35 research outputs found
Depletion potentials near geometrically structured substrates
Using the recently developed so-called White Bear version of Rosenfeld's
Fundamental Measure Theory we calculate the depletion potentials between a
hard-sphere colloidal particle in a solvent of small hard spheres and simple
models of geometrically structured substrates: a right-angled wedge or edge. In
the wedge geometry, there is a strong attraction beyond the corresponding one
near a planar wall that significantly influences the structure of colloidal
suspensions in wedges. In accordance with an experimental study, for the edge
geometry we find a free energy barrier of the order of several which
repels a big colloidal particle from the edge.Comment: 7 pages, 7 figure
Discrete solvent effects on the effective interaction between charged colloids
Using computer simulations of two charged colloidal spheres with their
counterions in a hard sphere solvent, we show that the granular nature of the
solvent significantly influences the effective colloidal interaction. For
divalent counterions, the total effective force can become attractive generated
by counterion hydration, while for monovalent counterions the forces are
repulsive and well-described by a solvent-induced colloidal charge
renormalization. Both effects are not contained in the traditional "primitive"
approaches but can be accounted for in a solvent-averaged primitive model.Comment: 4 pages, 3 figure
Depletion forces near curved surfaces
Based on density functional theory the influence of curvature on the
depletion potential of a single big hard sphere immersed in a fluid of small
hard spheres with packing fraction \eta_s either inside or outside of a hard
spherical cavity of radius R_c is calculated. The relevant features of this
potential are analyzed as function of \eta_s and R_c. There is a very slow
convergence towards the flat wall limit R_c \to \infty. Our results allow us to
discuss the strength of depletion forces acting near membranes both in normal
and lateral directions and to make contact with recent experimental results
Theory of asymmetric non-additive binary hard-sphere mixtures
We show that the formal procedure of integrating out the degrees of freedom
of the small spheres in a binary hard-sphere mixture works equally well for
non-additive as it does for additive mixtures. For highly asymmetric mixtures
(small size ratios) the resulting effective Hamiltonian of the one-component
fluid of big spheres, which consists of an infinite number of many-body
interactions, should be accurately approximated by truncating after the term
describing the effective pair interaction. Using a density functional treatment
developed originally for additive hard-sphere mixtures we determine the zero,
one, and two-body contribution to the effective Hamiltonian. We demonstrate
that even small degrees of positive or negative non-additivity have significant
effect on the shape of the depletion potential. The second virial coefficient
, corresponding to the effective pair interaction between two big spheres,
is found to be a sensitive measure of the effects of non-additivity. The
variation of with the density of the small spheres shows significantly
different behavior for additive, slightly positive and slightly negative
non-additive mixtures. We discuss the possible repercussions of these results
for the phase behavior of binary hard-sphere mixtures and suggest that
measurements of might provide a means of determining the degree of
non-additivity in real colloidal mixtures
Entropic torque
Quantitative predictions are presented of a depletion-induced torque and
force acting on a single colloidal hard rod immersed in a solvent of hard
spheres close to a planar hard wall. This torque and force, which are entirely
of entropic origin, may play an important role for the key-lock principle,
where a biological macromolecule (the key) is only functional in a particular
orientation with respect to a cavity (the lock)
Depletion potential in hard-sphere mixtures: theory and applications
We present a versatile density functional approach (DFT) for calculating the
depletion potential in general fluid mixtures. In contrast to brute force DFT,
our approach requires only the equilibrium density profile of the small
particles {\em before} the big (test) particle is inserted. For a big particle
near a planar wall or a cylinder or another fixed big particle the relevant
density profiles are functions of a single variable, which avoids the numerical
complications inherent in brute force DFT. We implement our approach for
additive hard-sphere mixtures. By investigating the depletion potential for
high size asymmetries we assess the regime of validity of the well-known
Derjaguin approximation for hard-sphere mixtures and argue that this fails. We
provide an accurate parametrization of the depletion potential in hard-sphere
fluids which should be useful for effective Hamiltonian studies of phase
behavior and colloid structure
Can Polymer Coils be modeled as "Soft Colloids"?
We map dilute or semi-dilute solutions of non-intersecting polymer chains
onto a fluid of ``soft'' particles interacting via a concentration dependent
effective pair potential, by inverting the pair distribution function of the
centers of mass of the initial polymer chains. A similar inversion is used to
derive an effective wall-polymer potential; these potentials are combined to
successfully reproduce the calculated exact depletion interaction induced by
non-intersecting polymers between two walls. The mapping opens up the
possibility of large-scale simulations of polymer solutions in complex
geometries.Comment: 4 pages, 3 figures ReVTeX[epsfig,multicol,amssymb] references update
Effective forces in colloidal mixtures: from depletion attraction to accumulation repulsion
Computer simulations and theory are used to systematically investigate how
the effective force between two big colloidal spheres in a sea of small spheres
depends on the basic (big-small and small-small) interactions. The latter are
modeled as hard-core pair potentials with a Yukawa tail which can be both
repulsive or attractive. For a repulsive small-small interaction, the effective
force follows the trends as predicted by a mapping onto an effective
non-additive hard-core mixture: both a depletion attraction and an accumulation
repulsion caused by small spheres adsorbing onto the big ones can be obtained
depending on the sign of the big-small interaction. For repulsive big-small
interactions, the effect of adding a small-small attraction also follows the
trends predicted by the mapping. But a more subtle ``repulsion through
attraction'' effect arises when both big-small and small-small attractions
occur: upon increasing the strength of the small-small interaction, the
effective potential becomes more repulsive. We have further tested several
theoretical methods against our computer simulations: The superposition
approximation works best for an added big-small repulsion, and breaks down for
a strong big-small attraction, while density functional theory is very accurate
for any big-small interaction when the small particles are pure hard-spheres.
The theoretical methods perform most poorly for small-small attractions.Comment: submitted to PRE; New version includes an important quantitative
correction to several of the simulations. The main conclusions remain
unchanged thoug
Management of visual clutter in annotated 3D CAD models: A comparative study
The use of annotations in CAD models has been an active area of research because of their ability to connect design information to specific aspects of the model s geometry. The effectiveness of annotations is determined by the ability to clearly communicate information. However, annotations can quickly create clutter and confusion as they increase both in number and complexity. Consequently, efficient interaction and visualization mechanisms become crucial. Despite recent standardizations of procedures for the presentation of textual information in CAD models, no explicit guidelines are available as to how to make annotated models more readable and manageable. In this paper, we present the results of a comparative study of different mechanisms to manage visual clutter in annotated 3D CAD models and offer recommendations based on our findings. Our results show that even basic interaction mechanisms have a substantial impact on user s performanceCamba, J.; Contero, M.; Johnson, M. 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