Permeation process in columnar discotic liquid crystal subjected to steady dilation and oscillatory shear

The response of homeotropically aligned columnar hexagonal mesophases to steady dilation and rectilinear oscillatory shear are described. At steady dilation the LC structure relax the stress with times dependent of deformation amplitude. At small dilation amplitude we have an elastic reply. For extension above δ⩾100 Å the transmitted stress was non-linear and the characteristic relaxation times are about τ≈100 ms. The response of rectilinear oscillatory shear is investigated for frequencies between 10−2 and 100 Hz and cell thicknesses L between 20 and 100 μm. At threshold shear amplitude, one notes the observation of spatial pattern, consisting of stationary bands, perpendicular to the direction of shear and localized in the bulk of the columnar sample. For fixed thickness and at threshold shear amplitude, we observe band structure for several frequencies corresponding to integer harmonics ν0, 2ν0, 3ν0. We tempt to explain the mechanism of the two instabilities and to give an estimation of the characteristic relaxation time in both cases. We interpret the experimental results by the disks permeation through the columns.