886 research outputs found
Self-Assembly on a Cylinder: A Model System for Understanding the Constraint of Commensurability
A crystal lattice, when confined to the surface of a cylinder, must have a
periodic structure that is commensurate with the cylinder circumference. This
constraint can frustrate the system, leading to oblique crystal lattices or to
structures with a chiral seam known as a "line slip" phase, neither of which
are stable for isotropic particles in equilibrium on flat surfaces. In this
study, we use molecular dynamics simulations to find the steady-state structure
of spherical particles with short-range repulsion and long-range attraction far
below the melting temperature. We vary the range of attraction using the
Lennard-Jones and Morse potentials and find that a shorter-range attraction
favors the line-slip. We develop a simple model based only on geometry and bond
energy to predict when the crystal or line-slip phases should appear, and find
reasonable agreement with the simulations. The simplicity of this model allows
us to understand the influence of the commensurability constraint, an
understanding that might be extended into the more general problem of
self-assembling particles in strongly confined spaces.Comment: 12 pages, 9 figures. Submitted for publication, 201
Determination of the Superconductor-Insulator Phase Diagram for One-Dimensional Wires
We establish the superconductor-insulator phase diagram for quasi-one
dimensional wires by measuring a large set of MoGe nanowires. This diagram is
consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely with
the critical resistance being equal to R_Q = h/4e^2. We find that transport
properties of insulating nanowires exhibit a weak Coulomb blockade behavior.Comment: 5 pages, 4 figure
Forced motion of a probe particle near the colloidal glass transition
We use confocal microscopy to study the motion of a magnetic bead in a dense
colloidal suspension, near the colloidal glass transition volume fraction
. For dense liquid-like samples near , below a threshold force
the magnetic bead exhibits only localized caged motion. Above this force, the
bead is pulled with a fluctuating velocity. The relationship between force and
velocity becomes increasingly nonlinear as is approached. The
threshold force and nonlinear drag force vary strongly with the volume
fraction, while the velocity fluctuations do not change near the transition.Comment: 7 pages, 4 figures revised version, accepted for publication in
Europhysics Letter
Hard Spheres in Vesicles: Curvature-Induced Forces and Particle-Induced Curvature
We explore the interplay of membrane curvature and nonspecific binding due to
excluded-volume effects among colloidal particles inside lipid bilayer
vesicles. We trapped submicron spheres of two different sizes inside a
pear-shaped, multilamellar vesicle and found the larger spheres to be pinned to
the vesicle's surface and pushed in the direction of increasing curvature. A
simple model predicts that hard spheres can induce shape changes in flexible
vesicles. The results demonstrate an important relationship between the shape
of a vesicle or pore and the arrangement of particles within it.Comment: LaTeX with epsfig; ps available at
http://dept.physics.upenn.edu/~nelson/index.shtml Phys Rev Lett in press
(1997
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
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