Estimation of microscopic rubbing alignment parameters

We report an examination of the cloth-rubbing process, widely used to effect liquid crystal alignment, from a simplified microscopic perspective. We define strength of rubbing by the average applied force per rubbing fibre (approximately 11-22 mu N under our conditions, assuming all fibres passing the surface make contact), and extent of rubbing by the fraction of total surface area contacted during the process. Fibre-surface microscopic contact widths estimated from atomic force microscopy images of rubbed alignment polymer surfaces were in the range 10-500 nm. Taking 100 nm as an average value, we show that the entire alignment surface may be contacted several times during a typical rubbing process. Fibre-surface contact shear stresses can approach the GPa range, several orders of magnitude greater than the macroscopic rubbing pressure.