关键词： Vesicles   Lipids   Membranes   Monolayers   Chemical structure  

摘要： We have developed an algorithm to determine membrane structure, area per lipid, and bending rigidity from molecular dynamics simulations of lipid vesicles. Current methods to extract structure from vesicle simulations define densities relative to the global center of mass of the vesicle. This approach ignores the long-wavelength fluctuations (undulations) that develop across the sphere and broaden the underlying structure. Our method establishes a local reference frame by defining a radially undulating reference surface (URS) and thereby removes the broadening effect of the undulations. Using an arc-length low-pass filter, we render the URS by defining the bilayer midplane on an equi-angular ?, ?-grid (colatitude, longitude). This surface is then expanded onto a truncated series of spherical harmonics. The spherical harmonic coefficients characterize the long-wavelength fluctuations that define both the local reference frameused to determine the bilayers structureand the area per lipid (AL) along the undulating surface. Additionally, the resulting power spectrum of spherical harmonic coefficients can be fit to a Helfrich continuum model for membrane bending in spherical geometry to extract bending rigidity (k(c)). k(c) values determined for both DMPC and DMPC + cholesterol (30 mol %) vesicles are consistent with values from corresponding flat-patch systems determined using an independent, previously published spectral method. These new tools to accurately extract structure, AL, and k(c) should prove invaluable in evaluating the construction and equilibration of lipid vesicle simulations.

关键词： Amorphous alloys   Ab initio molecular dynamics   Structure and mechanical properties  

摘要： Understanding the correlation of deformation and local structures in bulk metal glasses (BMGs) is essential for the developing new types of amorphous alloys and their engineering applications. We investigate the relation of local structures and mechanical properties for Mg-54(Cu1-xAgx)(35)Y-11 (0 < x <= 0.5) alloys by combining ab initio molecular dynamics simulations with density functional theory calculations. It is found that local structures defined by the common-neighbor analysis and mechanical properties display a development as a function of composition, and the calculated results with respect to the Ag composition is well consistent with experiments. We show a strong interplay between local structures and mechanical properties in that high icosahedral ordering and low free volume are helpful to improve the strength, while the incorporation of crystalline ordering and high free volume can initiate a better intrinsic plasticity. Physical implications of these results are discussed and our research is of great value to the development of multi-components amorphous alloys. (C) 2014 Elsevier B.V. All rights reserved.

关键词： Hamiltonian vector fields   Homogeneous polynomial potentials   Global flow  

摘要： In this work we study mechanical systems defined by polynomial potentials of degree on the plane, when the potential has a definite or semi-definite sign and the energy is non-negative. We give a global description of the flow for the non-negative potential case. Some partial results are obtained for the more complicated case of non-positive potentials.

关键词： 15A18   34A12   Expectation value dynamics   Hausdorff moment problem   Kronecker power series   Ordinary differential equations   Probabilistic evolution approach   Quantum mechanics  

摘要： This is the first one of two companion papers focusing on the establishment of a new path for the expectation value dynamics of the quantum mechanical operators. The main goal of these studies is to do quantum mechanics without explicitly solving Schrödinger wave equation, in other words, without using wave functions except their initially given forms. This goal is achieved by using Ehrenfest theorem and utilizing probabilistic evolution approach (PEA). PEA, first introduced by Metin Demiralp, is a method providing solutions to the nonlinear ordinary differential equations by transforming them to a set of linear ODEs at the cost of denumerably infinite dimensionality. It is recently shown that this method produces analytic solutions, if the initial conditions are given appropriately at some special cases. However, generalization of these conditions to the quantum mechanical applications is not straightforward due to the dispersion of the quantum mechanical systems. For this purpose, multivariate moment problems for the integral representation of the Kronecker power series are introduced and then solved yielding to more specific and precise convergence analysis for the quantum mechanical applications. [ABSTRACT FROM AUTHOR]

关键词： Bat   Efficiency   Flight   Mechanical   Model   Power  

摘要： Previous bat aerodynamic power models are refined by (1) varying the value of wing profile drag with lift coefficient, which varies with both flight speed and Reynolds number, (2) allowing for the aerodynamic cleanliness of head, body, ears and tail in calculating parasitic drag values at various speeds and according to airframe type, (3) incorporating models of wingbeat amplitude and frequency in the power calculations, and, (4) upgrading the allometric, phylogenetically corrected relationship between basal metabolic rate and body mass using data from 98 bat species. The fidelity of the aerodynamic power model is assessed using published wind tunnel data on a bat in steady glide. By comparing empirical published metabolic power (P_met) values with values derived using the new aerodynamic model, we update estimates of inflight musculoskeletal mechanical efficiency (g) for the airframes of eight bat species at steady level flight speeds. Furthermore, we calculate the increase in g at high speeds. The bats assessed range in body mass from 0.01 to 1 kg, and the comparison covers the speed range normally used by free-flying bats during their excursions. At their best endurance flight speeds (Vend), g — 1.52 Ln (m_bat) +11.44 (%). At speeds > V_end, η — η@_Vend+ 1.3 (V— V_end) (%). These equations yield accurate P_met estimates for flight speeds within the range used for the steady level flight. © Deutsches Zentrum für Luft- und Raumfahrt e.V. 2014

关键词： The worst resonant response   Multi-dimensional harmonic balance method   Quasi-Periodic Solution   The MultiStart algorithm   Uncertainty  

摘要： An efficient method to obtain the worst quasi-periodic vibration response of nonlinear dynamical systems with uncertainties is presented. Based on the multi-dimensional harmonic balance method, a constrained, nonlinear optimization problem with the nonlinear equality constraints is derived. The MultiStart optimization algorithm is then used to optimize the vibration response within the specified range of physical parameters. In order to illustrate the efficiency and ability of the proposed method, several numerical examples are illustrated. The proposed method is then applied to a rotor system with multiple frequency excitations (unbalance and support) under several physical parameters uncertainties. Numerical examples show that the proposed approach is valid and effective for analyzing strongly nonlinear vibration problems with different types of nonlinearities in the presence of uncertainties. (C) 2014 Elsevier B. V. All rights reserved.

关键词： peptide hydrogel   beta-hairpin peptide   residue substitution   gelation kinetics   gel rigidity   biomedical scaffold  

摘要： Self-assembling peptide hydrogels with faster gelation kinetics and higher mechanical rigidity are favorable for their practical applications. A design strategy to control the folding, self-assembly, and hydrogelation of beta-hairpin peptides via hydrophobic amino acid substitutions has been explored in this study. Isoleucine has higher hydrophobicity and stronger propensity for beta-sheet hydrogen bonding than valine. After the valine residues of MAXI ((VKVKVKVKVPPTKVKVKVKV)-P-D-NH2) were replaced with isoleucines, oscillatory rheometry and circular dichroism (CD) spectroscopy characterizations indicated that the variants had clearly faster self-assembly and hydrogelation rates and that the resulting gels displayed higher mechanical stiffiiess. Transmission electron microscopy (TEM) indicated the parent MAXI and its variants all formed networks of long and entangled fibrils with the similar diameters of similar to 3 nm, suggesting little effect of hydrophobic substitutions on the self-assembled morphology. The MAXII8 (IKIKIKIKVDPPTKIKIKIKI-NH2) hydrogel showed the fastest gelation rate (within 5 min) and the highest gel rigidity with the series, supporting the homogeneous cell distribution within its 3D scaffold. In addition, the MAX118 hydrogel showed quick shear-thinning and rapid recovery upon cessation of shear strain, and the MTT and immunological assays indicated its low cytotoxicity and good biocompatibility. These features are highly attractive for its widespread use in 3D cell culturing and regenerative medical treatments.

关键词： mechanical   system   presented   Uncertainty   Wheels   Mechanical systems   Mechanical   Unpredictable behavior   friction force   dynamics   friction force model   Singularity   Determinism   angular force   INITIAL CONDITIONS  

摘要： A mechanical system is presented exhibiting a nondeterministic singularity, that is, a point in an otherwise deterministic system where forward time trajectories become nonunique. A Coulomb friction force applies linear and angular forces to a wheel mounted on a turntable. In certain configurations, the friction force is not uniquely determined. When the dynamics evolves past the singularity and the mechanism slips, the future state becomes uncertain up to a set of possible values. For certain parameters, the system repeatedly returns to the singularity, giving recurrent yet unpredictable behavior that constitutes nondeterministic chaotic dynamics. The robustness of the phenomenon is such that we expect it to persist with more sophisticated friction models, manifesting as extreme sensitivity to initial conditions, and complex global dynamics attributable to a local loss of determinism in the limit of discontinuous friction.

关键词： Imidazole   Hydrogen bond   Vibrational spectra   Quantum-mechanical calculations   Car-Parrinello molecular dynamics  

摘要： Theoretical simulation of the bandshape and fine structure of the N-H(D) stretching band is presented for imidazole and its deuterated derivative taking into account adiabatic coupling between the high-frequency N-H(D) stretching and the low-frequency N center dot center dot center dot N stretching vibrations, anharmonicity of the potentials for the low-frequency vibrations in the ground and excited state of the N-H(D) stretching mode, Fermi resonance between the N-H(D) stretching and the first overtone of the N-H(D) bending vibrations, and electric anharmonicity. The vibrational potential functions describing N-H and N center dot center dot center dot N stretching modes have been obtained from ab initio calculations. The effect of deuteration has been successfully reproduced by our model calculations. Infrared, far-infrared, Raman and low-frequency Raman spectra of the polycrystalline imidazole have been recorded. The geometry and experimental frequencies are compared with the results of harmonic MP2/6-311++G** and anharmonic B3LYP/6-311++G** calculations. Car-Parrinello molecular dynamics was used to calculate geometry, power and infrared spectra of crystalline imidazole. The results were compared with the results of ab initio MP2/6-311++G** static calculations previously performed for the imidazole dimer. The reconstruction of the v(N-H) bandshape obtained by Car-Parrinello molecular dynamics method was compared with the results of quantum mechanical model of vibrational couplings in hydrogen-bonded dimer and with the experimental data. (C) 2013 Elsevier B.V. All rights reserved.

关键词： SOLITONS   OPTICAL bistability   NONLINEAR waves   METAMATERIALS   ELASTIC wave propagation   ANALYTICAL solutions (Mathematical analysis)   DESIGN & construction  

摘要： We investigate the nonlinear dynamics of a periodic chain of bistable elements consisting of masses connected by elastic springs whose constraint arrangement gives rise to a large-deformation snap-through instability. We show that the resulting negative-stiffness effect produces three different regimes of (linear and nonlinear) wave propagation in the periodic medium, depending on the wave amplitude. At small amplitudes, linear elastic waves experience dispersion that is controllable by the geometry and by the level of precompression. At moderate to large amplitudes, solitary waves arise in the weakly and strongly nonlinear regime. For each case, we present closed-form analytical solutions and we confirm our theoretical findings by specific numerical examples. The precompression reveals a class of wave propagation for a partially positive and negative potential. The presented results highlight opportunities in the design of mechanical metamaterials based on negative-stiffness elements, which go beyond current concepts primarily based on linear elastic wave propagation. Our findings shed light on the rich effective dynamics achievable by nonlinear small-scale instabilities in solids and structures.