220 research outputs found
Structure of liquid and glassy methanol confined in cylindrical pores
We present a neutron scattering analysis of the density and the static
structure factor of confined methanol at various temperatures. Confinement is
performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24
angstrom and D=35 angstrom. A change of the thermal expansivity of confined
methanol at low temperature is the signature of a glass transition, which
occurs at higher temperature for the smallest pore. This is an evidence of a
surface induced slowing down of the dynamics of the fluid. The structure factor
presents a systematic evolution with the pore diameter, which has been analyzed
in terms of excluded volume effects and fluid-matrix cross-correlation.
Conversely to the case of Van der Waals fluids, it shows that stronger
fluid-matrix correlations must be invoked most probably in relation with the
H-bonding character of both methanol and silicate surface.Comment: version March 12 200
New complexity results for parallel identical machine scheduling problems with preemption, release dates and regular criteria
In this paper, we are interested in parallel identical machine scheduling
problems with preemption and release dates in case of a regular criterion to be
minimized. We show that solutions having a permutation flow shop structure are
dominant if there exists an optimal solution with completion times scheduled in
the same order as the release dates, or if there is no release date. We also
prove that, for a subclass of these problems, the completion times of all jobs
can be ordered in an optimal solution. Using these two results, we provide new
results on polynomially solvable problems and hence refine the boundary between
P and NP for these problems
Dynamics of 8CB confined into porous silicon probed by incoherent neutron backscattering experiments
Confinement in the nanochannels of porous silicon strongly affects the phase
behavior of the archetype liquid-crystal 4-n-octyl-4-cyanobiphenyl (8CB). A
very striking phenom- enon is the development of a short-range smectic order,
which occurs on a very broad temperature range. It suggests in this case that
quenched disorder effects add to usual finite size and surface interaction
effects. We have monitored the temperature variation of the molecular dynamics
of the confined fluid by incoherent quasielastic neutron scat- tering. A
strongly reduced mobility is observed at the highest temperatures in the liquid
phase, which suggests that the interfacial molecular dynamics is strongly
hindered. A continuously increasing slowdown appears on cooling together with a
progressive growth of the static correlation lengt
Methyl group dynamics in a confined glass
We present a neutron scattering investigation on methyl group dynamics in
glassy toluene confined in mesoporous silicates of different pore sizes. The
experimental results have been analysed in terms of a barrier distribution
model, such a distribution following from the structural disorder in the glassy
state. Confinement results in a strong decreasing of the average rotational
barrier in comparison to the bulk state. We have roughly separated the
distribution for the confined state in a bulk-like and a surface-like
contribution, corresponding to rotors at a distance from the pore wall
respectively larger and smaller than the spatial range of the interactions
which contribute to the rotational potential for the methyl groups. We have
estimated a distance of 7 Amstrong as a lower limit of the interaction range,
beyond the typical nearest-neighbour distance between centers-of-mass (4.7
Amstrong).Comment: 5 pages, 3 figures. To be published in European Physical Journal E
Direct. Proceedings of the 2nd International Workshop on Dynamics in
Confinemen
Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores
We present a neutron and X-rays scattering study of the phase transitions of
4-n-octyl-4'-cyanobiphenyl (8CB) confined in unidirectional nanopores of porous
alumina and porous silicon (PSi) membranes with an average diameter of 30 nm.
Spatial confinement reveals a rich polymorphism, with at least four different
low temperature phases in addition to the smectic A phase. The structural study
as a function of thermal treatments and conditions of spatial confinement
allows us to get insights into the formation of these phases and their relative
stability. It gives the first description of the complete phase behavior of 8CB
confined in PSi and provides a direct comparison with results obtained in bulk
conditions and in similar geometric conditions of confinement but with reduced
quenched disorder effects using alumina anopore membranesComment: Accepted in EPJ E - Soft Matte
First evidence of anisotropic quenched disorder effects on a smectic liquid crystal confined in porous silicon
We present a neutron scattering analysis of the structure of the smectic
liquid crystal octylcyanobiphenyl (8CB) confined in one-dimensional nanopores
of porous silicon films (PS). The smectic transition is completely suppressed,
leading to the extension of a short-range ordered smectic phase aligned along
the pore axis. It evolves reversibly over an extended temperature range, down
to 50 K below the \textit{N-SmA} transition in pure 8CB. This behavior strongly
differs from previous observations of smectics in different one-dimensional
porous materials. A coherent picture of this striking behavior requires that
quenched disorder effects are invoked. The strongly disordered nature of the
inner surface of PS acts as random fields coupling to the smectic order. The
one-dimensionality of PS nano-channels offers new perspectives on quenched
disorder effects, which observation has been restricted to homogeneous random
porous materials so far.Comment: Submitted to Phys. Rev.
Recovering Beam Search Approach for the Multi-Skill Project Scheduling Problem
International audienc
Evolution of the magnetic phase transition in MnO confined to channel type matrices. Neutron diffraction study
Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type
matrices with channel diameters 24-87 A demonstrate a continuous magnetic phase
transition in contrast to a discontinuous first order transition in the bulk.
The character of the magnetic transition transforms with decreasing channel
diameter, showing the decreasing critical exponent and transition temperature,
however the latter turns out to be above the N\'eel temperature for the bulk.
This enhancement is explained within the framework of Landau theory taking into
consideration the ternary interaction of the magnetic and associated structural
order parameters.Comment: 6 pages pdf file, including 4 figures, uses revtex4.cl
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