2,337 research outputs found
Hexagonal convection patterns in atomistically simulated fluids
Molecular dynamics simulation has been used to model pattern formation in
three-dimensional Rayleigh--Benard convection at the discrete-particle level.
Two examples are considered, one in which an almost perfect array of
hexagonally-shaped convection rolls appears, the other a much narrower system
that forms a set of linear rolls; both pattern types are familiar from
experiment. The nature of the flow within the convection cells and quantitative
aspects of the development of the hexagonal planform based on automated polygon
subdivision are analyzed. Despite the microscopic scale of the system,
relatively large simulations with several million particles and integration
timesteps are involved.Comment: 4 pages, 6 figures (color figures have low resolution, high
resolution figures available on author's website) Minor changes to text. To
appear in PRE (Rapid Comm
Correlations in a two-dimensional Bose gas with long range interactions
We study the correlations of two-dimensional dipolar excitons in coupled
quantum wells with a dipole -- dipole repulsive interaction. We show that at
low concentrations, the Bose degeneracy of the excitons is accompanied by
strong multi-particle correlations and the system behaves as a Bose liquid. At
high concentration the particles interaction suppresses quantum coherence and
the system behaves similar to a classical liquid down to a temperature lower
than typical for a Bose gas. We evaluate the interaction energy per particle
and the resulting blue shift of the exciton luminescence that is a direct tool
to measure the correlations. This theory can apply to other systems of bosons
with extended interaction.Comment: 11 pages including 2 figure
Two-dimensional order in β-sheet peptide monolayers
Amphiphilic peptides comprising alternating hydrophilic and hydrophobic amino acid residues were designed to form super-secondary structures composed of self-assembled β-strands as monolayers at the air−water interface. Insights provided by in situ grazing-incidence X-ray diffraction (GIXD), surface pressure vs area isotherms, and Fourier transform infrared spectroscopy allow structural characterization of the assembled nanostructures and rational correlation with the peptide sequence. Peptides seven to seventeen amino acids in length were found to form crystalline arrays with coherence lengths in the range of 100 to 1000 Å. Two-dimensional registry of the self-assembled peptides was induced by placement of proline residues at the peptide termini. The films were found to intercalate ordered arrays of ions between juxtaposed β-sheet ribbons to generate peptide−ion composite phases
Shear-induced criticality near a liquid-solid transition of colloidal suspensions
We investigate colloidal suspensions under shear flow through numerical
experiments. By measuring the time-correlation function of a bond-orientational
order parameter, we find a divergent time scale near a transition point from a
disordered fluid phase to an ordered fluid phase, where the order is
characterized by a nonzero value of the bond-orientational order parameter. We
also present a phase diagram in the plane,
where is the density of the colloidal particles and
is the shear rate of the solvent. The transition
line in the phase diagram terminates at the equilibrium transition point, while
a critical region near the transition line vanishes continuously as
.Comment: 4 pages, 8 figure
Leaving College: Why Students Withdrew from a University
The purpose of this study was to determine the reasons why students withdrew during a semester from a mid-sized, comprehensive university located in the Midwest. Six hundred forty-five students were asked to complete the ACT Withdrawing/Non-returning Student Survey during the 1992-93 academic year and summer semester. Three hundred sixty-five completed surveys were returned for a 57% response rate.
Respondents indicated many different reasons for leaving which varied by year in school and whether or not the respondent was a graduate or undergraduate student. There was no typical withdrawing student and there were many reasons students withdrew over which the university has little or no control. The report concludes with a discussion of Vincent Tinto\u27s (1993) ideas concerning institutional departure.
The retention and persistence of students in higher education has been the focus of serious intellectual inquiry for many years. Various concepts of institutional departure, persistence and models for programmatic interventions to reduce departure have been developed. (For example, see Pascarella & Terenzini, 1991; Stage & Rushin, 1993; Steele, Kennedy, & Gordon, 1993; Tinto, 1993; Wolfe, 1993.) The purpose of this study was to focus on one aspect of student attrition, and. to investigate the reasons and general trends as . to why students withdrew during a semester from a midsized comprehensive university located in the Midwest. This information could then be used to guide institutional action
Molecular dynamics simulations of ballistic annihilation
Using event-driven molecular dynamics we study one- and two-dimensional
ballistic annihilation. We estimate exponents and that describe
the long-time decay of the number of particles () and of
their typical velocity (). To a good accuracy our results
confirm the scaling relation . In the two-dimensional case our
results are in a good agreement with those obtained from the Boltzmann kinetic
theory.Comment: 4 pages; some changes; Physical Review E (in press
Star-graph expansions for bond-diluted Potts models
We derive high-temperature series expansions for the free energy and the
susceptibility of random-bond -state Potts models on hypercubic lattices
using a star-graph expansion technique. This method enables the exact
calculation of quenched disorder averages for arbitrary uncorrelated coupling
distributions. Moreover, we can keep the disorder strength as well as the
dimension as symbolic parameters. By applying several series analysis
techniques to the new series expansions, one can scan large regions of the
parameter space for any value of . For the bond-diluted 4-state
Potts model in three dimensions, which exhibits a rather strong first-order
phase transition in the undiluted case, we present results for the transition
temperature and the effective critical exponent as a function of
as obtained from the analysis of susceptibility series up to order 18. A
comparison with recent Monte Carlo data (Chatelain {\em et al.}, Phys. Rev.
E64, 036120(2001)) shows signals for the softening to a second-order transition
at finite disorder strength.Comment: 8 pages, 6 figure
Memory effects on the statistics of fragmentation
We investigate through extensive molecular dynamics simulations the
fragmentation process of two-dimensional Lennard-Jones systems. After
thermalization, the fragmentation is initiated by a sudden increment to the
radial component of the particles' velocities. We study the effect of
temperature of the thermalized system as well as the influence of the impact
energy of the ``explosion'' event on the statistics of mass fragments. Our
results indicate that the cumulative distribution of fragments follows the
scaling ansatz , where is
the mass, and are cutoff parameters, and is a scaling
exponent that is dependent on the temperature. More precisely, we show clear
evidence that there is a characteristic scaling exponent for each
macroscopic phase of the thermalized system, i.e., that the non-universal
behavior of the fragmentation process is dictated by the state of the system
before it breaks down.Comment: 5 pages, 8 figure
Role of reversibility in viral capsid growth: A paradigm for self-assembly
Self-assembly at submicroscopic scales is an important but little understood
phenomenon. A prominent example is virus capsid growth, whose underlying
behavior can be modeled using simple particles that assemble into polyhedral
shells. Molecular dynamics simulation of shell formation in the presence of an
atomistic solvent provides new insight into the self-assembly mechanism,
notably that growth proceeds via a cascade of strongly reversible steps and,
despite the large variety of possible intermediates, only a small fraction of
highly bonded forms appear on the pathway.Comment: 4 pages, 4 figures (slightly shorter version, new Fig.2); further
minor change
Universal scaling dynamics in a perturbed granular gas
We study the response of a granular system at rest to an instantaneous input
of energy in a localised region. We present scaling arguments that show that,
in dimensions, the radius of the resulting disturbance increases with time
as , and the energy decreases as , where the
exponent is independent of the coefficient of restitution. We
support our arguments with an exact calculation in one dimension and event
driven molecular dynamic simulations of hard sphere particles in two and three
dimensions.Comment: 5 pages, 5 figure
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