Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Montag
DY 15 Fluids I
Zeit: Montag 14:30–16:15 Raum: H3
DY 15.1 Mo 14:30 H3
Different Modes of Spinodal Dewetting in a Two-Layer system
— •Andrey Pototsky 1 , Michael Betsehorn 1 , Domnic Merkt 1
und Uwe Thiele 2 — 1 Lehrstuhl fuer Theoretische Physik II, BTU Cottbus,
Erich-Weinert Str. 1, D-03046 Cottbus, Germany — 2 MPI komplexer
Systeme, Noethnitzer Str. 38, D-01187, Dresden, Germany
We consider two ultra thin layers of different liquids on a solid substrate.
Using long wave theory coupled evolution equations for the free
liquid-liquid and liquid-gas interfaces are derived taken into account nonretarded
Van der Waals forces. Analisis of their linear and non-linear behavior
shows that, both, varicose and zigzag modes can be unstable and
lead to film rupture at the inner interface or substrate. The results are
exemplified using a Si/PS/PMMA/air system.
DY 15.2 Mo 14:45 H3
Dynamics of self-excited droplet oscillations in electrowetting
— •Jean-Christophe Baret 1,2 , Dagmar Steinhauser 1 , Ralf
Seemann 1 , Stephan Herminghaus 1,3 , and Frieder Mugele 1
— 1 Universitaet Ulm, Abt. Angewandte Physik, D-89069 Ulm —
2 Philips Research, Eindhoven (NL) — 3 Max Planck Institut fuer
Stroemungsforschung, D-37018 Goettingen
Under suitable conditions, liquid droplets in electrowetting experiments
can perform self-excited oscillations, as shown recently in Ref.
1. Here, we present an extension of these previous experiments to higher
and much better defined oscillation frequencies. We deposited droplets
of a conductive liquid (≈ 1 nl) on top of a Si wafer, with an insulation
layer (1 µm SiO2) and a monolayer of octadecyltrichlorosilane. We applied
an AC voltage (10 kHz) between the substrate and a Pt wire that
was immersed into the droplet at a distance d above the surface. Upon
increasing the voltage U from zero to 100 V, the contact angle decreased
from 130 deg to 70 deg. If d was chosen close to the height of the droplet
at zero voltage, the droplet detached from the Pt wire upon increasing
the voltage. Within a certain range of d and U, droplets periodically
jumped on and off the wire. The oscillation frequency varied between 50
Hz and 120 Hz for droplet sizes between 1mm and 0.1mm. Close to the
critical values of d and U, where the oscillations cease, we found chaotic
behavior. Furthermore, we demonstrate microfluidic mixing in oscillating
droplets.
(1) A. Klingner, S. Herminghaus, and F. Mugele, Appl. Phys. Lett. 82,
4187 (2003)
DY 15.3 Mo 15:00 H3
Holes and fingers in vertically vibrated aqueous suspensions.*
— •Florian S. Merkt 1,2 , Robert D. Deegan 1 , Daniel I. Goldman
1 , Erin C. Rericha 1 , and Harry L. Swinney 1 — 1 University of
Texas at Austin — 2 presently at the Fritz-Haber-Institut der MPG
We have observed stable holes in a sinusoidally oscillated 0.5 cm deep
aqueous suspension of cornstarch for accelerations a above 10g and frequencies
of 50-200 Hz. Holes appear only if a finite perturbation is applied
to the layer. They are circular and approximately 0.5 cm wide and can
persist for more than 10 5 oscillation periods. Above a ≈ 17g the rim
of the hole becomes unstable producing finger-like protrusions or hole
division. At even higher accelerations, the hole delocalizes and grows to
cover the entire surface with erratic undulations. We have found similar
behavior in an aqueous suspension of glass microspheres and argue that
shear thickening fluids in general exhibit these patterns.
*Supported by DOE Grant DE-FG03-93ER14312
DY 15.4 Mo 15:15 H3
On the formation of the primary bead in the droplet detachment
process of an elastic liquid — •Christian Wagner 1 ,
Yacine Amarouchene 2 , Daniel Bonn 2 , and Jens Eggers 3 —
1 Experimentalphysik, Universität des Saarlandes, Postfach 151150, 66041
Saarbrücken, Germany — 2 Laboratoire de Physique Statistique, UMR
CNRS 8550, Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris
Cedex 05, France — 3 School of Mathematics, University of Bristol, University
Walk Bristol BS8, 1TW United Kingdom
The droplet detachment process of an elastic liquid is characterized
by the suppression of the finite time singularity and appearance of a
cylindrical filament between the droplet and the nozzle. Synchronously
to the appearance of the filament secondary droplets, so called beads
on the string can be observed too. Most reports concern the appearance
of beads at the final stage of the droplet detachment process but under
certain conditions they are to observe from the beginning of the filament
forming process. In this study we present the phase boundaries for these
beads. Their appearance is connected with a transition from a symmetric
to an asymmetric shape of the droplet neck in the Newtonian regime
before the onset of the elastic behavior. This transition is is properly
described with a crossover from the symmetric instability solution of the
pending droplet to a self similar solution for the pinch of behavior of low
viscous Newtonian liquids and the phase boundaries are determined by
the ratio of the elongational rate and the Reynoldsnumber.
DY 15.5 Mo 15:30 H3
Phase-field model for evaporation with convection in two-layer
systems — •Rodica Borcia and Michael Bestehorn — Lehrstuhl
für Theoretische Physik II, Brandenburgische Technische Universität
Cottbus, Erich-Weinert-Straße 1, 03046, Cottbus, Germany
We propose a phase-field model for analyzing the influence of evaporation
phenomenon on Marangoni convection in liquid-vapor systems. The
phase-field model treats the problem continuously (all the system parameters
vary continuously from one medium to another), and avoids interface
conditions. Therefore, evaporation with convection in multi-layers
structures can be discussed in a more natural way. The theoretical description
is based on the Navier-Stokes equation with some extra-terms
responsible for describing Marangoni convection [1], the heat equation
with a supplementary term responsible for describing evaporation phenomena
[2], and the continuity equation. We report on two-dimensional
simulations for both Marangoni instabilities in linear approximation and
we compare the results with the literature.
[1] R. Borcia, M. Bestehorn, Phys. Rev. E 67, 066307 (2003).
[2] R.J. Braun, B.T. Murray, J. Cryst. Growth 174, 41 (1997).
DY 15.6 Mo 15:45 H3
Schwingungsverhalten und innere Strömung bei verdampfenden
Wassertropfen — •Frank Rietz, Wolfgang Jantoß und
Stefan C. Müller — Universität Magdeburg, Fakultät für Naturwissenschaften,
Abteilung Biophysik
Wird eine kleine Menge Wasser auf eine heiße Platte gegeben, so
schwebt der sich bildende Tropfen aufgrund des Leidenfrost-Effekts.
Dies ist jedem bekannt. Dass sich aber unter bestimmten Bedingungen
faszinierende Strukturbildungsphänomene des Wassertropfens beobachten
lassen, entzieht sich der allgemeinen Kenntnis. Es entstehen
nämlich sternförmige Oszillationen, die durch Schwankungen der Oberflächenspannung
und des Temperaturgradienten angeregt werden. Der
Tropfen kann mit bis zu 4 cm Durchmesser in zahlreichen Moden schwingen,
deren Kenngrößen vorgestellt werden sollen. Neben diesen Ergebnissen
liegt das Augenmerk auf die durch Particle Image Velocimetry beobachtbare
Konvektion im Inneren des Tropfens. Die zugrunde liegenden
Mechanismen verleiten zu dem Schluss, die Schwingungen in Zusammenhang
mit der Marangoni-Instabilität zu betrachten.
DY 15.7 Mo 16:00 H3
Dynamics in spatially confined dipolar liquids — •Sabine H.L.
Klapp — Stranski-Laboratorium für Physikalische und Theoretische
Chemie, Sekr. TC 7, Fakultät II, Technische Universität Berlin, Straße
des 17 Juni 124, 10623 Berlin, Germany
Based on Molecular Dynamics simulations we investigate the interplay
between static and dynamic properties of confined films of simple dipolar
fluids, which can be considered as model systems for confined polar
liquids or ferrocolloid films. Previous Monte Carlo studies (1,2) on such
systems have shown that the specific properties of dipolar interactions,
combined with the presence of confining walls, yields interesting orientational
effects such as wall–induced chain formation in the contact layer
and spontaneous global polarization at pressures lower than in the bulk.
Here we discuss the implications of these features on translational and
rotational time correlation functions and on resulting transport coefficients.
1) S. H. L. Klapp and M. Schoen, J. Chem. Phys. 117, 8050 (2002).
2) S. H. L. Klapp and M. Schoen, in press (2003).