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

## TL;DR

This study investigates how defect structures in liquid crystals transform and retain geometric memory across the nematic-smectic A phase transition, combining experimental observations with numerical modeling.

## Contribution

It reveals the topological and geometrical rules governing defect transformations and demonstrates the preservation of geometric memory during phase transitions.

## Key findings

- Defect configurations are correlated across phase transition.
- Focal conic domains retain geometric memory of nematic boojum configurations.
- Numerical modeling elucidates topological rules of defect transformations.

## Abstract

We study transformations of self-organized 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 polarized 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 organized over large scales in patterns that retain a geometric memory of the nematic boojum configuration, which can be recovered with remarkable fidelity.

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Source: https://tomesphere.com/paper/1904.05507