Defect Transformations in Nematic Liquid Crystal Research Note

Abstract

The polarizing microscopic textural changes in a thin film (12p) of a nematic liquid crystal (NLC) p-ethoxy benzylidene -p′-n-butylaniline (EBBA) as it is cooled from its isotropic to the crystal phase, have been examined. No alignment was given to the substrates; no external field was applied. Upon cooling the isotropic (1) melt, the nematic phase (N) begins to separate at the clearing point (77°C) in the form of droplets. Each drop displays a black cross the iums of which are parallel to the vibration directions of the polarizers. Interference fringes are also observed within each drop. The structure of a nematic droplet floating in an isotropic liquid medium have been studied earlier both theoretically and exprimentaly1–13.A theoretical investigation of the structure of droplets of nematic materials, floating in an isotropic liquid has been carried out by Dubois-Violette and Parodi1.1 By assuming an anisotropic surface energy, these authors predict both the possible configurations depending on whether surface tension induces a normal or a tangential orientation of the molecules at the N-I interface. The surface tension of the N-I interface has been calculated in a mean field approximation for the system of rod-like molecules intracting via attraction as well as hard-core repulsion14. It is found that the excluded volume effect favours the planar orientation of molecules at the N-I interface in contrast with the case of the nematic free surface at which the effect favours the normal alignment. Thus one can expect that at the N-I interface of EBBA, the surface tension imposes tangential boundary condition on the droplet surface. As a result two point defects, classified as boojurns are obtained at the poles of the droplets. When such droplets are observed through a polarizing microscope with the droplet axis (direction connecting two poles of the droplet) parallel to the direction of incoming light, the resulting texture is an extinction cross with its arms parallel to analyzer and polarizer respectively, as can be seen in Plate 1. Certain droplets are surrounded by black ring indicating thereby that molecules on the equator of the droplets are exactly parallel to the direction of incoming light.