Smectic order-driven total wall defect formation

Abstract

We study experimentally and theoretically liquid crystal structure of smectic oily streaks, focusing on planar wall defects hosted within smectic flattened hemicylinders (SFHs). The wall configuration is singular both in orientational and translational order and we refer to it as the Total Wall Defect (TWD). Here “singular” refers to nematic director field and smectic phase field. In theoretical analysis of the TWD we use a mesoscopic Landau-de Gennes-Ginsburg approach in terms of the nematic tensor order parameter and smectic A (SmA) complex order parameter field. The smectic layer structure is experimentally determined using polarizing optical microscopy and X-ray diffraction measurements at Synchrotron facilities. We demonstrate theoretically and numerically that the experimentally observed abrupt change of the SmA layering in the centre of the wall defect is realized via nematic order reconstruction mechanism. Our experiments reveal that smectic layer spacing above and below the wall are almost similar. The theoretical analysis suggests that lateral SFH boundary conditions determine the vertical position of TWD.