229 research outputs found

    Cooperative motion and growing length scales in supercooled confined liquids

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    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

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    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

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    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

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    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

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    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

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    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 Oberflächengewässer-Grundwasser-Interaktion am Beispiel künstlicher Grundwasseranreicherung sowie variabler In-/Exfiltration der Birs (Schweiz)

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    Zusammenfassung: Der Nutzungsdruck auf Grundwasserressourcen in intensiv genutzten Flussebenen wird zunehmend größer. Ziel dieses Beitrages ist, anhand eines repräsentativen, instationären Datensatzes eines urbanen Grundwassersystems (Unteres Birstal CH) aufzuzeigen, wie mit Ansätzen des adaptiven Grundwassermanagements die Voraussetzungen für die nachhaltige Nutzung von Grundwasserressourcen geschaffen werden können. Mithilfe eines instationären Grundwassermodells können spezifische Fragen der Fluss-Grundwasser-Interaktion und dem Betrieb künstlicher Grundwasseranreicherung beantwortet werden. Die Instationarität der Fluss-Grundwasser-Interaktion und eine damit zusammenhängende Änderung von In - und Exfiltrationsverhältnissen konnte für verschiedene Flussabschnitte ermittelt werden. Die Datenauswertung eines Jahrhunderthochwassers trug wesentlich zum Verständnis dieser Prozesse bei. Durch ein Experiment mit der längerfristigen Außerbetriebnahme einer künstlichen Grundwasseranreicherung im Untersuchungsgebiet konnten die Auswirkungen von geplanten Nutzungsänderungen abgeschätzt werden. Die Untersuchungen tragen zum Prozessverständnis des Grundwassersystems bei und liefern die Grundlage für eine Diskussion über 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

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    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 WW and dimerization parameter δ\delta. New correction (``η\eta''-)terms to the current operator are identified. A broad band-to-band transition is found in the frequency range Wδ<ω<WW\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

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    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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