We report on measurements of single-molecule Brownian motion in liquid crystals, unravelling the anisotropic mobility of individual dye molecules. This anisotropic Brownian motion is directly correlated with the structural properties in a smectic A (8CB) and a nematic (5CB) liquid crystal sample cell on the micrometer scale using polarization contrast microscopy. A considerably slower mobility of dye molecules is found as compared to self-diffusion measurements by NMR, while anisotropy values compare well to recent literature data. This is suggested to be related to local distortions of the director structure around the dye molecules.

A method for analyzing single molecule tracking data is presented, which allows the measurement of flow profiles in shear flows of ultrathin liquid films. The results show that the velocity profile detected by single molecule tracking is in general different from the analytical solution of the Navier−Stokes equation due to the diffusion of the probe molecules. However, the theory presented allows the extraction of even slip boundary phenomena, which for the first time provides access to boundary conditions on a molecular scale. A verification of the theory and its capabilities is presented by means of tracking single polystyrene particles in a 500 nm thin water film confined in a surface forces apparatus.