229 research outputs found

### Cooperative motion and growing length scales in supercooled confined liquids

Using molecular dynamics simulations we investigate the relaxation dynamics
of a supercooled liquid close to a rough as well as close to a smooth wall. For
the former situation the relaxation times increase strongly with decreasing
distance from the wall whereas in the second case they strongly decrease. We
use this dependence to extract various dynamical length scales and show that
they grow with decreasing temperature. By calculating the frequency dependent
average susceptibility of such confined systems we show that the experimental
interpretation of such data is very difficult.Comment: 7 pages of Latex, 3 figure

### Glass transition and layering effects in confined water: a computer simulation study

Single particle dynamics of water confined in a nanopore is studied through
Computer Molecular Dynamics. The pore is modeled to represent the average
properties of a pore of Vycor glass. Dynamics is analyzed at different
hydration levels and upon supercooling. At all hydration levels and all
temperatures investigated a layering effect is observed due to the strong
hydrophilicity of the substrate. The time density correlators show, already at
ambient temperature, strong deviations from the Debye and the stretched
exponential behavior. Both on decreasing hydration level and upon supercooling
we find features that can be related to the cage effect typical of a
supercooled liquid undergoing a kinetic glass transition. Nonetheless the
behavior predicted by Mode Coupling Theory can be observed only by carrying out
a proper shell analysis of the density correlators. Water molecules within the
first two layers from the substrate are in a glassy state already at ambient
temperature (bound water). The remaining subset of molecules (free water)
undergoes a kinetic glass transition; the relaxation of the density correlators
agree with the main predictions of the theory. From our data we can predict the
temperature of structural arrest of free water.Comment: 14 pages, 15 figures inserted in the text, to be published in J.
Chem. Phys. (2000

### The relaxation dynamics of a simple glass former confined in a pore

We use molecular dynamics computer simulations to investigate the relaxation
dynamics of a binary Lennard-Jones liquid confined in a narrow pore. We find
that the average dynamics is strongly influenced by the confinement in that
time correlation functions are much more stretched than in the bulk. By
investigating the dynamics of the particles as a function of their distance
from the wall, we can show that this stretching is due to a strong dependence
of the relaxation time on this distance, i.e. that the dynamics is spatially
very heterogeneous. In particular we find that the typical relaxation time of
the particles close to the wall is orders of magnitude larger than the one of
particles in the center of the pore.Comment: 9 pages of Latex, 4 figure

### On the Adam-Gibbs-Wolynes scenario for the viscosity increase in glasses

We reformulate the interpretation of the mean-field glass transition scenario
for finite dimensional systems, proposed by Wolynes and collaborators.
This allows us to establish clearly a temperature dependent length xi* above
which the mean-field glass transition picture has to be modified. We argue in
favor of the mosaic state introduced by Wolynes and collaborators, which leads
to the Adam-Gibbs relation between the viscosity and configurational entropy of
glass forming liquids.
Our argument is a mixture of thermodynamics and kinetics, partly inspired by
the Random Energy
Model: small clusters of particles are thermodynamically frozen in low energy
states, whereas large clusters are kinetically frozen by large activation
energies. The relevant relaxation time is that of the smallest `liquid'
clusters. Some physical consequences are discussed.Comment: 8 page

### Low Velocity Granular Drag in Reduced Gravity

We probe the dependence of the low velocity drag force in granular materials
on the effective gravitational acceleration (geff) through studies of spherical
granular materials saturated within fluids of varying density. We vary geff by
a factor of 20, and we find that the granular drag is proportional to geff,
i.e., that the granular drag follows the expected relation Fprobe = {\eta}
{\rho}grain geff dprobe hprobe^2 for the drag force, Fprobe on a vertical
cylinder with depth of insertion, hprobe, diameter dprobe, moving through
grains of density {\rho}grain, and where {\eta} is a dimensionless constant.
This dimensionless constant shows no systematic variation over four orders of
magnitude in effective grain weight, demonstrating that the relation holds over
that entire range to within the precision of our data

### Imbibition in mesoporous silica: rheological concepts and experiments on water and a liquid crystal

We present, along with some fundamental concepts regarding imbibition of
liquids in porous hosts, an experimental, gravimetric study on the
capillarity-driven invasion dynamics of water and of the rod-like liquid
crystal octyloxycyanobiphenyl (8OCB) in networks of pores a few nanometers
across in monolithic silica glass (Vycor). We observe, in agreement with
theoretical predictions, square root of time invasion dynamics and a sticky
velocity boundary condition for both liquids investigated.
Temperature-dependent spontaneous imbibition experiments on 8OCB reveal the
existence of a paranematic phase due to the molecular alignment induced by the
pore walls even at temperatures well beyond the clearing point. The ever
present velocity gradient in the pores is likely to further enhance this
ordering phenomenon and prevent any layering in molecular stacks, eventually
resulting in a suppression of the smectic phase in favor of the nematic phase.Comment: 18 pages, 8 figure

### Adaptives Grundwassermanagement in urbanen Gebieten: Einfluss der OberflĂ¤chengewĂ¤sser-Grundwasser-Interaktion am Beispiel kĂĽnstlicher Grundwasseranreicherung sowie variabler In-/Exfiltration der Birs (Schweiz)

Zusammenfassung: Der Nutzungsdruck auf Grundwasserressourcen in intensiv genutzten Flussebenen wird zunehmend grĂ¶Ăźer. Ziel dieses Beitrages ist, anhand eines reprĂ¤sentativen, instationĂ¤ren Datensatzes eines urbanen Grundwassersystems (Unteres Birstal CH) aufzuzeigen, wie mit AnsĂ¤tzen des adaptiven Grundwassermanagements die Voraussetzungen fĂĽr die nachhaltige Nutzung von Grundwasserressourcen geschaffen werden kĂ¶nnen. Mithilfe eines instationĂ¤ren Grundwassermodells kĂ¶nnen spezifische Fragen der Fluss-Grundwasser-Interaktion und dem Betrieb kĂĽnstlicher Grundwasseranreicherung beantwortet werden. Die InstationaritĂ¤t der Fluss-Grundwasser-Interaktion und eine damit zusammenhĂ¤ngende Ă„nderung von In - und ExfiltrationsverhĂ¤ltnissen konnte fĂĽr verschiedene Flussabschnitte ermittelt werden. Die Datenauswertung eines Jahrhunderthochwassers trug wesentlich zum VerstĂ¤ndnis dieser Prozesse bei. Durch ein Experiment mit der lĂ¤ngerfristigen AuĂźerbetriebnahme einer kĂĽnstlichen Grundwasseranreicherung im Untersuchungsgebiet konnten die Auswirkungen von geplanten NutzungsĂ¤nderungen abgeschĂ¤tzt werden. Die Untersuchungen tragen zum ProzessverstĂ¤ndnis des Grundwassersystems bei und liefern die Grundlage fĂĽr eine Diskussion ĂĽber lang-, mittel- und kurzfristige Ziele hinsichtlich der regionalen Bewirtschaftung urbaner Wasserressource

### Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling

In this first of three articles on the optical absorption of electrons in
half-filled Peierls-distorted chains we present analytical results for
non-interacting tight-binding electrons. We carefully derive explicit
expressions for the current operator, the dipole transition matrix elements,
and the optical absorption for electrons with a cosine dispersion relation of
band width $W$ and dimerization parameter $\delta$. New correction
(``$\eta$''-)terms to the current operator are identified. A broad band-to-band
transition is found in the frequency range $W\delta < \omega < W$ whose shape
is determined by the joint density of states for the upper and lower Peierls
subbands and the strong momentum dependence of the transition matrix elements.Comment: 17 pages REVTEX 3.0, 2 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine

### Slow dynamics of a confined supercooled binary mixture II: Q space analysis

We report the analysis in the wavevector space of the density correlator of a
Lennard Jones binary mixture confined in a disordered matrix of soft spheres
upon supercooling. In spite of the strong confining medium the behavior of the
mixture is consistent with the Mode Coupling Theory predictions for bulk
supercooled liquids. The relaxation times extracted from the fit of the density
correlator to the stretched exponential function follow a unique power law
behavior as a function of wavevector and temperature. The von Schweidler
scaling properties are valid for an extended wavevector range around the peak
of the structure factor. The parameters extracted in the present work are
compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys.
Rev.

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