execution is achieved without any lack of generality and thus a wide variety of capabilities are present in the code, including standard bond, pair, angle, dihedral and improper potentials, along with the common NPT, NVE, NVT, and Brownian dynamics integration routines. The code is object-oriented, well documented, and easy to modify. We are constantly adding new features and looking for new developers to contribute to this fast maturing, open-source code [1]. In this talk, we present an overview of HOOMD-blue and give examples of its current and planned capabilities and speed over traditional CPU-based codes. [1] Find HOOMD-blue online at: http://codeblue.umich.edu/hoomd-blue/ [Preview Abstract] |
Friday, March 19, 2010 12:51PM - 1:03PM |
Z18.00009: Quantifying and Minimizing Lattice Anisotropy Qiang Wang We have quantified the anisotropy of various lattice models used in polymer simulations based on two quantities: the Fourier transform of the normalized Boltzmann factor of allowable bonds on a lattice (which is the central quantity for describing lattice chain conformations), and the bulk lamellar period at the mean-field order-disorder transition of symmetric diblock copolymers on a lattice (which is pertinent to the study of microphase separation). This allowed us to compare the anisotropy of different lattices and to design new lattice models that minimize the quantified anisotropy. [Preview Abstract] |
Friday, March 19, 2010 1:03PM - 1:15PM |
Z18.00010: Quantitative test of polymer field theories by fast lattice Monte Carlo simulations Xinghua Zhang, Pengfei Zhang, Baohui Li, Qiang Wang Recently, one of us proposed the so-called fast lattice Monte Carlo (FLMC) simulations,\footnote{\textit{Q. Wang}, \textbf{Soft Matter, 5}, 4564 (2009).} where, instead of the self- and mutual-avoiding walks used in conventional lattice Monte Carlo simulations, multiple occupancy of lattice sites is allowed with a proper Boltzmann weight. FLMC simulations give orders of magnitude faster/better sampling of configurational space for multi-chain polymeric systems, and further allow stringent test, without any parameter-fitting, of the widely used polymer field theories. Taking homopolymer solutions and brushes as examples, we formulate the field theories (the self-consistent field theory, Gaussian-fluctuation theory, and the self-consistent Hartree approximation) on the same lattice and with the same Hamiltonian as used in FLMC simulations. Direct comparisons between the simulations and theories therefore unambiguously and quantitatively reveal the consequences of approximations used in the latter. [Preview Abstract] |
Friday, March 19, 2010 1:15PM - 1:27PM |
Z18.00011: Cracks in Ductile Polymers Using Cohesive Zone Modeling Derek Reding Ductile polymer fracture is studied by using a relatively new technique in which cohesive elements are placed between elastic solid elements, along the mesh boundaries. Polymer chain elongation is described using cohesive model parameters that are calibrated to simulate the conical crack observed in a single fiber fragmentation experiment that uses a ductile polyester matrix. This approach limits the crack trajectory to align with the mesh, thus severely limiting the accuracy. We propose a new crack trajectory method to describe polymer chain elongation by incorporating both normal and shear traction contributions in a strictly cohesive zone model approach. Our formulation shows that local polymer chain orientation depends on the ratio of mode I and mode II stiffness penalty parameters and tractions. The corresponding stress state reaches a critical value that is represented by a material parameter. The new crack tip extends to a location where the critical stress is reached at a maximum distance from the existing crack tip. Implementation is performed by adding the proposed crack trajectory method to an extended finite element code (X-FEM) with cohesive element modeling. [Preview Abstract] |
Friday, March 19, 2010 1:27PM - 1:39PM |
Z18.00012: What exactly is measured by passive microbead rheology? Jay Schieber, Ekaterina Pilyugina The dynamic modulus $G^*$ of a viscoelastic medium is often measured by following the trajectory of a small bead subject to Brownian motion in a method called ``passive microbead rheology". In the pioneering manuscript that introduced the idea [T. G. Mason and D. A. Weitz, Phys. Rev. Lett. 74, 1250 (1995)], this equivalence between the autocorrelation function and $G^*$ was assumed via the generalized Stokes-Einstein relation (GSER). We show here that this expression does not satisfy the correct initial condition. Also, earlier derivations of the GSER use an initial condition that freezes the bead in space until measurements begin, which is not typical for experiments. We use here an analytic solution of the forces on a sphere undergoing arbitrary displacement in an arbitrary viscoelastic medium combined with the fluctuation-dissipation theorem to derive what is actually measured in the microbead rheology experiment. We find that a convolution of $G^*$ is indeed measured in bead-displacement statistics, which is similar to GSER but obeys the correct initial conditions. The result includes inertial effects, and allows for the presence of an optical trap, allowing a more general technique to extract the dynamic modulus from microrheology. [Preview Abstract] |
Friday, March 19, 2010 1:39PM - 1:51PM |
Z18.00013: Measuring colloid interactions and dynamics with digital holographic microscopy and multi-particle scattering theory Jerome Fung, David Kaz, Ryan McGorty, Guangnan Meng, K. Eric Martin, Vinothan N. Manoharan We describe an \textit{in situ}, nonperturbative optical technique for measuring the pair potential between two colloidal particles in bulk suspension. We image clusters of colloidal spheres at or near contact with digital holographic microscopy and fit the resulting holograms to the exact numerical solution for electromagnetic scattering from the clusters. We measure the depletion interaction between two 1 $\mu$m polystyrene spheres in a bulk suspension by studying the thermal fluctuations of their $\sim$ 50 nm separation distance with $\sim$ 5 nm spatial resolution. Our method does not require the use of optical tweezers and thus may be useful for studying interactions between colloids that are too small or too nearly index-matched to be optically trapped. We also use our methods for recording and fitting holograms to simultaneously measure the 3D translational and rotational Brownian motion of sphere clusters. [Preview Abstract] |
Friday, March 19, 2010 1:51PM - 2:03PM |
Z18.00014: Probing the Physics of Organic-Organic Heterojunctions using Laterally defined Organic Field Effect Diodes Bal Mukund Dhar, Geetha Kini, Nina Markovic, Howard Katz Scanning Probe Microscopy has been used extensively to probe the physics of metal/organic semiconductor contacts. However, a similar level of knowledge for Organic/Organic interfaces has been elusive because of the buried nature of the junction. We have invented a novel lithographic technique to fabricate a lateral heterojunction diode, the characteristics of which can be tuned by use of third terminal in a thin film transistor configuration. Kelvin Probe microscopy reveals the built in potential at the junction and its modulation by use of the third gate terminal. Changes in potential are consistent with changes in rectification behavior as evidenced by I-V plots. ~We also report the use of such a junction to investigate the energetics of doped organic interfaces, including the density of states profile on each side of the junction. [Preview Abstract] |
Friday, March 19, 2010 2:03PM - 2:15PM |
Z18.00015: Positron Annihilation Spectroscopy as a Novel Interfacial Probe for Thin Polymeric Films and Nano-Composites Somia Awad, Hongmin Chen, Grace Maina, L. James Lee, Xiaohong Gu, Y.C. Jean Positron annihilation spectroscopy (PAS) has been developed as a novel probe to characterize the sub-nanometer defect, free volume, profile from the surface, interfaces, and to the bulk in polymeric materials when a variable mono-energy slow positron beam is used. Free-volume hole sizes, fractions, and distributions are measurable as a function of depth at the high precision. PAS has been successfully used to study the interfacial properties of polymeric nanocomposites at different chemical bonding. In nano-scale thin polymeric films, such as in PS/SiO$_{2}$, and PU/ZnO, significant variations of T$_{g}$ as a function of depth and of wt{\%} oxide are observed. Variations of T$_{g}$ are dependent on strong or weak interactions between polymers and nano-scale oxides surfaces. [Preview Abstract] |
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