Thesis (pdf) - Espci

2.5 Discussion 19
ratio v−/v+ can be transformed, using Eq. 2.10, to yield α vs. Π and, therefore,
the ratio of the elastic constants as a function of Π (Fig. 2.15). Our measurements
can be combined with the results of Feder et al. (Feder et al., 1997)
whose analysis of orientation fluctuations in this system state that the value
for the geometric mean of KS and KB is of about K ≡ √ KSKB ∼ (40±25)kBT ,
and insensitive to pressure variation. This way, we can estimate KS and KB as
a function of Π, (Fig. 2.15), using
− K2
α = K2 S
. (2.11)
+ K2
K 2 S
In the zero-Π limit (lowest anisotropy) KS KB 10 −19 J. As Π increases,
there is a rapid increase (resp. decrease) of KS (resp. KB) until Π 1 mN/m,
where further variation of these parameters cannot be resolved. These values
are consistent with estimations found in the literature (Tabe et al., 1999) and
values extrapolated for thin suspended liquid crystal films (Rosenblatt et al.,
1979).
v−/v+
1
0.8
0.6
0.4
0.2
0
0 1 2 3 4
Π (mN m −1 )
Fig. 2.14. Experimental measurements of the relative velocities of boundary defects moving
towards their annihilation as a function of the lateral surface pressure.
We can qualitatively relate these results to the known tendency of azobenzene
derivatives to form supramolecular aggregates (H-aggregates), whose
presence can be revealed here by spectroscopy methods (Pedrosa et al., 2002).
In short, aggregates form in such a way that the azobenzene planes of neighboring
molecules are parallel, stacking perpendicularly to the molecular tilting
direction. Because of this, a splay arrangement of the molecular field
inside the axi-symmetric droplets would be energetically more demanding
than its bend counterpart, since the former would impose a certain curvature