RAMAN APPLICATIONS IN FOAMS - Raman Spectroscopy for Soft Matter Applications - page 185

(a) (b)

Elastic mlodulus
Yield stress

Liquid
Bubble separation

Plastic solid

Bubble rearrangement

Elastic solid

Bubble deformation

E

Dry foam Wet foam

Liquid content

Strain

Figure 7.8 Schematic presentation of (a) the typical stress-strain relationship in
foam and (b) the typical effect of foam liquid content on both elastic modulus (E) and
yield stress.

liquid fl ow is associated with bubble separation. Figure 7.8a shows a
schematic stress-strain relationship for a typical dry foam illustrating
the behavior change among the three distinct states of the mechanical
behavior of foam. Mechanical constants of foam have been found to
be very sensitive to the liquid content of the foam. As the liquid content
of the foam increases and the foam transforms from a dry foam to a
wet foam, both elastic constant and yield stress have been found to
drop dramatically as shown in Figure 7.8b.
Thus far, it is clear that the foam chemistry and interaction at the
molecular level both dictate the fi lm properties which in turn control
the cell structure and affect the continuum-level mechanical behavior
of the foam. For example, Overaker et al. [31] modeled the effect of
cell strut angle on the mechanical behavior of 2- D foams. Their simula-
tion showed that the elastic modulus of the foam is highest when the
strut angle is 100 ° .

7.6 EXPERIMENTAL TECHNIQUES FOR
FOAM INVESTIGATION

In spite of the fundamental importance of understanding foam struc-
ture on all length scales mentioned above, very few experimental tech-
niques have been utilized in its investigation. Recent simulation studies
[32-36] aiming at elucidating the mechanics of foams under shear
stresses have pointed out the need for more precise experimental tech-