The dissipative particle dynamics (DPD) simulation method has been used to study mesophase formation of linear (AmBn) diblock copolymer melts. The polymers are represented by relatively short strings of soft spheres, connected by harmonic springs. These melts spontaneously form a mesocopically ordered structure, depending on the length ratio of the two blocks and on the Flory–Huggins χ-parameter. The main emphasis here is on validation of the method and model by comparing the predicted equilibrium phases to existing mean-field theory and to experimental results. The real strength of the DPD method, however, lies in its capability to predict the dynamical pathway along which a block copolymer melt finds its equilibrium structure after a temperature quench. The present work has led to the following results: (1) As the polymer becomes more asymmetric, we qualitatively find the order of the equilibrium structures as lamellar, perforated lamellar, hexagonal rods, micelles. Qualitatively this is in agreement with experiments and existing mean-field theory. After taking fluctuation corrections to the mean field theory into account, a quantitative match for the locations of the phase transitions is found. (2) Where mean-field theory predicts the gyroid phase to be stable, the simulations evolve toward the hexagonally perforated lamellar phase. (3) When a melt is quenched the stable structure emerges via a nontrivial pathway, where a series of metastable phases can be formed before equilibrium is reached. The pathway to equilibrium involves a percolation of the minority phase into a network of tubes, which is destabilized by a nematic or smectic transition. (4) We conclude that either hydrodynamic interactions, or the precise form of the Onsager kinetic coefficient play an important role in the evolution of the mesophases.

A melt of linear diblock copolymers (AnBm) can form a diverse range of microphase separated structures. The detailed morphology of the microstructure depends on the length of the polymer blocks An and Bm and their mutual solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts is simulated using two continuum methods: dissipative particle dynamics (DPD) and Brownian dynamics (BD). Although both methods produce the correct equilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophase structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the system. In the DPD simulation, a melt of asymmetric block copolymers rapidly evolves towards the hexagonal structure that is predicted by mean-field theory, and that is observed in experiments. In contrast, the BD simulation remains in a metastable state consisting of interconnected tubes, and fails to reach equilibrium on a reasonable time scale. This demonstrates that the hydrodynamic forces play a critical part in the kinetics of microphase separation into the hexagonal phase. For symmetric block copolymers, hydrodynamics appears not to be crucial for the evolution. Consequently, the lamellar phase forms an order of magnitude faster than the hexagonal phase does, and thus it would be reasonable to infer a higher viscosity for the hexagonal phase than for the lamellar phase. The simulations suggest that the underlying cause of this difference is that the hexagonal phase forms via a metastable gyroid-like structure, and therefore forms via a nucleation-and-growth mechanism, whereas the lamellar phase is formed via spinodal decomposition.

We have developed a system to automatically acquire large numbers of acceptable quality images from specimens of negatively stained catalase, a biological protein which forms crystals. In this paper we will describe the details of the system architecture and analyze the performance of the system as compared to a human operator. The ultimate goal of the system if to automate the process of acquiring cryo-electron micrographs.

We present detailed numerical simulations of modal instabilities in high-power Yb-doped fiber amplifiers using a time-dependent temperature solver coupled to the optical fields and population inversion equations. The temperature is computed by solving the heat equation in polar coordinates using a 2D second-order alternating direction implicit method. We show that the higher-order modal content rises dramatically in the vicinity of the threshold and we recover the three power-dependent regions that are characteristic of the transfer of energy. We also investigate the dependence of the threshold on the seed power and the modal content ratio of the seed. The latter has a minimal effect on the threshold while it is shown that for the fiber configuration investigated, the modal instability threshold scales linearly over a wide range with the seed power. In addition, two different gain-tailored core designs are investigated and are shown to have higher thresholds than that of a uniformly doped core. Finally, we show that this full time-dependent model which does not assume a frequency offset between the modes a priori, predicts a reduced threshold when the seed is modulated at the KHz level. This is in agreement with the steady-periodic approach to this phenomenon.

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Molecular dynamics are performed on systems of two-dimensional periodicity composed of 64 ionic dioctadecyldimethylammonium chloride amphiphiles arranged in a monolayer at 298 K with surface coverages ranging from 57 A2 per amphiphile to 150 A2 per amphiphile. Bond lengths are constrained whereas valence and torsional angles interactions are described by conventional expressions. Nonbonded interactions are introduced through an anisotropic united atom method due to Toxvaerd. The amphiphiles adhere to the surface through electrostatic and nonbonded interactions between the amphiphiles and an underlying substrate. Results are presented for the density normal to the monolayer plane, order parameters, and conformational transition rates. The structure of the two-dimensional layer at the lowest headgroup area studied is quite disordered. Small islands of empty surface surrounded by amphiphiles are formed. As the headgroup area increases, we observed further translational disordering and an increase in the number of amphiphiles aligned with the surface. The g+g+tg+g+ defect observed in one of the chains is stable on a timescale of 250 ps. The equilibrium structures of the two chains are quite different but the conformational dynamics as observed by the transition rates are almost indistinguishable.