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Plenarvorträge - DPG-Tagungen

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Dynamik und Statistische Physik Montag<br />

DY 15 Fluids I<br />

Zeit: Montag 14:30–16:15 Raum: H3<br />

DY 15.1 Mo 14:30 H3<br />

Different Modes of Spinodal Dewetting in a Two-Layer system<br />

— •Andrey Pototsky 1 , Michael Betsehorn 1 , Domnic Merkt 1<br />

und Uwe Thiele 2 — 1 Lehrstuhl fuer Theoretische Physik II, BTU Cottbus,<br />

Erich-Weinert Str. 1, D-03046 Cottbus, Germany — 2 MPI komplexer<br />

Systeme, Noethnitzer Str. 38, D-01187, Dresden, Germany<br />

We consider two ultra thin layers of different liquids on a solid substrate.<br />

Using long wave theory coupled evolution equations for the free<br />

liquid-liquid and liquid-gas interfaces are derived taken into account nonretarded<br />

Van der Waals forces. Analisis of their linear and non-linear behavior<br />

shows that, both, varicose and zigzag modes can be unstable and<br />

lead to film rupture at the inner interface or substrate. The results are<br />

exemplified using a Si/PS/PMMA/air system.<br />

DY 15.2 Mo 14:45 H3<br />

Dynamics of self-excited droplet oscillations in electrowetting<br />

— •Jean-Christophe Baret 1,2 , Dagmar Steinhauser 1 , Ralf<br />

Seemann 1 , Stephan Herminghaus 1,3 , and Frieder Mugele 1<br />

— 1 Universitaet Ulm, Abt. Angewandte Physik, D-89069 Ulm —<br />

2 Philips Research, Eindhoven (NL) — 3 Max Planck Institut fuer<br />

Stroemungsforschung, D-37018 Goettingen<br />

Under suitable conditions, liquid droplets in electrowetting experiments<br />

can perform self-excited oscillations, as shown recently in Ref.<br />

1. Here, we present an extension of these previous experiments to higher<br />

and much better defined oscillation frequencies. We deposited droplets<br />

of a conductive liquid (≈ 1 nl) on top of a Si wafer, with an insulation<br />

layer (1 µm SiO2) and a monolayer of octadecyltrichlorosilane. We applied<br />

an AC voltage (10 kHz) between the substrate and a Pt wire that<br />

was immersed into the droplet at a distance d above the surface. Upon<br />

increasing the voltage U from zero to 100 V, the contact angle decreased<br />

from 130 deg to 70 deg. If d was chosen close to the height of the droplet<br />

at zero voltage, the droplet detached from the Pt wire upon increasing<br />

the voltage. Within a certain range of d and U, droplets periodically<br />

jumped on and off the wire. The oscillation frequency varied between 50<br />

Hz and 120 Hz for droplet sizes between 1mm and 0.1mm. Close to the<br />

critical values of d and U, where the oscillations cease, we found chaotic<br />

behavior. Furthermore, we demonstrate microfluidic mixing in oscillating<br />

droplets.<br />

(1) A. Klingner, S. Herminghaus, and F. Mugele, Appl. Phys. Lett. 82,<br />

4187 (2003)<br />

DY 15.3 Mo 15:00 H3<br />

Holes and fingers in vertically vibrated aqueous suspensions.*<br />

— •Florian S. Merkt 1,2 , Robert D. Deegan 1 , Daniel I. Goldman<br />

1 , Erin C. Rericha 1 , and Harry L. Swinney 1 — 1 University of<br />

Texas at Austin — 2 presently at the Fritz-Haber-Institut der MPG<br />

We have observed stable holes in a sinusoidally oscillated 0.5 cm deep<br />

aqueous suspension of cornstarch for accelerations a above 10g and frequencies<br />

of 50-200 Hz. Holes appear only if a finite perturbation is applied<br />

to the layer. They are circular and approximately 0.5 cm wide and can<br />

persist for more than 10 5 oscillation periods. Above a ≈ 17g the rim<br />

of the hole becomes unstable producing finger-like protrusions or hole<br />

division. At even higher accelerations, the hole delocalizes and grows to<br />

cover the entire surface with erratic undulations. We have found similar<br />

behavior in an aqueous suspension of glass microspheres and argue that<br />

shear thickening fluids in general exhibit these patterns.<br />

*Supported by DOE Grant DE-FG03-93ER14312<br />

DY 15.4 Mo 15:15 H3<br />

On the formation of the primary bead in the droplet detachment<br />

process of an elastic liquid — •Christian Wagner 1 ,<br />

Yacine Amarouchene 2 , Daniel Bonn 2 , and Jens Eggers 3 —<br />

1 Experimentalphysik, Universität des Saarlandes, Postfach 151150, 66041<br />

Saarbrücken, Germany — 2 Laboratoire de Physique Statistique, UMR<br />

CNRS 8550, Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris<br />

Cedex 05, France — 3 School of Mathematics, University of Bristol, University<br />

Walk Bristol BS8, 1TW United Kingdom<br />

The droplet detachment process of an elastic liquid is characterized<br />

by the suppression of the finite time singularity and appearance of a<br />

cylindrical filament between the droplet and the nozzle. Synchronously<br />

to the appearance of the filament secondary droplets, so called beads<br />

on the string can be observed too. Most reports concern the appearance<br />

of beads at the final stage of the droplet detachment process but under<br />

certain conditions they are to observe from the beginning of the filament<br />

forming process. In this study we present the phase boundaries for these<br />

beads. Their appearance is connected with a transition from a symmetric<br />

to an asymmetric shape of the droplet neck in the Newtonian regime<br />

before the onset of the elastic behavior. This transition is is properly<br />

described with a crossover from the symmetric instability solution of the<br />

pending droplet to a self similar solution for the pinch of behavior of low<br />

viscous Newtonian liquids and the phase boundaries are determined by<br />

the ratio of the elongational rate and the Reynoldsnumber.<br />

DY 15.5 Mo 15:30 H3<br />

Phase-field model for evaporation with convection in two-layer<br />

systems — •Rodica Borcia and Michael Bestehorn — Lehrstuhl<br />

für Theoretische Physik II, Brandenburgische Technische Universität<br />

Cottbus, Erich-Weinert-Straße 1, 03046, Cottbus, Germany<br />

We propose a phase-field model for analyzing the influence of evaporation<br />

phenomenon on Marangoni convection in liquid-vapor systems. The<br />

phase-field model treats the problem continuously (all the system parameters<br />

vary continuously from one medium to another), and avoids interface<br />

conditions. Therefore, evaporation with convection in multi-layers<br />

structures can be discussed in a more natural way. The theoretical description<br />

is based on the Navier-Stokes equation with some extra-terms<br />

responsible for describing Marangoni convection [1], the heat equation<br />

with a supplementary term responsible for describing evaporation phenomena<br />

[2], and the continuity equation. We report on two-dimensional<br />

simulations for both Marangoni instabilities in linear approximation and<br />

we compare the results with the literature.<br />

[1] R. Borcia, M. Bestehorn, Phys. Rev. E 67, 066307 (2003).<br />

[2] R.J. Braun, B.T. Murray, J. Cryst. Growth 174, 41 (1997).<br />

DY 15.6 Mo 15:45 H3<br />

Schwingungsverhalten und innere Strömung bei verdampfenden<br />

Wassertropfen — •Frank Rietz, Wolfgang Jantoß und<br />

Stefan C. Müller — Universität Magdeburg, Fakultät für Naturwissenschaften,<br />

Abteilung Biophysik<br />

Wird eine kleine Menge Wasser auf eine heiße Platte gegeben, so<br />

schwebt der sich bildende Tropfen aufgrund des Leidenfrost-Effekts.<br />

Dies ist jedem bekannt. Dass sich aber unter bestimmten Bedingungen<br />

faszinierende Strukturbildungsphänomene des Wassertropfens beobachten<br />

lassen, entzieht sich der allgemeinen Kenntnis. Es entstehen<br />

nämlich sternförmige Oszillationen, die durch Schwankungen der Oberflächenspannung<br />

und des Temperaturgradienten angeregt werden. Der<br />

Tropfen kann mit bis zu 4 cm Durchmesser in zahlreichen Moden schwingen,<br />

deren Kenngrößen vorgestellt werden sollen. Neben diesen Ergebnissen<br />

liegt das Augenmerk auf die durch Particle Image Velocimetry beobachtbare<br />

Konvektion im Inneren des Tropfens. Die zugrunde liegenden<br />

Mechanismen verleiten zu dem Schluss, die Schwingungen in Zusammenhang<br />

mit der Marangoni-Instabilität zu betrachten.<br />

DY 15.7 Mo 16:00 H3<br />

Dynamics in spatially confined dipolar liquids — •Sabine H.L.<br />

Klapp — Stranski-Laboratorium für Physikalische und Theoretische<br />

Chemie, Sekr. TC 7, Fakultät II, Technische Universität Berlin, Straße<br />

des 17 Juni 124, 10623 Berlin, Germany<br />

Based on Molecular Dynamics simulations we investigate the interplay<br />

between static and dynamic properties of confined films of simple dipolar<br />

fluids, which can be considered as model systems for confined polar<br />

liquids or ferrocolloid films. Previous Monte Carlo studies (1,2) on such<br />

systems have shown that the specific properties of dipolar interactions,<br />

combined with the presence of confining walls, yields interesting orientational<br />

effects such as wall–induced chain formation in the contact layer<br />

and spontaneous global polarization at pressures lower than in the bulk.<br />

Here we discuss the implications of these features on translational and<br />

rotational time correlation functions and on resulting transport coefficients.<br />

1) S. H. L. Klapp and M. Schoen, J. Chem. Phys. 117, 8050 (2002).<br />

2) S. H. L. Klapp and M. Schoen, in press (2003).

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