Topological defects and geometric memory across the nematic–smectic A liquid crystal phase transition

Abstract

We study transformations of self-organised defect arrays at the nematic–smectic A liquid crystal phase transition, and show that these defect configurations are correlated, or “remembered”, across the phase transition. A thin film of thermotropic liquid crystal is subjected to hybrid anchoring by an air interface and a water substrate, and viewed under polarised optical microscopy. Upon heating from smectic-A to nematic, a packing of focal conic domains melts into a dense array of boojums—nematic surface defects—which then coarsens by pair-annihilation. With the aid of Landau–de Gennes numerical modeling, we elucidate the topological and geometrical rules underlying this transformation. In the transition from nematic to smectic-A, we show that focal conic domain packings are organised over large scales in patterns that retain a geometric memory of the nematic boojum configuration, which can be recovered with remarkable fidelity.