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

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 $\xi$ and $\gamma$ that describe the long-time decay of the number of particles ($n(t)\sim t^{-\xi}$) and of their typical velocity ($v(t)\sim t^{-\gamma}$). To a good accuracy our results confirm the scaling relation $\xi + \gamma =1$. 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

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 $F(m)\propto m^{-\alpha}\exp{[-(m/m_0)^\gamma]}$, where $m$ is the mass, $m_0$ and $\gamma$ are cutoff parameters, and $\alpha$ is a scaling exponent that is dependent on the temperature. More precisely, we show clear evidence that there is a characteristic scaling exponent $\alpha$ 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

Hard Spheres: Crystallization and Glass Formation

Motivated by old experiments on colloidal suspensions, we report molecular dynamics simulations of assemblies of hard spheres, addressing crystallization and glass formation. The simulations cover wide ranges of polydispersity s (standard deviation of the particle size distribution divided by its mean) and particle concentration. No crystallization is observed for s > 0.07. For 0.02 < s < 0.07, we find that increasing the polydispersity at a given concentration slows down crystal nucleation. The main effect here is that polydispersity reduces the supersaturation since it tends to stabilise the fluid but to destabilise the crystal. At a given polydispersity (< 0.07) we find three regimes of nucleation: standard nucleation and growth at concentrations in and slightly above the coexistence region; "spinodal nucleation", where the free energy barrier to nucleation appears to be negligible, at intermediate concentrations; and, at the highest concentrations, a new mechanism, still to be fully understood, which only requires small re-arrangement of the particle positions. The cross-over between the second and third regimes occurs at a concentration, around 58% by volume, where the colloid experiments show a marked change in the nature of the crystals formed and the particle dynamics indicate an "ideal" glass transition

Stratified horizontal flow in vertically vibrated granular layers

A layer of granular material on a vertically vibrating sawtooth-shaped base exhibits horizontal flow whose speed and direction depend on the parameters specifying the system in a complex manner. Discrete-particle simulations reveal that the induced flow rate varies with height within the granular layer and oppositely directed flows can occur at different levels. The behavior of the overall flow is readily understood once this novel feature is taken into account.Comment: 4 pages, 6 figures, submitte

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