Precursor-mediated crystallization process in suspensions of hard spheres

We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real and reciprocal space data, a picture of a two-step crystallization process is supported: First dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that has no metastable fluid-fluid coexistence) crystallizes via the same mechanism.Comment: 4 figure

Electronic properties of graphene nano-flakes: Energy gap, permanent dipole, termination effect and Raman spectroscopy

The electronic properties of graphene nano-flakes (GNFs) with different edge passivation is investigated by using density functional theory. Passivation with F and H atoms are considered: C$_{N_c}$ X$_{N_x}$ (X=F or H). We studied GNFs with $10<N_c<56$ and limit ourselves to the lowest energy configurations. We found that: i) the energy difference $\Delta$ between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with $N_c$, ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination and bilayer on the electronic properties of small GNFs.This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy.Comment: 23 pages, 14 figures, accepted in J. Chem. Phy

Computation of thermodynamic and transport properties to predict thermophoretic effects in an argon-krypton mixture

Thermophoresis is the movement of molecules caused by a temperature gradient. Here we report the results of a study of thermophoresis using non-equilibrium molecular dynamics simulations of a confined argon-krypton fluid subject to two different temperatures at thermostated walls. The resulting temperature profile between the walls is used along with the Soret coefficient to predict the concentration profile that develops across the channel. We obtain the Soret coefficient by calculating the mutual diffusion and thermal diffusion coefficients. We report an appropriate method for calculating the transport coefficients for binary systems, using the Green-Kubo integrals and radial distribution functions obtained from equilibrium molecular dynamics simulations of the bulk fluid. Our method has the unique advantage of separating the mutual diffusion and thermal diffusion coef- ficients, and calculating the sign and magnitude of their individual contributions to thermophoresis in binary mixtures

Second-tier community forestry organisations and equitable resource management in the Maya Biosphere Reserve, Guatemala

Whilst community forestry programmes have combined sustainable forestry with community empowerment and poverty alleviation since the late 1970s, the role of intermediary organisations in shaping the technical and political capacities of forest user groups has rarely been systematically studied. The long-term durability of community forestry groups has been linked with the congruence between local and national determinations of tenure rights, and the involvement of local communities in determining the ‘rules' that govern the management of the forests. However, the role of intermediary organisations in negotiating such rights and rules is of central analytical importance - especially in contexts where pre-existing power and gender relationships influence who makes which decisions and how, and where long-standing conflicts over access to land constantly over-determine the possibilities for upscaling successful projects. As part of an interdisciplinary project on community forestry in Mesoamerica led by Bioversity International, this paper focuses on the case of the Asociación de Comunidades Forestales de Petén [ACOFOP], a second-tier community organisation founded in the mid-1990s to coordinate and represent first-tier organisations that were granted community concessions in the Maya Biosphere Reserve in the Petén region of Guatemala. Drawing on interviews, the outcomes of participative workshops, and a review of key literature, we trace ACOFOP's emergence as a forestry organisation from existing agricultural and non-timber product cooperatives, and its subsequent consolidation as a platform for regional coordination and advocacy. Using a ‘Sociology of Knowledge' approach to discourse analysis (SKAD), we highlight three important dimensions of ACOFOP's evolving strategy that facilitate the development of both local autonomies and collective coherence: the mobilisation of concepts of ‘environmental justice' and associated international rights frameworks to link local concerns with national governance issues; the elaboration of legal knowledges and mechanisms in these terms to negotiate improved tenure relationships; and a concentration of resources in the development of local leadership with an emphasis on elaborating the fixed ‘rules' of forestry management in a manner that is appropriate to each community's individual characteristics. We conclude by highlighting key strategies that may be applied to strengthen community forestry in other regional settings

Velocity autocorrelation functions of hard-sphere fluids: Long-time tails upon undercooling

Molecular dynamics simulations are employed to obtain the velocity autocorrelation function (VAF) for hard spheres, spanning a wide range of volume fractions from dilute to high-density metastable fluids. For all volume fractions below freezing, Alder&#039;s classical positive 3/2 long-time tail is observed. For volume fractions from 0.45 to 0.48 the VAF becomes negative, before becoming positive and decaying with the positive 3/2 long-time tail. At the freezing volume fraction (0.494) the Alder 3/2 tail is not observed. At higher volume fractions a negative tail with an exponent of 5/2 emerges, which coincides with the long-time tail of a Lorentz gas

Dispelling the myth of robotic efficiency: why human space exploration will tell us more about the Solar System than will robotic exploration alone

There is a widely held view in the astronomical community that unmanned robotic space vehicles are, and will always be, more efficient explorers of planetary surfaces than astronauts (e.g. Coates, 2001; Clements 2009; Rees 2011). Partly this is due to a common assumption that robotic exploration is cheaper than human exploration (although, as we shall see, this isn't necessarily true if like is compared with like), and partly from the expectation that continued developments in technology will relentlessly increase the capability, and reduce the size and cost, of robotic missions to the point that human exploration will not be able to compete. I will argue below that the experience of human exploration during the Apollo missions, more recent field analogue studies, and trends in robotic space exploration actually all point to exactly the opposite conclusion.Comment: 12 pages; 5 figures. Published, with minor modifications, in Astronomy and Geophysics, Vol. 53, pp. 2.22-2.26, 201

The relationship between adiposity and stature in prepubertal children with celiac disease

Background and Aim: The pathogenesis of short stature in celiac disease (CD) is unknown. Obese children are generally taller than their non-obese peers; however, the role of adiposity on stature in CD is unclear. Our aim was to determine the association between adiposity and stature in CD. Subjects and methods: We compared the anthropometric characteristics of prepubertal children of ages 3-12 years, with biopsy-proven CD (n=40) and who were not on gluten-free diet, to same aged, prepubertal non-CD children (n=50). Body mass index (BMI) was calculated using the formula weight/height2. Sex-adjusted midparental target height (MPTH) standard deviation score (SDS) was calculated using National Children Health Statistics data for 18-year-old adults. Data were expressed as mean±standard deviation. Results: CD subjects had significantly lower BMI SDS than controls (0.61±1.22 vs. 1.28±1.60, p=0.027) but were not significantly shorter than the controls (-0.05±1.21 vs. 0.21±1.71, p=0.41). When the patients were subdivided into the normal-weight and overweight/obese groups, the normal-weight CD patients were of similar height as the normal-weight controls (p=0.76) but were significantly shorter than both the overweight/obese controls (p=0.003). The MPTH SDS did not differ between the groups. Conclusions: Overweight/obese prepubertal children with CD were taller than both their normal-weight CD peers and the normal-weight controls, but were of similar height as the overweight/obese control subjects

Nonequilibrium Temperature and Thermometry in Heat-Conducting Phi-4 Models

We analyze temperature and thermometry for simple nonequilibrium heat-conducting models. We show in detail, for both two- and three-dimensional systems, that the ideal gas thermometer corresponds to the concept of a local instantaneous mechanical kinetic temperature. For the Phi-4 models investigated here the mechanical temperature closely approximates the local thermodynamic equilibrium temperature. There is a significant difference between kinetic temperature and the nonlocal configurational temperature. Neither obeys the predictions of extended irreversible thermodynamics. Overall, we find that kinetic temperature, as modeled and imposed by the Nos\'e-Hoover thermostats developed in 1984, provides the simplest means for simulating, analyzing, and understanding nonequilibrium heat flows.Comment: 20 pages with six figures, revised following review at Physical Review

How hard is a colloidal &#039;hard-sphere&#039; interaction?

Poly-12-hydroxystearic acid (PHSA) is widely used as a coating on colloidal spheres to provide a &quot;hard-sphere-type&quot; interaction. These hard spheres have been widely used in fundamental studies of nucleation, crystallization, and glass formation. Most authors describe the interaction as &quot;nearly&quot; hard sphere. In this paper we directly measure this interaction, using layers of PHSA adsorbed onto mica sheets in a surfaces force apparatus. We find that the layers, in appropriate solvents, have no long-range interaction. When the solvent is decahydronaphthalene (decalin), the repulsion rises from zero to the maximum measurable over a distance range of 15-20 nm. The data is converted to equivalent forces between spheres of different diameters, and modeled using a hard core potential. Using zeroth-order perturbation theory and computer simulation, we demonstrate that the equation of state does not deviate from that of a perfect hard-sphere system under any relevant experimental conditions

New Langevin and Gradient Thermostats for Rigid Body Dynamics

We introduce two new thermostats, one of Langevin type and one of gradient (Brownian) type, for rigid body dynamics. We formulate rotation using the quaternion representation of angular coordinates; both thermostats preserve the unit length of quaternions. The Langevin thermostat also ensures that the conjugate angular momenta stay within the tangent space of the quaternion coordinates, as required by the Hamiltonian dynamics of rigid bodies. We have constructed three geometric numerical integrators for the Langevin thermostat and one for the gradient thermostat. The numerical integrators reflect key properties of the thermostats themselves. Namely, they all preserve the unit length of quaternions, automatically, without the need of a projection onto the unit sphere. The Langevin integrators also ensure that the angular momenta remain within the tangent space of the quaternion coordinates. The Langevin integrators are quasi-symplectic and of weak order two. The numerical method for the gradient thermostat is of weak order one. Its construction exploits ideas of Lie-group type integrators for differential equations on manifolds. We numerically compare the discretization errors of the Langevin integrators, as well as the efficiency of the gradient integrator compared to the Langevin ones when used in the simulation of rigid TIP4P water model with smoothly truncated electrostatic interactions. We observe that the gradient integrator is computationally less efficient than the Langevin integrators. We also compare the relative accuracy of the Langevin integrators in evaluating various static quantities and give recommendations as to the choice of an appropriate integrator.Comment: 16 pages, 4 figure