MICROCAD 2005
EXPERIMENTS ON PARTICLES STABILIZED AQUEOUS FOAMS
Somosvári Béla1, Martin Meier2
1
PhD student, 2Dr. – Ing.
1
University of Miskolc, Department of Polymer Engineering, 2Brandenburg
University of Technology Cottbus
1. INTRODUCTION
Foams are disperse systems, in which gas filled bubbles are distributed in a liquid or
solid. If the bubbles fill more than 50% of the volume, we can talk about foams.
„Fluid foams” consist of gas bubbles separated by a continuous film of liquid.
These kinds of materials stimulate truly interdisciplinary fundamental research with
a wide range of industrial applications. [1]
According to their low density and high specific surface per unit mass, they have
many applications. Many types of foam contain tiny solid particles in varied size
and concentration. These kind of foams can be observed in nature (hot streams near
volcanoes) and in human activity, too. Foams loaded with particles frequently occur
from fire fighting to food industry. The froth flotation technique in separating ores
takes advantage of the particles’ different wetting behaviour.
Particles can play a crucial role in stabilizing foams. In metal foam industry, it is a
generally established fact that without them metallic foams cannot exist. They can
act through enhancing the bulk viscosity of the liquid thus slowing down the
drainage, and through their wetting behaviour by forming ordered structures on the
surface and obstruct thinning and rupturing. The effect of particles can highly
depend on the concentration, size (dispersity), shape and interfacial feature. [5]
Particle loaded aqueous foams have to be given much attendance due to their
important role in industry. They are also a good model system for other kind of
foams because they are transparent and can be formed at room temperature. [1][2]
2. OBJECTIVES
2.1. The role of particles in aqueous foams
The stabilizing effect is primarily due to interfacial forces and secondarily driven by
the enhancement of bulk viscosity. Particles with a given range of size (nano -or
micron) and wetting behaviour can modify the properties of the fluid in a good
direction.
The effect of particles in foam strongly depends on size, concentration and wetting
behaviour. Stabilization of liquid foams by solid particles takes place only if the
contact angle is at a certain value. A multi-layered network of particles can take
shape at the liquid-gas interface. This network can separate elastically the
neighbouring liquid-gas interfaces with a relatively stable liquid film in between.
[3]
One of the ESA MAP project’s (called Development of Advanced Foams under
Microgravity, AO-99075) main task is making ground-based investigation of
particle stabilization on bubble surface and thin liquid films with special attention
on foaming phenomena. The aim of this experimental study referring to the abovementioned task is to investigate whether real foam can or cannot be made with pure
water and particles only, in the lack, or at a very low concentration of surface-active
agent. Experimental series are part In order to investigate different effects of
particles on the foam structure and stability, a foam monitoring facility is under
development in the Technical University of Brandenburg, Cottbus, with the leading
of Dr. Martin Meier. [4]
The experimental method is in situ monitoring of the foaming process (by gas inlet)
and the foam decay. Measuring foam life and foaming time can be a simple and
clear method to deduce the role of particles in stabilizing foam structures.
2.2 Foam monitoring instrument
The aim of the development is to give the possibility of fully automated
measurements on different properties of foams. The main part of the facility is the
foam column where the evolving structure and the decay of the foam can be
monitored. Webcamera is used for following the foaming and decay process and a
high-resolution CCD camera with a frame grabber card is also involved for making
pictures of the cell walls, Plateau-borders and the motion of particles.
Other parts of the device are the mixing vessel (7l storage capacity) with magnetic
stirrer, gas container (air), pumps and valves for conducting the liquid flow, light
and infrared source for enhancing the contrast and sensitivity of the cameras.
The pumps, valves and the light can be controlled from a PC through a LabviewTM
virtual instrument (VI).
The polycarbonate foam columns are exchangeable so one can change the diameter
of the investigated foam and the nozzle plates can be also varied.
3. EXPERIMENTS
3.1. Materials
High purity deionised water and SDS (Sodium – Dodecyl – Sulphate, 90% purity in
crystalline state, CMC is 0,23 mass%) is used for preparing the solution. The
concentration of SDS was only enhanced if foam could not be created from the
mixture. For creating aqueous suspension foams both polydisperse and
monodisperse hollow sphere glass particles were chosen.
Table 1
Properties of different particles used in the experiments.
Particle’s name
SPHERICEL
110P8 “Potters”
Alpha
“AA5”
Aesar
Range of size
Sedimentation
tendency
Polydisperse, from Low
nm to 20microns
diameter
5 Monodisperse,
microns diameter
5 Low
Description
Potters Industries Inc.,
hollow glass spheres
(SiO2) hydrophilic
Johnson Mattley
hydrophilic
Inc.,
3.2. Foam life measurements
Polydisperse hollow sphere glass particles (“Potters”) were used for making
measurements on suspension foams. During the experiments, a multi-nozzle plate
with 1 mm size holes and a 100mm diameter foam column were used. Foaming gas
was compressed air, introducing from below. Overpressure changed between 0.250.5 bars. Foaming up was always made from approximately 1 litre of solution in the
foam column. The whole amount of the mixture was 7 litres in order to avoid
unhomogeneity in the composition. Each experiment was carried out seven times.
From the results, one can determine the mean value and the scattering. In the
following pictures shown in the document, the error bars represent the average
deviation from the average value. Error of time measuring was app. 1 second and
the error of measuring height was app. 0.5cm. Biggest errors (approximately 530%) occurred while determining the remained volume fraction of foam.
A 3D diagram showing the foam life in the function of SDS and particles
concentration is placed in Figure 1. Foam life is measured from stopping the gas
introduction until the foam reaches its 10-15% remnant volume fraction.
Figure 1
3D diagram of foam life measurement carried out on “Potters” – type particles. The
horizontal axes are SDS mass percentage, and Potters mass percentage and the
vertical one is the foam life, measured in minutes.
From this diagram, it is clear that these kinds of particles have a stabilizing effect to
the foam at higher concentrations. White colour of the Plateau-borders and nodes
can be observed at 4 and 8 mass percents of “Potters”.
Foam life increased with increasing concentration of particles. Mean scatter was
quite big. This uncertainty exists due to the following possible reasons:
Determining the end of the foam life highly depends on the observer because
it is hard to decide, that the foam reached 15% of its original volume fraction or not.
In order to avoid this effect further development of the instrument is needed.
Another thing is the aging of the particles, i.e. the particles may react with
the surfactants or the surfactant adsorb to the surface of the particles. This can be
solved only if one makes a lot of pre-experiment in order to map the aging
properties of the particles. Another solution is the changing of the mixture after
every experiment, but it would take a lot time and large amount of ingredients.
Sedimentation is another negative effect that can be avoided by constant
mixing of the solution. This happens in the mixing vessel, but not in the foam
column yet.
3.3 Foaming time measurements
A foaming time measurement starts at the beginning of the gas introduction and
ends when reaching the 30cm foam height. There were some cases where the foam
could not grow up to 30 cm even in 2 minutes. These cases were regarded as
instable foams. Each experiment was carried out 20 times and then their average
and standard deviation was calculated. The results of experiments carried out with
“AA5” type particles are shown below.
Figure 2
3D-plot of the change of foaming time by varying SDS and AA5 mass percents.
Foaming times were measured until the foam reached 30cm height. Elapsed time at
10 and 20 cms were also measured.
At low particle concentrations, the foam needed more time to reach the desired
height.
Figure 3 shows a foaming curve of a solution with 0.0075SDS mass% with different
concentrations of “AA5” particles.
Figure 3
Plot of foaming curves at 0,0075mass % SDS concentration. At large amount of
AA5 type particles inside the foaming time decreased significantly.
This big scatter would be strongly reduced after building in a gas pressure sensor or
a mass flow meter that could keep the gas flow constant. From Figure 3 one can
conclude that particles can significantly reduce foaming time. This means that there
are less ruptures occur in the foaming process at high particle concentration levels.
This effect slurs somehow at higher concentration of SDS, but the measurements
have to be also carried out in better instrumental conditions to see the truth.
5. CONCLUSION, FUTURE PLANS
A series of experiments were carried out with a foam monitoring facility at the
Technical University of Brandenburg, Cottbus, under the leadership of Dr. Martin
Meier. The aim of the investigations was to draw a development plan for the
instrument and to examine the role of particles in aqueous suspension foams.
Foam life measurements were taken with polydisperse hollow sphere glass particles
(“Potters”) at a low concentration of surfactant in the solution. These experiments
showed that at higher particle concentrations the foam exhibits higher stability. This
may due to the accumulation of particles in the Plateau-borders and nodes, thus
obstructing the drainage.
Another type of investigation was to measure the foaming time at different mass
percents of particles. The foaming time means the time elapsed between the 0cm
and the reaching of the 30cm height at a certain gas input level.
Results show that the particles have a quite clear effect on varying the foaming
time. Shorter times reached at high particle concentrations. In these ranges there
was much less ruptures during the foaming process.
All of the measurements have some contradictory results and quite poor
reproducibility which has to be cleared and reduced by developing the instrument
and using more sophisticated methods.
The instrument is planned to be developed in the following way:
- Building in easily controllable gas inlet and pressure sensors
- Enhancing the lightning conditions for the cameras
- Finding a way to determine easily the foam volume fraction in the column is
needed (Conductivity measurement).
Experimental research plans:
- Further investigation of particles with nm size and with well-defined
wettability at a low SDS mass % and even without surfactant
- Examination of particle arrangements on single cell walls
6. ACKNOWLEDGEMENTS
I would like thank the possibility for the one-month stay in BTU – Cottbus and for
the opportunity to use the foam monitoring facility to the following people: Dr.
Martin Meier (BTU-Cottbus), Andreas Stöckert (BTU-Cottbus), Dr. Babcsán
Norbert (TU-Berlin), Prof. Dr. Bárczy Pál (ME), Prof. Dr. Christoph Egbers (BTUCottbus), Kovács Árpád (ME).
REFERENCES
[1]
Dr. MEIER, Martin: The Bubble Bath Project. Technical University of
Cottbus, http://www.tu-cottbus.de/las/BubbleBath/indexover.htm
[2]
WEARIE, Dennis, HUTZLER, Stefan: Making, Modelling and Measuring
Foams. Europhysics News, May/June 1999
[3]
KAPTAY, György: Interfacial criteria for stabilization of liquid foams by
solid particles. Colloids and Surface A: Physicochem. Eng. Aspects 230 (2004) 6780
[4]
BÁRCZY, Pál: Development of advanced foams under microgravity,. ESA
PECS Project Meeting, BTU-Berlin, 2004. 15th October
[5]
MEIER, M., HILLE, D., WALLOT, G.: Experiments on the stability of solid
particle loaden aqueous foams. In: “Cellular Metals: Manufacture, Properties,
Applications” Editors: J. Banhart, N. A. Fleck, A. Mortensen (ISBN 3-935538-12X) Verl. MIT Publ. 2003, p 65 - 70