On the nonlocal viscosity kernel of mixtures

In this report we investigate the multiscale hydrodynamical response of a liquid as a function of mixture composition. This is done via a series of molecular dynamics simulations where the wave vector dependent viscosity kernel is computed for three mixtures each with 7-15 different compositions. We observe that the nonlocal viscosity kernel is dependent on composition for simple atomic mixtures for all the wave vectors studied here, however, for a model polymer melt mixture the kernel is independent of composition for large wave vectors. The deviation from ideal mixing is also studied. Here it is shown that a Lennard-Jones mixture follows the ideal mixing rule surprisingly well for a large range of wave vectors, whereas for both the Kob-Andersen mixture and the polymer melt large deviations are found. Furthermore, for the polymer melt the deviation is wave vector dependent such that there exists a critical length scale at which the ideal mixing goes from under-estimating to over-estimating the viscosity

How `sticky' are short-range square-well fluids?

The aim of this work is to investigate to what extent the structural properties of a short-range square-well (SW) fluid of range $\lambda$ at a given packing fraction and reduced temperature can be represented by those of a sticky-hard-sphere (SHS) fluid at the same packing fraction and an effective stickiness parameter $\tau$. Such an equivalence cannot hold for the radial distribution function since this function has a delta singularity at contact in the SHS case, while it has a jump discontinuity at $r=\lambda$ in the SW case. Therefore, the equivalence is explored with the cavity function $y(r)$. Optimization of the agreement between y_{\sw} and y_{\shs} to first order in density suggests the choice for $\tau$. We have performed Monte Carlo (MC) simulations of the SW fluid for $\lambda=1.05$, 1.02, and 1.01 at several densities and temperatures $T^*$ such that $\tau=0.13$, 0.2, and 0.5. The resulting cavity functions have been compared with MC data of SHS fluids obtained by Miller and Frenkel [J. Phys: Cond. Matter 16, S4901 (2004)]. Although, at given values of $\eta$ and $\tau$, some local discrepancies between y_{\sw} and y_{\shs} exist (especially for $\lambda=1.05$), the SW data converge smoothly toward the SHS values as $\lambda-1$ decreases. The approximate mapping y_{\sw}\to y_{\shs} is exploited to estimate the internal energy and structure factor of the SW fluid from those of the SHS fluid. Taking for y_{\shs} the solution of the Percus--Yevick equation as well as the rational-function approximation, the radial distribution function $g(r)$ of the SW fluid is theoretically estimated and a good agreement with our MC simulations is found. Finally, a similar study is carried out for short-range SW fluid mixtures.Comment: 14 pages, including 3 tables and 14 figures; v2: typo in Eq. (5.1) corrected, Fig. 14 redone, to be published in JC

An improved 2.5 GHz electron pump: single-electron transport through shallow-etched point contacts driven by surface acoustic waves

We present an experimental study of a 2.5 GHz electron pump based on the quantized acoustoelectric current driven by surface acoustic waves (SAWs) through a shallow-etched point contact in a GaAs/AlGaAs heterostructure. At low temperatures and with an additional counter-propagating SAW beam, up to n = 20 current plateaus at I=nef could be resolved, where n is an integer, e the electron charge, and f the SAW frequency. In the best case the accuracy of the first plateau at 0.40 nA was estimated to be dI/I = +/- 25 ppm over 0.25 mV in gate voltage, which is better than previous results.Comment: 11 pages, 4 figure

Origin of the Universal Roughness Exponent of Brittle Fracture Surfaces: Correlated Percolation in the Damage Zone

We suggest that the observed large-scale universal roughness of brittle fracture surfaces is due to the fracture process being a correlated percolation process in a self-generated quadratic damage gradient. We use the quasi-static two-dimensional fuse model as a paradigm of a fracture model. We measure for this model, that exhibits a correlated percolation process, the correlation length exponent nu approximately equal to 1.35 and conjecture it to be equal to that of uncorrelated percolation, 4/3. We then show that the roughness exponent in the fuse model is zeta = 2 nu/(1+2 nu)= 8/11. This is in accordance with the numerical value zeta=0.75. As for three-dimensional brittle fractures, a mean-field theory gives nu=2, leading to zeta=4/5 in full accordance with the universally observed value zeta =0.80.Comment: 4 pages RevTeX

Dielectric response of a polar fluid trapped in a spherical nanocavity

We present extensive Molecular Dynamics simulation results for the structure, static and dynamical response of a droplet of 1000 soft spheres carrying extended dipoles and confined to spherical cavities of radii $R=2.5$, 3, and 4 nm embedded in a dielectric continuum of permittivity $\epsilon' \geq 1$. The polarisation of the external medium by the charge distribution inside the cavity is accounted for by appropriate image charges. We focus on the influence of the external permittivity $\epsilon'$ on the static and dynamic properties of the confined fluid. The density profile and local orientational order parameter of the dipoles turn out to be remarkably insensitive to $\epsilon'$. Permittivity profiles $\epsilon(r)$ inside the spherical cavity are calculated from a generalised Kirkwood formula. These profiles oscillate in phase with the density profiles and go to a ``bulk'' value $\epsilon_b$ away from the confining surface; $\epsilon_b$ is only weakly dependent on $\epsilon'$, except for $\epsilon' = 1$ (vacuum), and is strongly reduced compared to the permittivity of a uniform (bulk) fluid under comparable thermodynamic conditions. The dynamic relaxation of the total dipole moment of the sample is found to be strongly dependent on $\epsilon'$, and to exhibit oscillatory behaviour when $\epsilon'=1$; the relaxation is an order of magnitude faster than in the bulk. The complex frequency-dependent permittivity $\epsilon(\omega)$ is sensitive to $\epsilon'$ at low frequencies, and the zero frequency limit $\epsilon(\omega=0)$ is systematically lower than the ``bulk'' value $\epsilon_b$ of the static primitivity.Comment: 12 pages including 17 figure

Three-body interactions in complex fluids: virial coefficients from simulation finite-size effects

A simulation technique is described for quantifying the contribution of three-body interactions to the thermodynamical properties of coarse-grained representations of complex fluids. The method is based on comparing the third virial coefficient $B_3$ for a complex fluid with that of an approximate coarse-grained model described by a pair potential. To obtain $B_3$ we introduce a new technique which expresses its value in terms of the measured volume-dependent asymptote of a certain structural function. The strategy is applicable to both Molecular Dynamics and Monte Carlo simulation. Its utility is illustrated via measurements of three-body effects in models of star polymer and highly size-asymmetrical colloid-polymer mixtures.Comment: 13 pages, 8 figure

Extremely correlated Fermi liquid theory meets Dynamical mean-field theory: Analytical insights into the doping-driven Mott transition

We consider a doped Mott insulator in the large dimensionality limit within both the recently developed Extremely Correlated Fermi Liquid (ECFL) theory and the Dynamical Mean-Field Theory (DMFT). We show that the general structure of the ECFL sheds light on the rich frequency-dependence of the DMFT self-energy. Using the leading Fermi-liquid form of the two key auxiliary functions introduced in the ECFL theory, we obtain an analytical ansatz which provides a good quantitative description of the DMFT self-energy down to hole doping level 0.2. In particular, the deviation from Fermi-liquid behavior and the corresponding particle-hole asymmetry developing at a low energy scale are well reproduced by this ansatz. The DMFT being exact at large dimensionality, our study also provides a benchmark of the ECFL in this limit. We find that the main features of the self-energy and spectral line-shape are well reproduced by the ECFL calculations in the O(\lambda^2) `minimal scheme', for not too low doping level >0.3. The DMFT calculations reported here are performed using a state-of-the-art numerical renormalization-group impurity solver, which yields accurate results down to an unprecedentedly small doping level 0.001.Comment: 21 pages, 18 figure

Fourth virial coefficients of asymmetric nonadditive hard-disc mixtures

The fourth virial coefficient of asymmetric nonadditive binary mixtures of hard disks is computed with a standard Monte Carlo method. Wide ranges of size ratio ($0.05\leq q\leq 0.95$) and nonadditivity ($-0.5\leq \Delta\leq 0.5$) are covered. A comparison is made between the numerical results and those that follow from some theoretical developments. The possible use of these data in the derivation of new equations of state for these mixtures is illustrated by considering a rescaled virial expansion truncated to fourth order. The numerical results obtained using this equation of state are compared with Monte Carlo simulation data in the case of a size ratio $q=0.7$ and two nonadditivities $\Delta=\pm 0.2$.Comment: 9 pages, 7 figures; v2: section on equation of state added; tables moved to supplementary material (http://jcp.aip.org/resource/1/jcpsa6/v136/i18/p184505_s1#artObjSF

Time scale for the onset of Fickian diffusion in supercooled liquids

We propose a quantitative measure of a time scale on which Fickian diffusion sets in for supercooled liquids and use Brownian Dynamics computer simulations to determine the temperature dependence of this onset time in a Lennard-Jones binary mixture. The time for the onset of Fickian diffusion ranges between 6.5 and 31 times the $\alpha$ relaxation time (the $\alpha$ relaxation time is the characteristic relaxation time of the incoherent intermediate scattering function). The onset time increases faster with decreasing temperature than the $\alpha$ relaxation time. Mean squared displacement at the onset time increases with decreasing temperature