Anisotropic particles from LC polymers for optical manipulation

Abstract

We have developed a method to convert 10 different LC acrylate monomers into colloids by dispersion polymerization. This yields nine different types of anisotropic colloids with nematic and different smectic phases. The diameter of these colloids mostly varied between 0.5 and 3.5 μm; it can be adjusted by variation of the solvent mixture and it can be systematically increased by seed polymerization. The polydispersity of the anisotropic colloids is thereby often below 10%. Polarizing microscopy shows that colloids of a size between about 2 to 4 μm appear to have a bipolar director configuration. Smaller colloids appear uniaxially oriented, the resolution does, however, not allow a more refined investigation of the director pattern. These anisotropic spheres (diameter between 0.7 and 3.7 μm) can be trapped with an optical tweezers. Circularly polarized light transfers a torque to the particles, enabling one to rotate them clockwise and anticlockwise, which makes these spheres attractive as actuators. The size dependence of their rotational frequency makes it additionally possible to determine changes of the director configuration with size. For a nematic colloid (P 6), it could be shown that the anisotropy stays constant from 1.6 to 3.4 μm.