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10
5
A
10
4
B
C
10
3
10
2
10
2
10
3
1
10
σ
(Pa)
Figure 4.3
Complex modulus
G
* versus shear stress of liquid crystal samples:
A
= cubic phase,
B
= lamellar phase, and
C
= hexagonal phase. The critical shear stress
is determined from the intersect between the linear and nonlinear viscoelastic regions.
[Adapted from An et al. (2006) ].
the water content. While not quantitative in terms of precise water-surfactant
composition, this simple measurement allows a “snapshot” of the major ele-
ments of the phase diagram to be recorded, without the need for laborious
phase diagram studies. This is particularly useful when searching for the exis-
tence of specifi c phases of interest.
Efforts to make the fl ooding technique more quantitative with respect to
phase composition have led to the development of a family of
diffusive inter-
facial transport
techniques (Laughlin and Munyon, 1987). Here, the fl ooding
experiment is combined with a spectroscopic technique that allows water
content to be determined within each of the observed phases. However, exper-
imental complexity has prevented widespread uptake of these techniques.
4.2.2
Rheology
Different LLC phases exhibit different rheological properties. Thus quantita-
tive rheological studies of LLC systems are supportive of other phase behavior
observations.
LLC phases typically show shear thinning behavior, while measurements
of the critical shear stress, at which LLC structure breaks down, can be related
to phase behavior (Cordobes et al., 2005). As shown in Figure 4.3, An et al.
(2006) have shown that the critical shear stress decreases in the order cubic
liquid crystal (330 Pa)
lamellar (27 Pa),
consistent with the greater interconnectivity (and hence resistence to shear)
of the cubic LLCs in particular.
>
hexagonal liquid crystal (64 Pa)
>
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