Laser manipulation in liquid crystals: an approach to microfluidics and micromachines

Laser trapping of particles in three dimensions can occur as a result of the refraction of
strongly focused light through micrometre-sized particles. The use of this effect to
produce laser tweezers is extremely common in fields such as biology, but it is only
relatively recently that the technique has been applied to liquid crystals (LCs). The
possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an
isotropic fluid medium, or both particles and defects can be trapped and manipulated
within a liquid crystalline medium. This paper considers both the possibilities. The
mechanism of transfer of optical angular momentum from circularly polarized light to
small droplets of nematic LCs is described. Further, it is shown that droplets of chiral
LCs can be made to rotate when illuminated with linearly polarized light and possible
mechanisms are discussed. The trapping and manipulation of micrometre-sized particles
in an aligned LC medium is used to provide a measure of local shear viscosity coefficients
and a unique test of theory at low Ericksen number in LCs.