1,632 research outputs found
Quenching of lamellar ordering in an n-alkane embedded in nanopores
We present an X-ray diffraction study of the normale alkane nonadecane
C_{19}H_{40} embedded in nanoporous Vycor glass. The confined molecular crystal
accomplishes a close-packed structure by alignment of the rod-like molecules
parallel to the pore axis while sacrificing one basic principle known from the
bulk state, i.e. the lamellar ordering of the molecules. Despite this disorder,
the phase transitions observed in the confined solid mimic the phase behavior
of the 3D unconfined crystal, though enriched by the appearance of a true
rotator phase known only from longer alkane chains.Comment: 7 pages, 3 figure
Percolation, depinning, and avalanches in capillary condensation of gases in disordered porous solids
We propose a comprehensive theoretical description of hysteresis in capillary
condensation of gases in mesoporous disordered materials. Applying mean-field
density functional theory to a coarse-grained lattice-gas model, we show that
the morphology of the hysteresis loops is influenced by out-of-equilibrium
transitions that are different on filling and on draining. In particular,
desorption may be associated to a depinning process and be percolation-like
without explicit pore-blocking effects.Comment: 4 pages, 5 figure
Soft disks in a narrow channel
The pressure components of "soft" disks in a two dimensional narrow channel
are analyzed in the dilute gas regime using the Mayer cluster expansion and
molecular dynamics. Channels with either periodic or reflecting boundaries are
considered. It is found that when the two-body potential, u(r), is singular at
some distance r_0, the dependence of the pressure components on the channel
width exhibits a singularity at one or more channel widths which are simply
related to r_0. In channels with periodic boundary conditions and for
potentials which are discontinuous at r_0, the transverse and longitudinal
pressure components exhibit a 1/2 and 3/2 singularity, respectively. Continuous
potentials with a power law singularity result in weaker singularities of the
pressure components. In channels with reflecting boundary conditions the
singularities are found to be weaker than those corresponding to periodic
boundaries
Structural properties of hard disks in a narrow tube
Positional ordering of a two-dimensional fluid of hard disks is examined in
such narrow tubes where only the nearest-neighbor interactions take place.
Using the exact transfer-matrix method the transverse and longitudinal pressure
components and the correlation function are determined numerically. Fluid-solid
phase transition does not occur even in the widest tube, where the method just
loses its exactness, but the appearance of the dramatic change in the equation
of state and the longitudinal correlation function shows that the system
undergoes a structural change from a fluid to a solid-like order. The pressure
components show that the collisions are dominantly longitudinal at low
densities, while they are transverse in the vicinity of close packing density.
The transverse correlation function shows that the size of solid-like domains
grows exponentially with increasing pressure and the correlation length
diverges at close packing. It is managed to find an analytically solvable model
by expanding the contact distance up to first order. The approximate model,
which corresponds to the system of hard parallel rhombuses, behaves very
similarly to the system of hard disks.Comment: Acceped in Journal of Statistical Mechanics: Theory and Experimen
Thermodynamics, Structure, and Dynamics of Water Confined between Hydrophobic Plates
We perform molecular dynamics simulations of 512 water-like molecules that
interact via the TIP5P potential and are confined between two smooth
hydrophobic plates that are separated by 1.10 nm. We find that the anomalous
thermodynamic properties of water are shifted to lower temperatures relative to
the bulk by K. The dynamics and structure of the confined water
resemble bulk water at higher temperatures, consistent with the shift of
thermodynamic anomalies to lower temperature. Due to this shift, our
confined water simulations (down to K) do not reach sufficiently low
temperature to observe a liquid-liquid phase transition found for bulk water at
K using the TIP5P potential. We find that the different
crystalline structures that can form for two different separations of the
plates, 0.7 nm and 1.10 nm, have no counterparts in the bulk system, and
discuss the relevance to experiments on confined water.Comment: 31 pages, 14 figure
Adsorption in non interconnected pores open at one or at both ends: A reconsideration of the origin of the hysteresis phenomenon
We report on an experimental study of adsorption isotherme of nitrogen onto
porous silicon with non interconnected pores open at one or at both ends in
order to check for the first time the old (1938) but always current idea based
on Cohan's description which suggests that the adsorption of gaz should occur
reversibly in the first case and irreversibly in the second one. Hysteresis
loops, the shape of which is usually associated to interconnections in porous
media, are observed whether the pores are open at one or at both ends in
contradiction with Cohan's model.Comment: 5 pages, 4 EPS figure
The Classic: Integration of Deoxyribonucleic Acid Specific for Rous Sarcoma Virus after Infection of Permissive and Nonpermissive Hosts: (RNA tumor viruses/reassociation kinetics/duck cells)
A relatively simple but stringent technique was developed to detect the integration of virus-specific DNA into the genomes of higher organisms. In both permissive (duck) and nonpermissive (mammalian) cells which normally contain no nucleotide sequences specific for Rous sarcoma virus, transformation by the virus results in the appearance of DNA specific for Rous sarcoma virus covalently integrated into strands of host-cell DNA containing reiterated sequences. Early after infection of mouse or duck cells by Rous sarcoma virus, unintegrated DNA specific for the virus can be demonstrated
Experimental Results of Concurrent Learning Adaptive Controllers
Commonly used Proportional-Integral-Derivative based UAV flight controllers are often seen to provide adequate trajectory-tracking performance only after extensive tuning. The gains of these controllers are tuned to particular platforms, which makes transferring controllers from one UAV to other time-intensive. This paper suggests the use of adaptive controllers in speeding up the process of extracting good control performance from new UAVs. In particular, it is shown that a concurrent learning adaptive controller improves the trajectory tracking performance of a quadrotor with baseline linear controller directly imported from another quadrotors whose inertial characteristics and throttle mapping are very di fferent. Concurrent learning adaptive control uses specifi cally selected and online recorded data concurrently with instantaneous data and is capable of guaranteeing tracking error and weight error convergence without requiring persistency of excitation. Flight-test results are presented on indoor quadrotor platforms operated in MIT's RAVEN environment. These results indicate the feasibility of rapidly developing high-performance UAV controllers by using adaptive control to augment a controller transferred from another UAV with similar control assignment structure.United States. Office of Naval Research. Multidisciplinary University Research Initiative (Grant N000141110688)National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 0645960)Boeing Scientific Research Laboratorie
Capillary condensation in disordered porous materials: hysteresis versus equilibrium behavior
We study the interplay between hysteresis and equilibrium behavior in
capillary condensation of fluids in mesoporous disordered materials via a
mean-field density functional theory of a disordered lattice-gas model. The
approach reproduces all major features observed experimentally. We show that
the simple van der Waals picture of metastability fails due to the appearance
of a complex free-energy landscape with a large number of metastable states. In
particular, hysteresis can occur both with and without an underlying
equilibrium transition, thermodynamic consistency is not satisfied along the
hysteresis loop, and out-of-equilibrium phase transitions are possible.Comment: 4 pages, 4 figure
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