21 research outputs found
Precursor-mediated crystallization process in suspensions of hard spheres
We report on a large scale computer simulation study of crystal nucleation in
hard spheres. Through a combined analysis of real and reciprocal space data, a
picture of a two-step crystallization process is supported: First dense,
amorphous clusters form which then act as precursors for the nucleation of
well-ordered crystallites. This kind of crystallization process has been
previously observed in systems that interact via potentials that have an
attractive as well as a repulsive part, most prominently in protein solutions.
In this context the effect has been attributed to the presence of metastable
fluid-fluid demixing. Our simulations, however, show that a purely repulsive
system (that has no metastable fluid-fluid coexistence) crystallizes via the
same mechanism.Comment: 4 figure
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
Coincidence of the freezing and the onset of caging in hard sphere and Lennard-Jones fluids
In this article, we examine the collective particle dynamics, as expressed by the time correlation function of the longitudinal particle current density, of several different fluids in the vicinity of their freezing points/lines. We consider and compare results obtained by dynamic light scattering for a suspension of hard spheres and by molecular dynamics for fluids with hard sphere and Lennard-Jones interactions. The latter are performed along both an isotherm and an isochore. In all cases, we find a qualitative change in the collective dynamics, within the resolution of the data, when their respective freezing lines are crossed. We associate this change with the onset of caging. The new results for the Lennard-Jones fluid reported here confirm that the occurrence of caging, found previously for systems of hard spheres, is a more general feature that distinguishes a metastable fluid from one in thermodynamic equilibrium