Thesis (pdf) - Espci

(a) (b)
Π
CCD CCD
Brewster Angle
Microscopy
Π
Chloroform
solution
Π
2.2 Experimental setup 7
Fig. 2.1. (a) Schematics of the experimental setup. Spread monolayers of the azobenzene amphiphile
8Az3COOH are prepared by depositing drops of a chloroform solution on a surface
of pure water contained in a Teflon cuvette. A lateral pressure Π is applied by adjusting the
surface area. Orientational order inside the trans droplets is measured by a Brewster angle
microscope (BAM) (Ignés-Mullol et al., 2004). (b) Images obtained using the experimental
setup courtesy of Jordi Ignés-Mullol.
Azobenzenes exhibit two main properties that can be exploited for our experimental
purposes. On one hand, transitions between trans and cis isoforms
can be induced by exposure to appropriate wavelengths of light (photoisomerization),
Fig. 2.2a, and by thermal relaxation towards the trans configuration
which has a favorable energetic configuration, Fig. 2.2b. Due to the dramatically
different geometrical properties of the trans and cis conformations, mixtures
of both isomers organize differently at the air/water interface. On the
other hand, mesophases of the trans isomer, appear at relatively low area densities.
As a consequence, monolayers composed of these azobenzenes exhibit
long-range orientational order from the trans isoform, but positional disorder
(Tabe and Yokoyama, 1994), which allow topological defects to easily
move.
(a) (b)
N
N
hν
Energy
N
hν kBT
N
′ , N
N
N N
Configuration
Fig. 2.2. Azobenzene photoisomerization. Transition between trans form (left) and cis form
(right) can be induced using appropriate wavelengths of light (a). Alternatively, the cis configuration
will thermally relax to the energetically stable trans configuration (b).