Selective imaging of 3D director fields and study of defects in biaxial smectic A liquid crystals

Eur. Phys. J. E 16, 179-191 (2005)
DOI: 10.1140/epje/e2005-00019-8

Selective imaging of 3D director fields and study of defects in biaxial smectic A liquid crystals

I.I. Smalyukh¹, R. Pratibha², N.V. Madhusudana² and O.D. Lavrentovich¹

¹ Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, Kent, OH44242, USA
² Raman Research Institute, 560080 Bangalore, India

odl@lci.kent.edu

(Received 16 October 2003 / Received in final form 11 October 2004 / Published online 22 February 2005 )

Abstract
We report on the selective imaging of different director fields in a biaxial smectic A (SmA $_{\rm b})$ liquid crystal using Fluorescence Confocal Polarizing Microscopy (FCPM) and Polarizing Microscopy (PM). The patterns of two directors, namely the director $\hat{n}_a$ perpendicular to the lamellae and the director $\hat{n}_b$ in their planes are visualized by doping the liquid crystal with two fluorescent dyes with different orientation of the transition dipoles with respect to the lamellar matrix. The properties of defects such as disclinations and focal conic domains (FCDs) are consistent with the non-polar D_2h-symmetry of the SmA $_{\rm b}$ mesophase in the studied mixture of bent-core and rod-like molecules: ([see full textsee full text]) majority of defects in the director $\hat{n}_b$ are half-integer " $\pm 1/2$ " disclinations; ([see full textsee full text]) the integer-strength " $\pm 1$ " defects tend to split into the " $\pm 1/2$ " disclinations. We compare the vertical cross-sections of the " $\pm 1$ " disclinations in the $\hat{n}_b$ field in SmA $_{\rm b}$ and uniaxial nematic samples. In SmA $_{\rm b}$ , the " $\pm 1$ " disclinations do not escape into the third dimension, while in the nematic samples with Schlieren textures they do despite the surface anchoring at the plates; the experimentally determined director field around the escaped disclination capped by a pair of surface point defects - boojums matches the one predicted recently [C. Chiccoli et al., Phys. Rev. E 66, 030701 (2002)]. The FCD structure in SmA $_{\rm b}$ is similar to that in SmA and SmC in terms of the normal to the layers but differs significantly in terms of the director field $\hat{n}_b$ parallel to the smectic layers. The FCDs in SmA $_{\rm b}$ can be associated with topologically non-trivial configurations of $\hat{n}_b$ in the surrounding matrix that are equivalent to the disclination lines.

PACS
61.30.-v - Liquid crystals.
87.64.Tt - Confocal microscopy.
61.30.Jf - Defects in liquid crystals.
61.30.Eb - Experimental determinations of smectic, nematic, cholesteric, and other structures .

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005