RAMAN APPLICATIONS IN FOAMS - Raman Spectroscopy for Soft Matter Applications - page 187

In addition to optical techniques, a number of electrical- based tech-
niques were utilized to investigate foam structure as well. Electrical-
based techniques were all based on the change in electrical properties,
namely, capacitance and resistance of the foam due to change in liquid
fraction, and were all employed to measure the liquid fraction of the
foam. The capabilities of such techniques ranged from a single point
measurement to multipoint imaging using segmented capacitance plates
[47]. To this end, it is clear that most of the experimental techniques
utilized in characterizing foams targeted the cellular and continuum
levels of foam investigation. More experimental techniques are required
to investigate foam on the nanoscale, or in other words the fi lm level
of observation.
7.7 RAMAN SPECTROSCOPY IN FOAM CHARACTERIZATION
In spite of its powerful characterization capabilities, Raman spectros-
copy has not been fully utilized in investigating foams. Since surfactant
molecules play a central role in the formation and stabilization of foam,
Raman spectroscopy with its ability to monitor molecular vibration can
provide vital information on their packing, mobility, and conformation
[48]; hence, a clear and informative picture of molecular structure
within the liquid fi lm inside foam can be obtained. Understanding the
structure and performance of the liquid fi lm (foam on the nano- or
molecular level) enables a broader and better understanding of foam
properties and behavior on macro and continuum levels, as we have
discussed before. It can be confi dently said that with its ability to extract
vital information about the chemical, thermal, electrical, and mechani-
cal properties of the system, Raman spectroscopy is still an unexplored
frontier in the fi eld of foam investigation.
As a laser beam impinges on a 3-D foam, the passage of light through
this scattering medium is essentially a random walk with a mean free
path l * termed diffusive propagation. Such diffusive propagation results
in what can be termed diffusive excitation , creating a distribution of
elementary Raman scattering centers in the bulk of the foam. The
Raman signal will, in turn, undergo a diffusive propagation in all direc-
tions, eventually reaching the foam cell boundaries, enabling the Raman
signal from the bulk foam to be detected. The specifi c dimension of the
Raman intensity distribution at scattering focal plane (usually the
surface of the foam) is proportional to the transport mean free path l * ,
which has been connected with the size of the foam bubble ( d ) accord-
ing to the relationship [49]