arXiv subjects

I. M. de Schepper

Publications and source records attributed to I. M. de Schepper.

3 recordsLinked to original sources

arXiv2003

Cage diffusion in liquid mercury

We present inelastic neutron scattering measurements on liquid mercury at room temperature for wave numbers $q$ in the range 0.3 $< q <$ 7.0 Å$^{-1}$. We find that the energy halfwidth of the incoherent part of the dynamic structure factor $S(q,E)$ is determinded by a self-diffusion process. The halfwidth of the coherent part of $S(q, E)$ shows the characteristic behavior expected for a cage diffusion process. We also show that the response function at small wave numbers exhibits a quasi-elastic mode with a time scale characteristic of cage diffusion, however, its intensity is larger by an order of magnitude than what would be expected for cage diffusion. We speculate on a scenario in which the intensity of the cage diffusion mode at small wave numbers is amplified through a valence fluctuation mechanism.

cond-mat.dis-nn↗

arXiv1997

Square root singularity in the viscosity of neutral colloidal suspensions at large frequencies

The asymptotic frequency $ω$, dependence of the dynamic viscosity of neutral hard sphere colloidal suspensions is shown to be of the form $η_0 A(ϕ) (ωτ_P)^{-1/2}$, where $A(ϕ)$ has been determined as a function of the volume fraction $ϕ$, for all concentrations in the fluid range, $η_0$ is the solvent viscosity and $τ_P$ the Péclet time. For a soft potential it is shown that, to leading order steepness, the asymptotic behavior is the same as that for the hard sphere potential and a condition for the cross-over behavior to $1/ωτ_P$ is given. Our result for the hard sphere potential generalizes a result of Cichocki and Felderhof obtained at low concentrations and agrees well with the experiments of van der Werff et al, if the usual Stokes-Einstein diffusion coefficient $D_0$ in the Smoluchowski operator is consistently replaced by the short-time self diffusion coefficient $D_s(ϕ)$ for non-dilute colloidal suspensions.

cond-mat.soft↗

arXiv1996

Viscosity of Colloidal Suspensions

Simple expressions are given for the Newtonian viscosity $η_N(ϕ)$ as well as the viscoelastic behavior of the viscosity $η(ϕ,ω)$ of neutral monodisperse hard sphere colloidal suspensions as a function of volume fraction $ϕ$ and frequency $ω$ over the entire fluid range, i.e., for volume fractions $0 < ϕ< 0.55$. These expressions are based on an approximate theory which considers the viscosity as composed as the sum of two relevant physical processes: $η(ϕ,ω) = η_{\infty}(ϕ) + η_{cd}(ϕ,ω)$, where $η_{\infty}(ϕ) = η_0 χ(ϕ)$ is the infinite frequency (or very short time) viscosity, with $η_0$ the solvent viscosity, $χ(ϕ)$ the equilibrium hard sphere radial distribution function at contact, and $η_{cd}(ϕ,ω)$ the contribution due to the diffusion of the colloidal particles out of cages formed by their neighbors, on the Péclet time scale $τ_P$, the dominant physical process in concentrated colloidal suspensions. The Newtonian viscosity $η_N(ϕ) = η(ϕ,ω= 0)$ agrees very well with the extensive experiments of Van der Werff et al and others. Also, the asymptotic behavior for large $ω$ is of the form $η_{\infty}(ϕ) + A(ϕ)(ωτ_P)^{-1/2}$, in agreement with these experiments, but the theoretical coefficient $A(ϕ)$ differs by a constant factor $2/χ(ϕ)$ from the exact coefficient, computed from the Green-Kubo formula for $η(ϕ,ω)$. This still enables us to predict for practical purposes the visco-elastic behavior of monodisperse spherical colloidal suspensions for all volume fractions by a simple time rescaling.

chao-dyn↗