Chemistry Reference
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Fig. 7 Bifocal contact lens
made from LC hydrogel.
involves the use of soft birefringent liquid crystal hydrogels for bifocal contact lenses
monomers, swollen with water, and having long-range orientational order. The binary
monomer-water mixture exhibits lamellar and smectic phases. In the smectic phase,
the director is aligned with a magnetic field in the plane of the circumference of the
lens, and the material is photopolymerized. The resulting materials are nontoxic and
hydrated, with good oxygen transport capability. They are strongly birefringent, and
the two refractive indices give rise, simultaneously, to two focal lengths.
3 Effects of Light on LCEs
One of the remarkable properties of LCEs is their mechanical response to light.
LCEs can be made photosensitive by the incorporation of light-absorbing materials.
They can then exhibit striking responses to visible radiation, ranging from large and
rapid changes in shape to light-driven rotary motion. Because of the versatility and
ease of implementation of such schemes, perhaps the greatest potential of LCEs for
technological applications lies in photoactuation.
3.1 Optomechanical Coupling Mechanisms
A variety of processes exist that produce mechanical stresses in LCEs as a result of
illumination. Such stresses can subsequently cause shape changes, deformation and
motion. Some possible processes are described in Sects. 3.1.1-3.1.3
3.1.1 Direct Optical Stresses and Torques
The radiation field exerts direct forces and torques on condensed matter via the
P=Ac
where
D
is the electric displacement,
I
is the irradiance,
c
is the speed of light,
P
is the incident power, and
A
is the area. If the direct optical force is to cause a
deformation, the radiation pressure would have to overcome the aligning effect of
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