I.M. de Schepper

Very-short-wavelength collective modes in fluids

The existence of very-short-wavelength collective modes in fluids is discussed. These collective modes are the extensions of the five hydrodynamic (heat, sound, viscous) modes to wavelengths of the order of the mean free path in a gas or to a fraction of the molecular size in a liquid. They are computed here explicitly on the basis of a model kinetic equation for a hard sphere fluid. At low densities all five modes are increasingly damped with decreasing wavelength till each ceases to exist at a cutoff wavelength. At high densities the extended heat mode softens very appreciably for wavelengths of the order of the size of the particles and becomes a diffusion-like mode that persists till much shorter wavelengths than the other modes. Except for the shortest wavelengths these collective modes and in particular the heat mode dominate the dynamical structure factorS(k, ω) for all densities. The agreement of the theory with experimentalS(k, ω) of liquid Ar seems to imply that very-short-wavelength collective modes also occur in real fluids.

Analogies between the dynamics of concentrated charged colloidal suspensions and dense atomic liquids

Abstract We summarize and discuss the short time analogies between concentrated charged colloidal suspensions and dense atomic liquids known in the literature and introduce a new analogy between both systems which is valid for long times. This new analogy agrees with experiment.

Viscosity and diffusion in hard-sphere-like colloidal suspensions

A large number of experimental data on the viscous and the diffusive behavior at short and long times in three classes of disordered hard-sphere-like colloidal suspensions with varying polydispersity is compared with theory. At low and intermediate concentrations (volume fraction φ 0.35) the dominant contributions, however, come from direct interactions in cage diffusion, whose rapidly increasing difficulty with increasing concentration, leads to a singularity-dominated increase (decrease) of the viscosity (self- and collective diffusion coefficient).

The vibrational spectrum of solid ferrocene by inelastic neutron scattering

We calculate the spectrum of internal vibrations of a single ferrocene Fe(C5H5)2 molecule using ab initio density functional theory (without free parameters) and compare this with inelastic neutron scattering data on ferrocene in the solid state at 28 K. Due to the good agreement, we can assign each vibrational mode to each observed peak in the neutron spectrum and so remove ambiguities existing in the literature. There is also consistency between the calculated potential energy of a single ferrocene molecule for different orientations, φ, of the two cyclopentadienyl C5H5 rings with respect to each other, which shows a potential barrier of 0.9 kcal/mol, and electron diffraction, and between the calculated shallow minimum at φ=9 deg and x-ray diffraction.

Note on transport processes in dense colloidal suspensions

A new approach to transport processes in dense charged as well as neutral colloidal suspensions is presented. It is based on a far-reaching analogy between dense colloidal suspensions and dense hard-sphere fluids, implying, in turn, an analogy with atomic liquids. As a result, new expressions valid for a number of colloidal transport coefficients are predicted.

Non-analytic dispersion relations in liquid argon

Abstract Neutron scattering experiments on liquid argon reveal an anomalous approach to the hydrodynamic limit for the frequency of sound and for the ratio of the sound and heat mode damping factors. In both cases the behavior agrees with non-analytic dispersion relations predicted by the mode coupling theory.

The width of neutron spectra and the heat mode of fluids

The width of neutron spectra is studied on the basis of kinetic theory as a function of wavenumber and density and appears to be determined by the heat mode of the fluid. For high-density fluids an interference of static and dynamic effects in this mode is observed.

SELF-DIFFUSION BEYOND FICK'S LAW

The Van Hove self-correlation function, the intermediate incoherent scattering function and its Laplace transform are determined asymptotically for a one component fluid in equilibrium, using the mode coupling theory. The results reproduce in the hydrodynamic limit the predictions from Ficks law. The corrections to Ficks law are consistent with a long time tail in the velocity correlation function and with a diverging super Burnett coefficient in the linear diffusion equation.

Long time diffusion in suspensions of interacting charged colloids

A new expression is given for the long time diffusion coefficient DL(k) of charged interacting colloidal spheres in suspension, as a function of the wavenumber k, near k = km, where the static structure factor has a maximum. The expression is based on a physical analogy between a mode description of the behaviour of atomic fluids (as observed in neutron scattering) and of colloids (as observed in light scattering). Use of this expresssion in conjunction with a hard-sphere model yields good agreement with extant data on colloids.

The incoherent scattering function and related correlation functions in hard sphere fluids at short times

For a classical fluid of hard spheres and hard disks exact expressions for all densities and wave vectors are derived for the coefficients oftnin the short-time expansion of the incoherent intermediate scattering function (n = 0, 1,..., 4) and the velocity correlation function (n=0,1,2). Similarly, we obtain the coefficient of the leading term in the short-time behavior of the cumulants of the displacements. Furthermore,S(k, ω) has a high-frequency tail ∼ω−4, characteristic for the hard-sphere fluid, which leads to a modification of the standard sum rules. We present estimates for the frequency range, in which this tail may be observed in neutron scattering off noble gases. The results are also compared with Enskogs theory and molecular dynamics calculations.