708 research outputs found
Structure of polydisperse inverse ferrofluids: Theory and computer simulation
By using theoretical analysis and molecular dynamics simulations, we
investigate the structure of colloidal crystals formed by nonmagnetic
microparticles (or magnetic holes) suspended in ferrofluids (called inverse
ferrofluids), by taking into account the effect of polydispersity in size of
the nonmagnetic microparticles. Such polydispersity often exists in real
situations. We obtain an analytical expression for the interaction energy of
monodisperse, bidisperse, and polydisperse inverse ferrofluids. Body-centered
tetragonal (bct) lattices are shown to possess the lowest energy when compared
with other sorts of lattices and thus serve as the ground state of the systems.
Also, the effect of microparticle size distributions (namely, polydispersity in
size) plays an important role in the formation of various kinds of structural
configurations. Thus, it seems possible to fabricate colloidal crystals by
choosing appropriate polydispersity in size.Comment: 22 pages, 8 figure
Optimal packing of polydisperse hard-sphere fluids II
We consider the consequences of keeping the total surface fixed for a
polydisperse system of hard spheres. In contrast with a similar model (J.
Zhang {\it et al.}, J. Chem. Phys. {\bf 110}, 5318 (1999)), the Percus-Yevick
and Mansoori equations of state work very well and do not show a breakdown. For
high pressures Monte Carlo simulation we show three mechanically stable
polydisperse crystals with either a unimodal or bimodal particle-size
distributions.Comment: 17 pages, 8 figures, revtex (accepted by J. Chem. Phys.
Recommended from our members
Rheology of magmas with bimodal crystal size and shape distributions: insights from analog experiments
Magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts, as often indicated by volcanic rock textures. In this study, we present two different experiments that inves- tigate the flow behavior of these bidisperse systems. In the first experiments, rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller, prolate particles (microlites) and larger, equant particles (phenocrysts) in a bubbleâfree Newtonian liquid (silicate melt). Our data show that increasing the relative proportion of prolate microlites to equant pheno- crysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude. Consequently, the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles. We propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models. In a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup. The resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in wellâorganized strings, with sphereâdepleted shear bands in highâshear regions. These observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio. The implications for magma flow are discussed, and rheological results and tex- tural observations are compared with observations on natural samples
Jamming at Zero Temperature and Zero Applied Stress: the Epitome of Disorder
We have studied how 2- and 3- dimensional systems made up of particles
interacting with finite range, repulsive potentials jam (i.e., develop a yield
stress in a disordered state) at zero temperature and applied stress. For each
configuration, there is a unique jamming threshold, , at which
particles can no longer avoid each other and the bulk and shear moduli
simultaneously become non-zero. The distribution of values becomes
narrower as the system size increases, so that essentially all configurations
jam at the same in the thermodynamic limit. This packing fraction
corresponds to the previously measured value for random close-packing. In fact,
our results provide a well-defined meaning for "random close-packing" in terms
of the fraction of all phase space with inherent structures that jam. The
jamming threshold, Point J, occurring at zero temperature and applied stress
and at the random close-packing density, has properties reminiscent of an
ordinary critical point. As Point J is approached from higher packing
fractions, power-law scaling is found for many quantities. Moreover, near Point
J, certain quantities no longer self-average, suggesting the existence of a
length scale that diverges at J. However, Point J also differs from an ordinary
critical point: the scaling exponents do not depend on dimension but do depend
on the interparticle potential. Finally, as Point J is approached from high
packing fractions, the density of vibrational states develops a large excess of
low-frequency modes. All of these results suggest that Point J may control
behavior in its vicinity-perhaps even at the glass transition.Comment: 21 pages, 20 figure
Geometrical properties of rigid frictionless granular packings as a function of particle size and shape
Three-dimensional discrete numerical simulation is used to investigate the
properties of close-packed frictionless granular assemblies as a function of
particle polydispersity and shape. Unlike some experimental results,
simulations show that disordered packings of pinacoids (eight-face convex
polyhedron) achieve higher solid fraction values than amorphous packings of
spherical or rounded particles, thus fulfilling the analogue of Ulam's
conjecture stated by Jiao and co-workers for random packings [Y. Jiao and S.
Torquato, Phys. Rev. E , ()]. This seeming
discrepancy between experimental and numerical results is believed to lie with
difficulties in overcoming interparticle friction through experimental
densification processes. Moreover, solid fraction is shown to increase further
with bidispersity and peak when the volume proportion of small particles
reaches . Contrarywise, substituting up to of flat pinacoids for
isometric ones yields solid fraction decrease, especially when flat particles
are also elongated. Nevertheless, particle shape seems to play a minor role on
packing solid fraction compared to polydispersity. Additional investigations
focused on the packing microstructure confirm that pinacoid packings fulfill
the isostatic conjecture and that they are free of order except beyond to
of flat or flat \& elongated polyhedra in the packing. This order
increase progressively takes the form of a nematic phase caused by the
reorientation of flat or flat \& elongated particles to minimize the packing
potential energy. Simultaneously, this reorientation seems to increase the
solid fraction value slightly above the maximum achieved by monodisperse
isometric pinacoids, as well as the coordination number. Finally, partial
substitution of elongated pinacoids for isometric ones has limited effect on
packing solid fraction or order.Comment: 12 figures, 12 page
Predicting phase equilibria in polydisperse systems
Many materials containing colloids or polymers are polydisperse: They
comprise particles with properties (such as particle diameter, charge, or
polymer chain length) that depend continuously on one or several parameters.
This review focusses on the theoretical prediction of phase equilibria in
polydisperse systems; the presence of an effectively infinite number of
distinguishable particle species makes this a highly nontrivial task. I first
describe qualitatively some of the novel features of polydisperse phase
behaviour, and outline a theoretical framework within which they can be
explored. Current techniques for predicting polydisperse phase equilibria are
then reviewed. I also discuss applications to some simple model systems
including homopolymers and random copolymers, spherical colloids and
colloid-polymer mixtures, and liquid crystals formed from rod- and plate-like
colloidal particles; the results surveyed give an idea of the rich
phenomenology of polydisperse phase behaviour. Extensions to the study of
polydispersity effects on interfacial behaviour and phase separation kinetics
are outlined briefly.Comment: 48 pages, invited topical review for Journal of Physics: Condensed
Matter; uses Institute of Physics style file iopart.cls (included
Self-assembly of magnetic iron oxide nanoparticles into cuboidal superstructures
This chapter describes the synthesis and some characteristics of magnetic
iron oxide nanoparticles, mainly nanocubes, and focus on their self-assembly
into crystalline cuboids in dispersion. The influence of external magnetic
fields, the concentration of particles, and the temperature on the assembly
process is experimentally investigated
- âŠ