Plenarvorträge - DPG-Tagungen

Symposium Physics of Foams Mittwoch
Fachsitzungen
– Haupt-, Kurzvorträge und Posterbeiträge –
SYPF 1 Foams I
Zeit: Mittwoch 14:00–14:30 Raum: H16
Hauptvortrag SYPF 1.1 Mi 14:00 H16
The foam/emulsion analogy in structure and drainage —
•Stefan Hutzler, Nicolas Peron, Denis Weaire, and Wiebke
Drenckhan — Physics Department, Trinity College Dublin, Ireland
In recent years the study of the physics of foams has advanced across a
wide front. Often the analysis of a property will begin with the assertion
that the same description applies equally well to an emulsion. However,
this is rarely tested.
We have therefore revisited some of the key experiments, using a watersilicone
oil emulsion system (stabilised by surfactants) of similar scale to
familiar foam systems. Here we present illustrations of several ordered
SYPF 2 Foams II
structures in cylindrical tubes, essentially identical to those observed in
foams.
We have also performed the common foam experiment of “forced
drainage” on these emulsions, i.e. adding surfactant solution from the
top at constant flow rates. These experiments suggest that drainage is of
the Poiseuille type flow of liquid past the drops.
At high flow rates the drops exhibit oscillations within the emulsion,
eventually leading to a break-up of the arrangement. This differs from
the behaviour of foams, where varies types of convective instabilities set
in, once a critical flow rate, dependent on bubble size and geometry, is
exceeded.
Zeit: Mittwoch 14:30–15:50 Raum: H16
SYPF 2.1 Mi 14:30 H16
The role of geometrical optics for light transport in dry foams
— •Holger Stark 1 and MirFaez Miri 2 — 1 Universität Konstanz,
Fachbereich Physik, D-78457 Konstanz, Germany — 2 Institute For Advanced
Studies in Basic Sciences, Zanjan, Iran
Recent light-scattering experiments on foams suggest that the light
transport is diffusive. This means that single photons can be considered
as random walkers. However, there is a debate in literature about the
main mechanism underlying the random walk. One suggestion is that it
is based on light scattering from Plateau borders and/or vertices. On the
other hand, since the cells are much larger than the wavelength of light,
one can employ geometrical optics and follow a light beam or photon as
it is reflected by the liquid films with a probability r called the intensity
reflectance. This naturally leads to a random walk of the photons in
space.
Here we study the second mechanism based on geometrical optics in
two steps using numerical simulations. In a first model, we introduce
topological and geometrical disorder based on a 2D Voronoi foam model
and choose a constant r to investigate the influence of disorder. In a
second model, we use the realistic intensity reflectance r of thin films
and also consider disorder in the film thickness. Comparing our results
to recent experiments, we find that the geometrical optics approach reproduces
experimental features and the right order of magnitude for the
diffusion constant. This convinces us that geometrical optics is an important
mechanism for the diffusive light transport in foams.
SYPF 2.2 Mi 14:50 H16
Effect of colloidal charges on the thickness of foam films —
•Regine v. Klitzing, Branko Kolaric, and Tatjana Mauser
— Stranski-Laboratorium, TU Berlin, Str. d. 17. Juni 112, 10623 Berlin
Polymers can be used as stabilizers of foams. To use polymers in an
efficient way in surfactant systems, it is necessary to control and to manipulate
the building blocks, i.e. the single free-standing surfactant films
of a foam. In our experiments the film thickness is controlled on a mesoscopic
length scale (5-100 nm) by varying the outer pressure in a thin
film pressure balance. The sum of interactions (DLVO, steric, structural
forces) between the film surfaces is determined quantitatively by the disjoining
pressure. The addition of polyelectrolytes affects the (de)stability
of the film due to complexation between polymer and surfactant at the
film surfaces. In dependence on the charge combination of the surfactant
and polyelectrolyte either a Newton Black Film (NBF, stabilized by steric
repulsion) or a Common Black Film (CBF, stabilized by electrostatic repulsion)
occurs at high outer pressure. The influence of polyelectrolytes
on the film stability is compared to the effect of hard colloids.
SYPF 2.3 Mi 15:10 H16
Single foam films and macroscopic foams stabilized by CnTAB:
influence of the chain length — •Cosima Stubenrauch 1 , Judith
Schlarmann 1 , and Khristo Khristov 2 — 1 Institut für Physikalische
Chemie, Universität zu Köln, Luxemburgerstr. 116, D-50939 Köln,
Deutschland — 2 Institute of Physical Chemistry, Bulgarian Academy of
Sciences, u. Acad, G. Bonchev 11, Sofia 1113, Bulgarien
A quantitative comparison of single foam films and the corresponding
macroscopic foams is very difficult. One problem is the fact that investigations
of foam films are usually performed at constant capillary pressures
P, whereas in macroscopic foams P is a function of the height of the
foam column. A way out of this dilemma is to examine films and foams
at the same P. The method of choice for the foam films is the ”thin film
pressure balance ” (TFPB) with which the film thickness is measured as
a function of P. The corresponding investigation of macroscopic foams is
based on the ”foam pressure drop technique” (FPDT). As in the TFPB
a constant P is applied at which the rate of drainage as well as the lifetime
of the foam is determined. In order to study the correlation between
the properties of films and foams we investigated the homologous series
of the cationic alkyl trimethyl ammonium bromides CnTAB with n =
10,12,14, and 16. For this series a strong increase of foam film stability
was observed when the chain length was increased from n = 12 to n =
14. In order to show that this change is also reflected in the properties of
the macroscopic foam we measured the lifetime and the drainage of the
respective CnTAB foams with the FPDT as a function of P. The results
will be compared and discussed.
SYPF 2.4 Mi 15:30 H16
Foam Analysis by the Foam Pressure Drop Technique — •R.
Miller 1 , C. Stubenrauch 2 , Khr. Khristov 3 , L. Christov 3 ,
A.V. Makievski 4 , and D. Exerowa 3 — 1 MPI für Kolloid- und
Grenzfl¨chenforschung, Am Mühlenberg 1, D-14476 Golm, Germany
— 2 Institute Phys. Chem., University Cologne, Luxemburger Str.
116, D-50939 Cologne, Germany — 3 Institute of Physical Chemistry,
Bulgarian Academy of Sciences, Sofia 1113, Bulgaria — 4 SINTERFACE
Technologies, Volmer Str. 5-7, D-12489 Berlin, Germany
Foam is a complex capillary system and the processes of foam drainage
and foam destruction (stability) are important for its properties. These
processes are interrelated in a very peculiar way. The questions on which
would be the predominating mechanism or how would the processes impact
on each other depend entirely on the characteristics of the foam.
The complex relationships make it very hard to assess the different processes
independently. Moreover, for many application the drainage is important,
for example in firefighting, food industry, while for others the
lifetime (stability) plays the major role, such as in foam concentration,
foam separation.
The characterisation of foams by the foam pressure drop technique