RAMAN APPLICATIONS IN FOAMS - Raman Spectroscopy for Soft Matter Applications - page 177

emulsions [7], magnetic fl uids froth [8,9], Langmuir monolayer systems
[10], and noncoalescing systems under microgravity conditions [11].
More interestingly, realizing the fact that for liquid fi lms in foams, the
correlation length, which is also a measure of a liquid- gas interface and
a measure of the range of intermolecular forces, is usually in the range
of 1-5 nm, and that the ratio of the line tension to the surface tension
is usually in the range of 20 nm [12], raises another important aspect of
investigating foam systems. Thin foam fi lm systems provide an excep-
tional opportunity for the investigation of truly thermodynamic small
systems, the core of what is known as “nanotechnology.” In this chapter,
we will discuss the history, physics, and general applications of foams
as one of the important members of the soft matter family. We will also
discuss the efforts to utilize Raman spectroscopy in analyzing and
investigating foam systems and the potential of such characterization
technique in investigating foams. In other words, in this chapter, we will
try to discuss an important soft matter system that has its roots in the
Renaissance era and is still active, important, and not fully explored in
the nanotechnology era.

7.2 FOAM HISTORY

Foam is a two-phase material system in which the gas phase is enclosed
in a liquid phase. This is different from, but greatly analogous to, emul-
sions and binary liquid foams in which two liquid phases are dispersed
together. Liquid-gas foams are the focus of this chapter. The theory of
liquid-gas foams is not quite elementary. However, a single material
property, the surface tension, , of the liquid-gas interface, is all that

matters for a solid start toward understanding the subject. The science
of liquid foams emerged from the scientifi c study of liquid surfaces that
dates as early as the fi fteenth century, the time of Leonardo da Vinci,
who pioneered the study of what is known today as the capillarity
phenomenon [13]. The important concept of surface tension was intro-
duced by J.A. Segner in 1751, and the concept of contact angle was
introduced by Thomas Young in 1805, and independently by the famous
mathematician the Marquis de Laplace. Since the fi fteenth century,
researchers have tackled foams, in the form of soap bubbles, from two
different aspects. Researchers interested in physical, chemical, and bio-
logical sciences investigated soap bubbles for their macroscopic and
molecular surface properties and their similarities to other systems of
interest. Mathematicians, however, investigate foams due to their inter-

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