Table Of ContentFoam Films and Foams
Fundamentals and Applications
PROGRESS IN COLLOID AND INTERFACE SCIENCE
Series Editors
Reinhard Miller and Libero Liggieri
The Progress in Colloid and Interface Science series consists of edited volumes and monographs
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Interfacial Rheology
Reinhard Miller and Libero Liggieri
Bubble and Drop Interfaces
Reinhard Miller and Libero Liggieri
Drops and Bubbles in Contact with Solid Surfaces
Michele Ferrari, Libero Liggieri and Reinhard Miller
Colloid and Interface Chemistry for Nanotechnology
Peter Kralchevsky, Reinhard Miller and Francesca Ravera
Computational Methods for Complex Liquid–Fluid Interfaces
Mohammad Taeibi Rahni, Mohsen Karbaschi and Reinhard Miller
Physical-Chemical Mechanics of Disperse Systems and Materials
Eugene D. Shchukin and Andrei S. Zelenev
Foam Films and Foams
Fundamentals and Applications
Dotchi Exerowa, Georgi Gochev, Dimo Platikanov, Libero Liggieri and Reinhard Miller
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Foam Films and Foams
Fundamentals and Applications
Edited by
Dotchi Exerowa
Georgi Gochev
Dimo Platikanov
Libero Liggieri
Reinhard Miller
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Dedication
DOTCHI EXEROWA (1935–2017)
Dotchi Exerowa passed away on October 2, 2017, in Sofia. The Bulgarian and international colloid
and interface community lost a scientist of high professional authority and a prominent representative
of the Bulgarian School of Colloid and Interface Science.
Dotchi Exerowa was born on May 20, 1935, in the town of Varna,
Bulgaria. In 1958, she graduated in chemistry at Sofia University “St.
Kliment Ohridski” with honors and joined the newly founded Institute
of Physical Chemistry at the Bulgarian Academy of Sciences (IPC-
BAS), where the Department of Interfaces and Colloids was headed by
Professor Alexei Scheludko, founder of the Bulgarian School of Colloid
and Interface Science. Dotchi Exerowa completed her PhD thesis (“Free
thin liquid films and foams”) in 1969 and obtained habilitation in 1971.
In 1987, she completed her Doctor of Science Thesis (“Formation and
stability of black foam films”). In 1983, she succeeded Professor
Scheludko as Head of the Department of Interfaces and Colloids in
IPC-BAS and led the department until 2005. Parallel with her duties at
IPC-BAS, she also lectured on “Physicochemical methods in biology,”
“Foams and emulsions,” and “Physical chemistry of liquid interfaces” at Sofia University. In 1988,
she became Professor in Physical Chemistry and in 2004 was elected as a Full Member of the
Bulgarian Academy of Sciences.
The start of the scientific career of Dotchi Exerowa coincided with the brilliant period when
the basis of surface forces studies in the area of fluid interfaces was being researched all over the
world. Her research activity actually began in 1958 when she elaborated her MSc thesis entitled
“Electrostatic disjoining pressure in foam films” under the supervision of Alexei Scheludko. This
study was published soon after in Kolloid Zeitschrift [1]. Three conceptual papers that followed may
be considered as benchmarks for the further development of the Bulgarian School of Colloids and
Interface Science [2–4]. Her subsequent scientific achievements were related to the study of surface
forces in thin liquid films, as well as to adjoining aspects in the physics and chemistry of interfacial
phenomena. They might be grouped into three major trends: (i) new scientific instrumentation; (ii)
pioneering new research results; and (iii) innovative applications of the new scientific results.
New scientific instrumentation: Over the course of several decades, microscopic foam films
have been one of the most actively investigated research topics of colloids and interfaces. Their
specific kinetic and thermodynamic properties promote them as a simple and comprehensible model
for the investigation of surface forces and the stability of dispersive systems. They also have an
important independent role as a vital constituent of biological systems, thus bridging the fundamental
physicochemical knowledge among biology, biomedicine, biotechnologies, and life sciences. The
progress in this research field is based on the effectiveness of various experimental methods developed
for the formation and the investigation of thin liquid films. One of the most successful techniques for
the study of these films is the microinterferometric setup for the formation and investigation of thin
films, put forward in the 1960s by Dotchi Exerowa and Alexei Scheludko. The key element of this
setup is the so-called Scheludko–Exerowa cell. Using this cell, the first experimental verification of
the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO theory) was accomplished
[1,5]. Later on, this methodology was modified and further developed by Exerowa and her research
group and applied to the investigation of a variety of fluid systems. Particularly important is the novel
modification—Thin Liquid Film Pressure Balance Technique (TLF-PBT)—which allows the direct
measurement of interaction forces in the microscopic liquid films, gaining the disjoining pressure/
v
vi Dedication
film thickness isotherms (Π(h)) for a wide range of thickness values and pressures, including the
transition region of common black/Newton black films (CBF/NBF).
The Scheludko–Exerowa microinterferometric technique turned out to be a very reliable research
instrumentation and is now used worldwide in most cases where surface forces at fluid interfaces
are investigated. Variations of these setups are working in a number of laboratories: University
of California at Berkley (USA), Technical University of Virginia (USA), Max-Planck Institute of
Colloid and Interface Research in Potsdam (Germany), Moscow State University (Russia), Institute of
Surface Chemistry, Stockholm (Sweden), Ecole Normale, Paris (France), Institute of Food Research,
Norwich (UK), and so on. In 2004–2005, Dotchi Exerowa initiated the design of the latest version of
the microinterferometric technique with the additional option for electrochemical investigations of
thin liquid films. It was constructed by Exerowa’s collaborators [6,7] in partnership with Canadian
colleagues (University of Alberta and CanmetENERGY, Edmonton). The technique is applied to
the investigation of emulsion films stabilized with natural surfactants. These thin liquid films are
of crucial importance for understanding the properties of dispersive systems, which are obtained in
the oil, pharmaceutical, cosmetic, and food industries.
Pioneering new research results: Historically, the microscopic foam film was the first simple
model of two interacting fluid interfaces. Using the microscopic foam film model, Exerowa and
collaborators investigated the fundamental aspects of the formation and stability of foam films. It
was verified for the first time that there are two types of equilibrium films: common black films
(CBF) and Newton (bilayer) black films (NBF). One of the basic research trends to which Exerowa
had a significant contribution was the detailed investigation of the NBFs. A new theoretical approach
to explain their formation and properties was proposed with NBFs regarded as a 2D-ordered system.
The rupture of the bilayers was related to a fluctuation mechanism of nanoscale hole formation.
The hole formation was treated as a nucleation process of a new phase in the 2D-system (NBF).
The leading role of the amphiphile concentration and the short-range intermolecular forces for the
formation and stabilization of these films was precisely verified. One of the basic achievements
of Exerowa and her group was the promotion of the NBF as an efficient model for the study of
interaction forces in biological membrane configurations.
Innovative applications of the new scientific results: Although the primary accomplishments of
Dotchi Exerowa concerned the fundamental studies of fluid interfacial properties and interactions,
in many cases she transferred the results into innovative advances and applications. One indicative
example was the development of a new method for the estimation of the lung maturity of neonates.
As is well-known, the deficiency of the lung alveolar surfactant causes lung immaturity and leads
to so-called Respiratory Distress Syndrome (RDS) in newborn infants. Through the investigation
of bilayer films formed from phospholipids and the components of alveolar surfactant, Exerowa and
collaborators demonstrated that NBFs may be applied successfully for the exploration of alveolar
surfactant insufficiency. The microscopic foam film was proposed as a novel in vitro model for
the study of the alveolar interface and the estimation for the stability of the alveoli. Based on this
model, a new method for early diagnostics of lung maturity of neonates was proposed and a new
methodology for the design and optimization of therapeutic pulmonary preparations was developed.
The method was often called the Exerowa Black Film method [8] and is implemented in a number
of medical institutions in Bulgaria, Europe, and the USA. In recognition of these accomplishments,
Dotchi Exerowa was elected as a member of the International Association for Lung Surfactant
System and of the EURAIL Commission (“EURope Against Immature Lung”). The very good
correspondence of the clinical results and the parameters of the in vitro model suggested a new
hypothesis for the structure of the alveolar surfactant layer. This is the so-called “bilayer-monolayer
model”, which allows deeper understanding of the major physiological process of breathing [9].
Another significant application trend was the study of the properties and the stability of foams.
Starting from their experience on foam films, Dotchi Exerowa and her group succeeded in relating
their peculiarities to the stability of foam systems. The basic success here was the elucidation of
the role of the foam film type (common thin, common black, or bilayer) for the lifetime and the
Dedication vii
syneresis (drainage) of foams [10]. The obtained results were very useful for the optimization of real
foam-producing formulations, to be used, for example, in firefighting, food, and cosmetic industries.
Many of Exerowa’s research results were included in the monograph “Foam and Foam Films,”
first published in 1990 (Khimiya, Moscow). The second edition, extended and revised, was published
by Elsevier in 1998. She had also authored a number of chapters in the encyclopedic books edited
by: Hans Lyklema (“Fundamentals of Interface and Colloid Science,” Elsevier, 2005); Johan
Sjöblom (“Emulsions and Emulsion Stability,” CRC Taylor and Francis, 2005); Thаrwat Tadros
(“Emulsion Science and Technology,” Wiley-VCH, 2009); Victor Starov (“Nanoscience: Colloidal
and Interfacial Aspects,” CRC Press, Taylor and Francis, 2010); Hiroyuki Ohshima and Kimiko
Makino (“Colloid and Interface Science in Pharmaceutical Research and Development,” Elsevier,
2014). In 2007, the book “Colloid Stability—The Role of Surface Forces” (edited by Th. Tadros,
Wiley-VCH), was published in two volumes and dedicated to the 70th birthdays of Dotchi Exerowa
and Dimo Platikanov. In 2009, another book, “Highlights in Colloid Science” (Wiley-VCH), was
published, with Dotchi Exerowa as a co-editor.
During her long and successful scientific career, Dotchi Exerowa initiated and developed
numerous research contacts and scientific collaborations with leading scientists and research
laboratories in the field of physical chemistry of interfaces and colloids (France, Great Britain,
Germany, Spain, Poland, Russia, Canada, Italy, Japan, etc.). She was elected to the scientific
committees of many international conferences and symposia. She was member of the editorial
boards of leading colloid chemistry journals (Current Opinion in Colloid and Interface Science,
Advances in Colloid and Interface Science, Colloids and Surfaces A, Colloid and Polymer Science,
Colloid Journal (in Russian)). In 1997, she was a co-chair (together with Dimo Platikanov) of the
9th International Conference on Surface and Colloid Chemistry, organized under the aegis of the
International Association of Colloid and Interface Scientists (IACIS). Professor Exerowa had also
been elected twice as a member of the IACIS Council and was closely engaged in the development
and the organizational advance of the international colloid and interface scientific community. A
tribute to the high quality of her research achievements and fruitful international collaborations
was the international symposium (Dotchi Exerowa Symposium—Smart and Green Interfaces:
Fundamentals and Diagnostics) held in Sofia, October 29–31, 2015, and dedicated to her 80th
birthday. More than 110 researchers from 23 countries participated in this event and a special issue
with selected papers was published recently in Colloids and Surfaces A: Physicochemical and
Engineering Aspect, vol. 519, 2017.
The death of Dotchi Exerowa is a great loss to the colloid and interface community. She will be
remembered by her colleagues, collaborators, and former students for her deep passion for science,
the wide horizon of her research interests, and her keen intuition to address challenging problems,
thus advancing truly basic and pioneering studies. The legacy of the important results of her scientific,
teaching, and applied activities constituted a vital contribution to the productive development of the
Bulgarian School of Colloid and Interface Science and were key pillars supporting the progress and
international reputation of this school.
Elena Mileva
Sofia, Bulgaria
BIBLIOGRAPHY
1. Scheludko, A., Exerowa, D. Über den elektrostatischen Druck in Schaumfilmen aus wässringen
Ektrolytlösungen. Koll Z 1959; 165: 148.
2. Scheludko, A., Exerowa, D. Instrument for interferometric measuring of the thickness of microscopic
foam layers. Comm Dept Chem Bulg Acad Sci 1959; 7: 123.
3. Exerowa, D., Scheludko, A. Taches noires et stabilité des mousses. In: Overbeek, J.Th.G. (Ed.), Chemistry,
Physics and Application of Surface Active Substances, Vol. 2. Gordon & Breach Sci. Publ., London,
1964, p. 1097.
viii Dedication
4. Exerowa, D., Scheludko, A. Porous plate method for studying microscopic foam and emulsion films.
Compt Rend Acad Bulg Sci 1971; 24: 47.
5. Scheludko, A., Exerowa, D. Über den elektostatischen und van der Waalsschen zusätzlichen Druck in
wässeringen Schaumfilmen. Kolloid Zeitschrift 1960; 168: 24.
6. Panchev, N., Khristov, K., Czarnecki, J., Exerowa, D., Bhattacharjee, S., Masliyah, J. A new method for
water-in-oil emulsion film studies. Colloids Surf A 2008; 315: 74.
7. Mostowfi, F., Khristov, K., Czarnecki, J., Masliyah, J., Bhattacharjee, S. Electric field mediated
breakdown of thin liquid films separating microscopic emulsion droplets. Appl Phys Lett 2007; 90:
184102.
8. Cordova, M., Mautone, A.J., Scarpelli, E.M. Rapid in vitro tests of surfactant film formation: Advantages
of the Exerowa black film method. Pediatr Pulmonol 1996; 21: 373.
9. Kashchiev, D., Exerowa, D. Structure and Surface Energy of the Surfactant Layer on the Alveolar
Surface. Eur Biophys J 2001; 30: 34.
10. Khristov, K., Exerowa, D. Influence of the Foam Film Type on the Foam Drainage Process. Colloids
Surf 1995; 94: 303.
Dedication
DIMO PLATIKANOV (1936—2017)
Dimo Platikanov passed away on October 21, 2017, in Sofia. He enjoyed a long and distinguished
career at the University of Sofia, Bulgaria.
Dimo Platikanov was Professor of Physical Chemistry and a former
Head of the Department of Physical Chemistry of the University of
Sofia. This department was established in 1925 by Professor Iwan
Stranski (1897–1979) and was headed by Professor Rostislav Kaishev
(1908–2002) and Professor Alexei Scheludko (1920–1995).
Dimo Nikolov Platikanov was born on February 2, 1936, in Sofia. His
mother was Lyuba Zaharieva Trifonova (1908–1989), a philologist—
Germanist, and his father was Nikola Dimov Platikanov (1898–1984)
who was a well-known specialist in cattle breeding and professor in the
University of Sofia and Fellow of the Bulgarian Academy of Sciences.
Dimo’s sister, Vesselina Platikanova, is also a physical chemist, and
Professor Boyan Mutaftchiev (France)—a well-known specialist in
crystal growth, is his cousin.
Dimo Platikanov graduated from the University of Sofia in 1958. Regarded by his professors as
an unusually gifted student, Dimo was immediately employed as a lecturer in the Department of
Physical Chemistry of the University of Sofia. In that department, Dimo elaborated his PhD thesis,
then later presented his DSc thesis; there he became first an associate professor and later, a professor.
During that time, he worked for Professor Scheludko in his new Bulgaria colloid science research
area and taught physical chemistry to the students in chemistry.
Dimo was a brilliant experimenter; but he contributed to pure theory as well: “Disjoining Pressure
of Curved Films,” 1985 (with B.V. Toshev) [1], “Line Tension in Three-Phase Equilibrium Systems,”
1988 (with B. V. Toshev and A. Scheludko) [2], “Disjoining Pressure, Contact Angle and Line
Tension in Free Thin Liquid Films,” 1992 (with B.V. Toshev) [3], and “Wetting: Gibbs’ Superficial
Tension Revisited,” 2006 (with B.V. Toshev) [4].
The books on the modern development of colloid science as an extension to nano science appeared
under the editorship of Dimo Platikanov and Dochi Exerowa, his wife who passed away weeks before
him, are much appreciated by the international colloid science community: “Highlights in Colloid
Science” (Wiley, 2008) [5].
Two basic thermodynamic quantities that characterize the three-phase contact with a thin liquid
film are contact angle and line tension. Line tension, first introduced by Gibbs, is similar to surface
tension, but in contrast, it could be either positive or negative. It is extremely important to provide
reliable methodologies for its experimental measurement. The contact (wetting) angles measurment
and the description of the wetting phenomena have a prolonged history. In these systems, thin
liquid films usually exist. The disjoining pressure with van der Waals, electrostatic, steric, and other
components is the basic thermodynamic quantity that characterizes the behavior of such systems.
Symmetrical as well as asymmetrical thin liquid films exist. There are black films of two types:
thicker common films and thinner Newton films. All these systems have been examined carefully
by Professor Platikanov and his co-workers and more than 20 unique experimental techniques have
been elaborated. This was not a surprise because Dr. Platikanov had the talent to practice science
with accuracy and responsibility and it marked his long scientific career.
Anyone in the study of thin films knows the Sceludko–Exerowa microinterferometric method.
Platikanov used this method with several improvements to determine the shape of the three-phase
contact in the presence of films under dynamic and as well as equilibrium conditions. The initial
ix
