Brownian dynamics of polydisperse colloidal hard spheres: Equilibrium structures and random close packings

Oil & Gas Science and Technology - Rev. IFP

Aarts

2008

Résumé -Mesures directes des propriétés thermodynamiques de sphères dures colloïdales -Récemment, nous avons montré comment mesurer les propriétés de suspensions de sphères dures colloïdales par microscopie [Dullens et al. (2006) PNAS 103, 529]. Ici, nous présentons les détails expérimentaux complets sur la manière de capturer des images en trois dimensions d'une suspension de sphères dures colloïdales, couvrant une large gamme de densités, en utilisant la microscopie à laser confocal. De plus, nous approfondissons l'analyse des données et mesurons le volume disponible pour insérer une sphère additionnelle et l'aire de surface de ce volume. Ces propriétés géométriques sont liées aux grandeurs thermodynamiques clés par la géométrie statistique. Ceci nous permet de mesurer, d'une façon directe et non-intrusive, la pression, le potentiel chimique et la densité d'énergie libre d'une suspension de sphères dures ; ces mesures sont en bon accord avec les prédictions théoriques. Abstract -Direct Measurement of Thermodynamic Properties of Colloidal Hard SpheresRecently, we have shown how to measure thermodynamic properties of colloidal hard sphere suspensions by microscopy

Section: Methodsmentioning

confidence: 99%

Direct Measurement of Thermodynamic Properties of Colloidal Hard Spheres

Dullens

Oil & Gas Science and Technology - Rev. IFP

Aarts

2008

“…In the resulting colloidal glass, large-scale particle diffusion and crystallization by homogeneous nucleation stop, implying that crystal nucleation proceeds by large-scale particle diffusion [6,7]. The glass state is reached at a surprisingly small number density of the hard spheres: 0:57-0:58, a significantly smaller density than where (monodisperse) hard spheres are jammed at random close packing with 0:64 [8]. It is widely believed that the glass transition of hard spheres corresponds to permanent caging of the particles by their neighbors [7,9].…”

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confidence: 99%

“…Therefore, the only difference between the low gravity and the normal gravity system is the gravitational length. The volume fractions of the samples were determined relative to random close packing of 7% polydisperse hard spheres [8].…”

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confidence: 99%

Gravity-Induced Aging in Glasses of Colloidal Hard Spheres

Simeonova

2004

Phys. Rev. Lett.

The influence of gravity on the long-time behavior of the mean squared displacement in glasses of polydisperse colloidal hard spheres was studied by means of real-space fluorescent recovery after photobleaching. We present, for the first time, a significant influence of gravity on the mean squared displacements of the particles. In particular, we observe that systems which are glasses under gravity (with a gravitational length on the order of tens of micrometers) show anomalous diffusion over several decades in time if the gravitational length is increased by an order of magnitude. No influence of gravity was observed in systems below the glass transition density. We show that this behavior is caused by gravity dramatically accelerating aging in colloidal hard sphere glasses. This behavior explains the observation that colloidal hard sphere systems which are a glass on Earth rapidly crystallize in space. DOI: 10.1103/PhysRevLett.93.035701 PACS numbers: 64.70.Pf, 05.40.Jc, 82.70.Dd Hard spheres for which the potential energy is infinite on overlap and (in the absence of fields) zero otherwise freeze upon increasing number density from a fluid into a crystal at a volume fraction, , of 0.494 [1,2]. In between 0:494 and 0.545, fluid and crystal coexist [3], while, at even higher volume fractions, a single phase (fcc [4]) crystal is thermodynamically stable up to the close packed density corresponding to =3 2 p 0:74. Colloids may behave as hard spheres [6]. Besides the thermodynamic freezing transition referred to above, colloidal hard spheres can be brought in a (long-lasting) metastable fluid state by rapid quenching using centrifugation [6]. In the resulting colloidal glass, large-scale particle diffusion and crystallization by homogeneous nucleation stop, implying that crystal nucleation proceeds by large-scale particle diffusion [6,7]. The glass state is reached at a surprisingly small number density of the hard spheres: 0:57-0:58, a significantly smaller density than where (monodisperse) hard spheres are jammed at random close packing with 0:64 [8]. It is widely believed that the glass transition of hard spheres corresponds to permanent caging of the particles by their neighbors [7,9]. However, recent computer simulations [10] and experiments under microgravity [11] pose serious doubts on the very existence of a glass transition at < 0:64 of monodisperse hard spheres in the absence of a gravity field. These studies show that monodisperse [10] but also (about 5%) polydisperse [11] hard spheres crystallize in the absence of gravity up to volume fractions close to 0.64. In the last study, a system that is a glass on Earth rapidly crystallizes in space after homogenization by mixing.While microscopic particle movements do not seem to be affected by details of the particle size distribution [12], as far as we are aware, no quantitative studies of the influence of gravity on dynamics in concentrated colloidal suspensions have been reported. In this work we address the question as to what the influence of gravity ...

“…This fraction was set at 0.66 [10]. Confocal scanning laser microscopy was used to image the particles present in the first layer, parallel to the flat glass bottom of the container [11].…”

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confidence: 99%

Reentrant Surface Melting of Colloidal Hard Spheres

Dullens

2004

Phys. Rev. Lett.

Concentrated suspensions of model colloidal hard spheres at a wall were studied in real space by means of time-resolved fluorescence confocal scanning microscopy. Both structure and dynamics of these systems differ dramatically from their bulk analogs (i.e., far away from a wall). In particular, systems that are a glass in the bulk show significant hexagonal order at a wall. Upon increasing the volume fraction of the colloids, a reentrant melting transition involving a hexatic structure is observed. The last observation points to two-dimensional behavior of matter at walls. DOI: 10.1103/PhysRevLett.92.195702 PACS numbers: 64.60.Cn, 64.70.-p, 82.70.Dd Hard spheres under thermal excitation may be considered as the ''fruit flies'' of statistical mechanics, where the hard sphere potential is extensively used as a reference potential [1]. Therefore, rigorous insight into the behavior of hard spheres is imperative for understanding the wide class of systems where excluded volume interactions are important. In nature, all systems are bounded, either by walls or surfaces. These will tend to dominate the behavior as systems become smaller [2 -4]. Motivated by the increasing interest in nanometer-sized systems in science and technology, we address the question as to what surfaces do to the behavior of hard spheres. We expect our results to be relevant for the behavior of many other systems at walls or boundaries in general.Although the role of polydispersity is still being debated, most of the bulk behavior of hard spheres has been well established [5]. In contrast, much less is known about the influence of a single wall on the local structure and dynamics of hard spheres. In this Letter, we use colloidal model particles to address this issue. Colloidal suspensions are extensively used as experimental model hard spheres [5][6][7]. The colloidal model particles used here consist of a core of silica, 450 nm in diameter, labeled with the fluorescent dye fluorescein isothiocyanate [8]. The cores are covered with a thick shell of nonfluorescent polymethylmethacrylate (PMMA). The system is sterically stabilized by a layer of 12-poly-hydroxystearic acid that is covalently linked to the PMMA [9]. The spheres were dispersed in a mixture of tetralin, cis-decalin, and carbon tetrachloride, which (nearly) matches the refractive index and the mass density of the colloids. In this mixture the particles behave as hard spheres [6]. The particle diameter d is 1:4 m and the size polydispersity is 6%. Crystallization was absent in bulk [6].The volume fraction of the samples was defined relative to the volume fraction at random close packing of 6% polydisperse hard spheres. This fraction was set at 0.66 [10]. Confocal scanning laser microscopy was used to image the particles present in the first layer, parallel to the flat glass bottom of the container [11]. No signs of attraction between the particles and the glass wall were found. As the colloidal particles have a core-shell character, time-dependent particle coordinates could be obtai...