Dynamic simulations of multicomponent lipid membranes over long length and time scales

We present a stochastic phase-field model for multicomponent lipid bilayers that explicitly accounts for the quasi-two-dimensional hydrodynamic environment unique to a thin fluid membrane immersed in aqueous solution. Dynamics over a wide range of length scales (from nanometers to microns) for durations up to seconds and longer are readily accessed and provide a direct comparison to fluorescence microscopy measurements in ternary lipid-cholesterol mixtures. Simulations of phase separation kinetics agree with experiment and elucidate the importance of hydrodynamics in the coarsening process.     

Mechanical heterogeneities in model polymer glasses at small length scales

Molecular simulations of a model, deeply quenched polymeric glass show that the elastic moduli become strongly inhomogeneous at length scales comprising several tens of monomers; these calculations reveal a broad distribution of local moduli, with regions of negative moduli coexisting within a matrix of positive moduli. It is shown that local moduli have the same physical meaning as that traditionally ascribed to moduli obtained from direct measurements of local constitutive behaviors of macroscopic samples.     

Huge (but finite) time scales in slow relaxations: beyond simple aging

Experiments performed in the last years demonstrated slow relaxations and aging in the conductance of a large variety of materials. Here, we present experimental and theoretical results for conductance relaxation and aging for the case-study example of porous silicon. The relaxations are experimentally observed even at room temperature over time scales of hours, and when a strong electric field is applied for a time tw, the ensuing relaxation depends on tw. We derive a theoretical curve and show that all experimental data collapse onto it with a single time scale as a fitting parameter. This time scale is found to be of the order of thousands of seconds at room temperature. The generic theory suggested is not fine-tuned to porous silicon, and thus we believe the results should be universal, and the presented method should be applicable for many other systems manifesting memory and other glassy effects.     

Probing the electron delocalization in liquid water and ice at attosecond time scales

We determine electron delocalization rates in liquid water and ice using core-hole decay spectroscopy. The hydrogen-bonded network delocalizes the electrons in less than 500 as. Broken or weak hydrogen bonds--in the liquid or at the surface of ice--provide states where the electron remains localized longer than 20 fs. These asymmetrically bonded water species provide electron traps, acting as a strong precursor channel to the hydrated electron.     

Relaxation dynamics at different time scales in electrostatic complexes: time-salt superposition

In this Letter we show that in the rheology of electrostatically assembled soft materials, salt concentration plays a similar role as temperature for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheological data of a set of model electrostatic complexes at different salt concentrations to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.     

三个不同时间尺度的电耦合模型的组合簇放电

胰岛中间隙连接的胰腺β细胞的簇放电行为对胰岛素分泌起着重要的作用. 本文利用了最小的phantom 簇放电模型, 研究两个电耦合胰腺β细胞具有簇同步的组合簇放电, 其膜电位表现出一个长簇和几个短簇组成的放电簇集和振幅先减小后增大的小振幅阈下振荡的相互转迁. 在两个慢变量和快的膜电位的三维空间中, 分别考虑了中慢变量和慢慢变量作为分岔参数的多层次的快慢动力学分析, 研究这两个时间尺度不同的慢变量如何共同或单独地控制着这种组合簇放电的复杂动力学行为. 特别地, 探讨了耦合强度引起的组合簇放电的每个簇集中短簇个数变化的内在机理. 关键词： 电耦合 具有不同时间尺度的慢变量 组合簇放电 快慢动力学分析     

Novel heat conduction model for bridging different space and time scales

This exposition describes a novel heat transport model and an underlying unified theory emanating from the physics of the Boltzmann transport equation which acknowledges simultaneously the coexistence of that termed as slow processes (at low energies) and fast processes (at high energies) as heat carriers while describing the evolution of heat transport characteristics spanning both spatial scales (characterizing ballistic to diffusive limits), and also time scales (characterizing finite to infinite heat propagation speeds).     

Brownian dynamics in Fourier space: membrane simulations over long length and time scales

A simulation algorithm for elastic membrane sheets is presented. Overdamped stochastic dynamics including hydrodynamic coupling to surrounding solvent and arbitrary external forces are generated by employing Fourier modes of the sheet as the primary dynamic variables. Simulations over the micron length scale and second time scale are easily achieved. The dynamics of a lipid bilayer attached to an underlying network of cytoskeletal filaments is used to estimate the diffusion constant of membrane-bound proteins on the surface of the red blood cell.     

Interatomic potential effects on dynamical heterogeneities in liquid SiO2

Dynamical heterogeneities (DH) in low density liquid SiO₂ have been investigated by molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 particles with the pair interatomic potentials, which have a weak electrostatic interaction and a Morse type short range interaction (PMSI). We have evaluated the non-Gaussian parameter for the self part of the van Hove correlation function and we found a clear evidence of the existence of DH in low density liquid SiO₂. Moreover, the atomic displacement distribution (ADD) in a model has been obtained and it deviates from a Gaussian form. The results have been compared with those obtained in another liquid SiO₂ system with the Born-Mayer interatomic potentials (BMP) in order to observe the interatomic potential effects on the DH in the system and indeed, the effects are strong. Calculations showed that particles of extremely low or fast mobility have a tendency to form a cluster and mean cluster size of most mobile and immobile particles in PMSI models increases with decreasing temperature. In contrast, no systematic changes have been obtained for the most mobile and immobile particles in BMP models. Calculations show that there is no relation between local particle environment and particle mobility in the system.     

Concentrated suspensions of Brownian beads in water: dynamic heterogeneities through a simple experimental technique

Concentrated suspensions of Brownian hard-spheres in water are an epitome for understanding the glassy dynamics of both soft materials and supercooled molecular liquids. From an experimental point of view, such systems are especially suited to perform particle tracking easily, and, therefore, are a benchmark for novel optical techniques, applicable when primary particles cannot be resolved. Differential variance analysis(DVA) is one such novel technique that simplifies significantly the characterization of structural relaxation processes of soft glassy materials, since it is directly applicable to digital image sequences of the sample. DVA succeeds in monitoring not only the average dynamics, but also its spatio-temporal fluctuations, known as dynamic heterogeneities. In this work, we study the dynamics of dense suspensions of Brownian beads in water, imaged through digital video-microscopy, by using both DVA and single-particle tracking. We focus on two commonly used signatures of dynamic heterogeneities: the dynamic susceptibility, χ_4, and the non-Gaussian parameter, α_2. By direct comparison of these two quantities,we are able to highlight similarities and differences. We do confirm that χ_4 and α_2 provide qualitatively similar information, but we find quantitative discrepancies in the scalings of characteristic time and length scale on approaching the glass transition.     

Structural and compositional heterogeneities in liquid aluminosilicate: insight from a grain structure model

Network structure as well as structural and compositional heterogeneities in aluminosilicate (Al₂O₃-2SiO₂) under compression is investigated by analysis and visualization of simulation data. Structural and compositional heterogeneities are clarified through analysis of topology structure and size distribution of TO _x -clusters (T = Si, Al; x = 3, 4, 5, 6) as well as OT _y -clusters (y = 2, 3, 4). The TO _x -cluster can be considered as TO _x -grains. It appears that the structure of aluminosilicate is the mixture of TO _x -grains with a different short-range order structure and this is the origin of structural heterogeneity. Regarding their composition, the OSi _y - and OAl _y -clusters can be considered as silica- and alumina-grains respectively, and the structure of aluminosilicate can thus be considered to be formed from silica- and alumina-grains. This results in compositional heterogeneity. Moreover, the degree of polymerization and polyamorphism as well as dynamic heterogeneity is also discussed in detail.     

Predicting the long-time dynamic heterogeneity in a supercooled liquid on the basis of short-time heterogeneities

We report that the local Debye-Waller factor in a simulated 2D glass-forming mixture exhibits significant spatial heterogeneities and that these short-time fluctuations provide an excellent predictor of the spatial distribution of the long-time dynamic propensities. In contrast, the potential energy per particle of the inherent structure does not correlate well with the spatially distributed dynamics.     

Relaxation and lyapunov time scales in a one-dimensional gravitating sheet system

The relation between relaxation, the time scale of Lyapunov instabilities, and the Kolmogorov-Sinai time in a one-dimensional gravitating sheet system is studied. Both the maximum Lyapunov exponent and the Kolmogorov-Sinai entropy decrease as proportional to N(-1/5). The time scales determined by these quantities evidently differ from any type of relaxation time found in the previous investigations. The relaxation time to quasiequilibria (microscopic relaxation) is found to have the same N dependence as the inverse of the minimum positive Lyapunov exponent. The relaxation time to the final thermal equilibrium differs from the inverse of the Lyapunov exponents and the Kolmogorov-Sinai time.     

Bursting oscillations in a hydro-turbine governing system with two time scales

A fast-slow coupled model of the hydro-turbine governing system(HTGS)is established by introducing frequency disturbance in this paper.Based on the proposed model,the performances of two time scales for bursting oscillations in the HTGS are investigated and the effect of periodic excitation of frequency disturbance is analyzed by using the bifurcation diagrams,time waveforms and phase portraits.We find that stability and operational characteristics of the HTGS change with the value of system parameter k_d.Furthermore,the comparative analyses for the effect of the bursting oscillations on the system with different amplitudes of the periodic excitation a are carried out.Meanwhile,we obtain that the relative deviation of the mechanical torque mt rises with the increase of a.These methods and results of the study,combined with the performance of two time scales and the fast-slow coupled engineering model,provide some theoretical bases for investigating interesting physical phenomena of the engineering system.     

The -relaxation dynamics of a simple liquid

We present a detailed analysis of the -relaxation dynamics of a simple glass former, a binary Lennard-Jones system with a stochastic dynamics. By testing the various predictions of mode-coupling theory, including the recently proposed corrections to the asymptotic scaling laws, we come to the conclusion that in this time regime the dynamics is described very well by this theory. Received 5 February 1999 and Received in final form 7 June 1999