93 research outputs found
Observation of condensed phases of quasi-planar core-softened colloids
We experimentally study the condensed phases of repelling core-softened
spheres in two dimensions. The dipolar pair repulsion between superparamagnetic
spheres trapped in a thin cell is induced by a transverse magnetic field and
softened by suitably adjusting the cell thickness. We scan a broad density
range and we materialize a large part of the theoretically predicted phases in
systems of core-softened particles, including expanded and close-packed
hexagonal, square, chain-like, stripe/labyrinthine, and honeycomb phase.
Further insight into their structure is provided by Monte Carlo simulations
Aggregates of two-dimensional vesicles: Rouleaux and sheets
Using both numerical and variational minimization of the bending and adhesion
energy of two-dimensional lipid vesicles, we study their aggregation, and we
find that the stable aggregates include an infinite number of vesicles and that
they arrange either in a columnar or in a sheet-like structure. We calculate
the stability diagram and we discuss the modes of transformation between the
two types of aggregates, showing that they include disintegration as well as
intercalation.Comment: 4 figure
Limiting shapes of confined lipid vesicles
We theoretically study the shapes of lipid vesicles confined to a spherical cavity, elaborating a framework based on the so-called limiting shapes constructed from geometrically simple structural elements such as double-membrane walls and edges. Partly inspired by numerical results, the proposed non-compartmentalized and compartmentalized limiting shapes are arranged in the bilayer-couple phase diagram which is then compared to its free-vesicle counterpart. We also compute the area-difference-elasticity phase diagram of the limiting shapes and we use it to interpret shape transitions experimentally observed in vesicles confined within another vesicle. The limiting-shape framework may be generalized to theoretically investigate the structure of certain cell organelles such as the mitochondrion
Soft Spheres Make More Mesophases
We use both mean-field methods and numerical simulation to study the phase
diagram of classical particles interacting with a hard-core and repulsive, soft
shoulder. Despite the purely repulsive interaction, this system displays a
remarkable array of aggregate phases arising from the competition between the
hard-core and shoulder length scales. In the limit of large shoulder width to
core size, we argue that this phase diagram has a number of universal features,
and classify the set of repulsive shoulders that lead to aggregation at high
density. Surprisingly, the phase sequence and aggregate size adjusts so as to
keep almost constant inter-aggregate separation.Comment: 4 pages, 2 included figure
Casimir Torques between Anisotropic Boundaries in Nematic Liquid Crystals
Fluctuation-induced interactions between anisotropic objects immersed in a
nematic liquid crystal are shown to depend on the relative orientation of these
objects. The resulting long-range ``Casimir'' torques are explicitely
calculated for a simple geometry where elastic effects are absent. Our study
generalizes previous discussions restricted to the case of isotropic walls, and
leads to new proposals for experimental tests of Casimir forces and torques in
nematics.Comment: 4 pages, 1 figur
Axially symmetric membranes with polar tethers
Axially symmetric equilibrium configurations of the conformally invariant
Willmore energy are shown to satisfy an equation that is two orders lower in
derivatives of the embedding functions than the equilibrium shape equation, not
one as would be expected on the basis of axial symmetry. Modulo a translation
along the axis, this equation involves a single free parameter c.If c\ne 0, a
geometry with spherical topology will possess curvature singularities at its
poles. The physical origin of the singularity is identified by examining the
Noether charge associated with the translational invariance of the energy; it
is consistent with an external axial force acting at the poles. A one-parameter
family of exact solutions displaying a discocyte to stomatocyte transition is
described.Comment: 13 pages, extended and revised version of Non-local sine-Gordon
equation for the shape of axi-symmetric membrane
Interfaces in Diblocks: A Study of Miktoarm Star Copolymers
We study AB miktoarm star block copolymers in the strong segregation
limit, focussing on the role that the AB interface plays in determining the
phase behavior. We develop an extension of the kinked-path approach which
allows us to explore the energetic dependence on interfacial shape. We consider
a one-parameter family of interfaces to study the columnar to lamellar
transition in asymmetric stars. We compare with recent experimental results. We
discuss the stability of the A15 lattice of sphere-like micelles in the context
of interfacial energy minimization. We corroborate our theory by implementing a
numerically exact self-consistent field theory to probe the phase diagram and
the shape of the AB interface.Comment: 12 pages, 11 included figure
Soap Froths and Crystal Structures
We propose a physical mechanism to explain the crystal symmetries found in
macromolecular and supramolecular micellar materials. We argue that the packing
entropy of the hard micellar cores is frustrated by the entropic interaction of
their brush-like coronas. The latter interaction is treated as a surface effect
between neighboring Voronoi cells. The observed crystal structures correspond
to the Kelvin and Weaire-Phelan minimal foams. We show that these structures
are stable for reasonable areal entropy densities.Comment: 4 pages, RevTeX, 2 included eps figure
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