Drops and bubbles in interfacial research

729 Pages·1998·33.99 MB·English

by  D. MÖbius and R. Miller (Eds.)

STUDIES IN INTERFACE SCIENCE
Drops and Bubbles
in Interfacial Research
STUDIES IN INTERFACE SCIENCE
SERIES EDITORS
D. M6bius and R. Miller
Vol. I
Dynamics of Adsorption at Liquid Interfaces
Theory, Experiment, Application
by S.S. Dukhin, G. Kretzschmar and R. Miller
Vol. 2
An Introduction to Dynamics of Colloids
by J.K.G. Dhont
Vol. 3
Interracial Tensiometry
by A.I. Rusanov and V.A. Prokhorov
Vol. 4
New Developments in Construction
and Functions of Organic Thin Films
edited by T. Kajiyama and M. Aizawa
Vol. 5
Foam and Foam Films
by D. Exerowa and P.M. Kruglyakov
Vol. 6
Drops and Bubbles in Interfacial Research
edited by D. M6bius and R. Miller
Drops and Bubbles
in lnterfacial Research
Edited by
D. MOBIUS
Max-Planck-lnstitut fur Biophysikalische Chemie
P.O. Box 2841
G6ttingen
Germany
R. MILLER
Max-Planck-lnstitut fur Kolloid- und Grenzfl~ichenforschung
Rudower Chaussee 5
Berlin-Adlershof
Germany
I998
ELSEVIER
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Foreword
The shape of drops and bubbles and their specific properties make them a centre of interest of
many interfacial scientists. There are classical methods for measuring the surface and interfacial
tension, such as the maximum bubble pressure technique or the drop volume method.
Milestones in the development of the theoretical basis were set by Thomas Young in 1805 and
P.S. de Laplace in1806. On the basis of an analysis of the forces acting at an interface, they
derived a description of both the capillary elevation and the contact angle. While Young only
described the phenomena qualitatively, Laplace developed a very clear and well developed
physical and mathematical theory without any analytical solution, given in 1830 by Gauss. This
theory, named Gauss-Laplace-Equation is the basis for all coming methods on surface and
interfacial tension.
In the early years of this century many methods had been established as standard methods.
However, a number of physically principle questions were under discussion at that time.
Remember the discussion of Lohnstein and Tate in 1905 through 1913 about the correction
factors for the drop volume methods. In recent time this method was further developed to make
it applicable at drop times less than one second, which brings along additional problems
connected with the hydrodynamics of the drop formation and detachment. Also, in presence of
surfactants, surface rheological parameters come into play and have to be considered.
Another method, the maximum bubble pressure technique, used in literature for many decades,
has had a renaissance also very recently due to the availability of high precision pressure sensors
which made this method faster and more accurate and allow nowadays measurements of
dynamic surface tensions down to adsorption times of some hundred microseconds.
The shape of a pendent drop as another example gives access to the interfacial tension and
various developed sol, ware packages such as ADSA or ASTRA supply simultaneously the area
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and volume of the studied drops. This makes the pendent drop method very variable and allows
even interfacial relaxation experiments with a single liquid drop.
The spinning drop has been especially developed for ultra-low interfacial tensions. Experiments
have shown that such ultra-low tensions are important for the stability of emulsions and lead
under certain conditions even to a spontaneous emulsification. However this technique has a
number of limitations and needs attention when used at higher interfacial or surface tensions.
A completely new methodology for studying the dilational rheology of adsorption layers has
been developed only during the last 25 years, the oscillating bubble technique. This method
allows to determine the dilational rheology of adsorbed layers and simultaneously the exchange
of matter of these adsorbed molecules or other relaxation mechanisms. This technique has been
applied for so interesting scientific problems like the compression/expansion of layers formed
by lung surfactants. It is also a method of choice for fast matter exchange or interfacial
relaxation processes for example of mixed protein/surfactant systems the intrinsic mechanisms
of which is not understood so far.
The properties of rising bubbles in surfactants solutions is another highly interesting system in
which bubbles play a key role. The rising characteristics of a bubble in a liquid are significantly
influences by adsorbed layers of surfactants or polymers. This makes the phenomenon
interesting for example for the purity test of drinking water or for the progress of purification
procedures in water treatment plants. The same scenario is the basis for a technology of large
economic and ecological importance - the flotation. The efficiency of this technology is
controlled by the interfacial properties of the floating bubbles and their interaction with the
particles to be flotated.
Several of the described techniques are available as commercial instruments. The book will
serve as an up to date guide to understand the methods, to show their advantages and
disadvantages, to specify the range of application and to explain their experimental limits. For
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other methods just laboratory set-ups exist and the book will give insight into how to design
them and what are the key questions to be solved in order to get accurate results. Especially
these prototype instruments give access to completely new phenomena or allow studies of
particular properties, such as it is true for the oscillation mode analysis of drops, the force
measurements between two liquid phases or the shape analysis of growing drops using modern
light fibre techniques.
The are more fields of application of bubbles and drops, for example in medicine and biology.
The lungs are constructed by bubbles, by so-called alveols. The functionality of alveols and their
mechanical properties are still not fully understood, although live is based on the perfect
functioning of the lungs since millions of years.
Another very practical problem closely linked to drops is the metallurgy. Metals or alloys during
their processing are usually liquids. This fact is used to study their properties - to determine the
surface tension of metals from the shape of a molten metal drop, to understand reactions on the
drop surface etc.
The book contains 15 chapters dedicated to either of the above mentioned topics. In Volume 1
of this series (Dynamics of Adsorption at Liquid Interfaces) few of the classical methods have
been described more from the point of view of interfacial dynamics so than only a small overlap
with this books exists. Moreover, there is a basic introduction of the surface and interfacial
tension methods in Volume 3 (Surface Tensiometry). However, chapters on these methods
given here do not introduce into the physical basics but describe in detail the most recent
developments and how to use them correctly. Also experimental examples are given to
demonstrate the data interpretation properly.
After a general introduction into the topic the chapter about the mechanics of axisymmetric
liquid menisci gives an overview of the theoretical background for all drop and bubble
experiments. Subsequent chapters are dedicated to drop and bubble methods. The chapters
viii
about the three main drop experiments provide the theoretical basis, a description of
experimental set-ups, specific advantages and disadvantages, correction and calibration
problems, experimental examples, and their interpretation: pendent and sessile drop, drop
volume, and spinning drop technique.
The chapter about capillary pressure methods summarises different techniques and gives
examples of applications, for instance measurements under microgravity conditions in space
experiments. This type of methods comprises bubbles and drops.
The maximum bubble pressure technique as a particular capillary pressure method will be
described with emphasis on the decisive developments in the last five years, during which this
method was developed as the most reliable one for extremely short adsorption times, down to
the range of milliseconds and less. Problems connected with aerodynamics and hydrodynamics
are discussed and used to show the limits of this widely used standard method.
The oscillating bubble technique has been first developed about 25 years ago and does exist still
in form of individual set-ups only. However it provides a number of information not available by
other techniques, for example about the dilational rheology of adsorption layers and relaxation
processes at the interface. Especially the frequency interval spans a range which is of large
importance for many surfactants. The theory as well experimental details and results will be
presented and compared with other techniques.
The description of rising bubbles in surfactant solutions will contain the hydrodynamic basis as
well as the theoretical description of the effect of interfacial layers on the movement of bubbles.
Besides the theoretical basis also experimental data and the relevance for practical applications
will be presented, such as water purification, flotation processes etc.
The chapter about lung alveols demonstrates how important bubbles built by biological
membranes are in everyday life. The relevance for medicine and biology as well as model studies
will be discussed.
ix
An important example for the application of drops is the metallurgy, where the surface tension
of metals and alloys is an important parameter for many applications. Also the chapters on drop
shape analysis by using fibre technique and on force measurements between emulsion droplets
are of much practical relevance. These chapters however will close the present book although a
number of further interesting fields could be added, such as on formation and stabilisation of
emulsions, or drops in shear fields, thin films between drops or bubbles, are extensive enough to
fill separate books and thus will be excluded here. A book especially dedicated to "Foams and
Foam Films" has been published within this series.
This book will be interesting for all users of commercial instruments based on principles such as
drop volume, spinning drop and maximum bubble pressure techniques. Also for those scientists
and engineers using unique or home-made set-ups for example of the pendent or sessile drop
technique will find the state of the art of the respective methods and comparison with other
techniques. The different examples of application are interesting for a number of groups, in
material sciences as well as life sciences. For those scientists starting up research in the field of
characterisation of liquid interfaces this book will serve as guide to find the right methods and to
realise the certain peculiarities inherent in specific liquid systems.
For the very valuable support during the whole preparation of the book manuscript we want to
express our gratefulness to Dr. Martina Bree and Sabine Siegmund.
September 1997 D. M6bius R. Miller