Direct optical quantification of backflow in a 90 degrees twisted nematic cell

Optical guided mode observations of the transient director profile (optical tensor distribution) during the relaxation of a 90 degrees twisted nematic cell directly reveals backflow. In the first 6 ms of the relaxation process, after a voltage across the cell is removed, the midplane tilt of the director increases, reaching a maximum value of 101 degrees at 1.4 ms. This increase in midplane tilt is attributed to coupling between fluid flow (backflow) and director reorientation. A 270 degrees twisted state of the opposite handedness to the 90 degrees twisted state found at equilibrium is shown to exist during the backflow period. Good fits of theoretical models with experimentally determined time dependent director profiles yield the viscosity coefficients.