The static and dynamic properties of colloidal inclusions in nematic liquid crystals

The viscous and elastic properties of nematic liquid crystals are strongly dependent on the anisotropic orientational ordering of the fluid. In the past, standard rheological techniques for observing these phenomena have proven challenging due to the coupling between macroscopic shear forces and disturbances within the liquid crystalline order. A solution to this problem is to utilize micron-scale inclusions driven by gravitational and magnetic fields to investigate these rheological properties. By using slow moving colloids, the long range ordering of the liquid crystal can be maintained while the colloid moves through the fluid. Furthermore, by utilizing anisotropic particles of different shapes, the symmetry of the surrounding fluid can be broken allowing for a variety of different phenomena to come forth, including an anisotropic viscous drag that was found to be far more complex than in isotropic fluids. Additionally, the particle's surface chemistry can be modified, allowing for different anchoring conditions and subsequently different viscous drags to manifest. The elasticity of liquid crystals can also strongly affect anisotropic inclusions. By using magnetic fields to rotate the particles to specific orientations, energy can be stored within the liquid crystal much like the energy stored in the ambient electric field when a capacitor is charged. As with capacitors, this energy stored within can be suddenly released. Whereas in the case of capacitors this is seen via the newfound kinetic energy transferred to electrons; liquid crystals in contrast release this stored energy into the rotational kinetic energy of the anisotropic inclusions within.;This dissertation presents a series of three experimental studies that have proven useful in better understanding the elastic and viscous properties of liquid crystals. This has been explored using discoidal colloids to evaluate a nematic's elastic properties as well as spherical and cylindrically shaped colloids for studying the fluid's viscous properties. For the case of disk-shaped colloids, experiments revealed a quadratic dependence on the elastic energy stored within the fluid due to distortions generated by their rotation. The conversion of this elastic energy to kinetic energy and viscous dissipation was subsequently shown to quantitatively account for the dynamics of the disks within these fluids. The viscous properties of nematics in contrast were explored by observing the translation of spheres and cylinders. Spheres were found to move at an angle to the force driving the inclusion's motion, and this dynamic lift was shown to be consistent with measurements of the anisotropically dependent drag coefficients. Measurements on cylinders demonstrated a similar albeit more complicated effect since this dynamic lifting of the particle now depended on how the surface anchoring of the liquid crystal. The ratio of the viscosity for motion perpendicular and parallel to the director was found for various cylindrical orientations and anchoring conditions, which varied from approximately 0.88 to 2.38. This dissertation endeavors to expound upon this research as well as detail several additional experiments that compliment the primary focus described above.