Laser tweezers for determining anisotropic viscosity coefficients of nematic liquid crystals

The dynamic response characteristics of a liquid crystal (LC) device are dependent upon its viscosity coefficients. Local shear viscosity coefficients, or Miesowicz viscosity coefficients, ηi, are of particular importance for backflow effects and their optimisation allows for faster LC device response times. With such a wide range of LC materials available, information regarding their viscous properties is often incomplete. Micromanipulation with laser tweezers offers an alternative method for determining shear viscosity coefficients. Micron sized dielectric particles are dispersed in homeotropically and planarly aligned nematic LC, sandwiched between two coverslips. The microfluidic behaviour of the LC is investigated using a computer controlled laser tweezer system where particle tracking is performed using a high speed CMOS camera to record bead displacement for power spectral density analysis. We investigate the effective viscosity coefficients parallel and perpendicular to the director n̂, η∥ eff and η⊥ eff respectively. These are directly related to the Miesowicz viscosity coefficients for homeotropic alignment η1, and homogenous alignment η2 and η3. The results infer practically pertinent details about the viscoelastic properties of liquid crystals, and particles in LC systems. © 2010 SPIE.