66 research outputs found
Is there something of the MCT in orientationally disordered crystals ?
Molecular Dynamics simulations have been performed on the orientationally
disordered crystal chloroadamantane: a model system where dynamics are almost
completely controlled by rotations. A critical temperature T_c = 225 K as
predicted by the Mode Coupling Theory can be clearly determined both in the
alpha and beta dynamical regimes. This investigation also shows the existence
of a second remarkable dynamical crossover at the temperature T_x > T_c
consistent with a previous NMR and MD study [1]. This allows us to confirm
clearly the existence of a 'landscape-influenced' regime occurring in the
temperature range [T_c-T_x] as recently proposed [2,3].Comment: 4 pages, 5 figures, REVTEX
Structure and relaxations in liquid and amorphous Selenium
We report a molecular dynamics simulation of selenium, described by a
three-body interaction. The temperatures T_g and T_c and the structural
properties are in agreement with experiment. The mean nearest neighbor
coordination number is 2.1. A small pre-peak at about 1 AA^-1 can be explained
in terms of void correlations. In the intermediate self-scattering function,
i.e. the density fluctuation correlation, classical behavior, alpha- and
beta-regimes, is found. We also observe the plateau in the beta-regime below
T_g. In a second step, we investigated the heterogeneous and/or homogeneous
behavior of the relaxations. At both short and long times the relaxations are
homogeneous (or weakly heterogeneous). In the intermediate time scale, lowering
the temperature increases the heterogeneity. We connect these different domains
to the vibrational (ballistic), beta- and alpha-regimes. We have also shown
that the increase in heterogeneity can be understood in terms of relaxations
Model for performance prediction in multi-axis machining
This paper deals with a predictive model of kinematical performance in 5-axis
milling within the context of High Speed Machining. Indeed, 5-axis high speed
milling makes it possible to improve quality and productivity thanks to the
degrees of freedom brought by the tool axis orientation. The tool axis
orientation can be set efficiently in terms of productivity by considering
kinematical constraints resulting from the set machine-tool/NC unit. Capacities
of each axis as well as some NC unit functions can be expressed as limiting
constraints. The proposed model relies on each axis displacement in the joint
space of the machine-tool and predicts the most limiting axis for each
trajectory segment. Thus, the calculation of the tool feedrate can be performed
highlighting zones for which the programmed feedrate is not reached. This
constitutes an indicator for trajectory optimization. The efficiency of the
model is illustrated through examples. Finally, the model could be used for
optimizing process planning
Glass transition in systems without static correlations: a microscopic theory
We present a first step toward a microscopic theory for the glass transition
in systems with trivial static correlations. As an example we have chosen N
infinitely thin hard rods with length L, fixed with their centers on a periodic
lattice with lattice constant a. Starting from the N-rod Smoluchowski equation
we derive a coupled set of equations for fluctuations of reduced k-rod
densities. We approximate the influence of the surrounding rods onto the
dynamics of a pair of rods by introduction of an effective rotational diffusion
tensor D and in this way we obtain a self-consistent equation for D. This
equation exhibits a feedback mechanism leading to a slowing down of the
relaxation. It involves as an input the Laplace transform v_0(l/r) at z=0,
l=L/a, of a torque-torque correlator of an isolated pair of rods with distance
R=ar. Our equation predicts the existence of a continuous ergodicity-breaking
transition at a critical length l_c=L_c/a. To estimate the critical length we
perform an approximate analytical calculation of v_0(l/r) based on a
variational approach and obtain l_c^{var}=5.68, 4.84 and 3.96 for an sc, bcc
and fcc lattice. We also evaluate v_0(l/r) numerically exactly from a two-rod
simulation. The latter calculation leads to l_c^{num}=3.45, 2.78 and 2.20 for
the corresponding lattices. Close to l_c the rotational diffusion constant
decreases as D(l) ~ (l_c - l)^\gamma with \gamma=1 and a diverging time scale
t_\epsilon ~ |l_c - l|^{-\delta}, \delta=2, appears. On this time scale the t-
and l-dependence of the 1-rod density is determined by a master function
depending only on t/t_\epsilon. In contrast to present microscopic theories our
approach predicts a glass transition despite the absence of any static
correlations.Comment: 22 pages, 7 figures (minor revisions in the text, corrected figures
Tool path deformation in 5-axis flank milling using envelope surface
International audienc
Subterahertz characterization of ethanol hydration layers by microfluidic system
International audienceCharacterizations of ethanol hydration layers are examined through subterahertz spectroscopy of water/ethanol mixtures by using a microfluidic system. A three-component model is used to explain measurements discrepancies with the Lambert–Beer law and to determine ethanol hydration shell absorption. Moreover, the hydration shell distribution is compared with molecular dynamics simulations with a good agreement. Ethanol hydration number is then computed and it can quickly characterize only the first water hydration layer or the whole hydration shell, depending on the chosen extraction mode
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