Complex light with optical singularities induced by nanocomposites

TL;DR

This study investigates the self-organization, aggregation, and electro-optical effects of multi-walled carbon nanotubes in nematic liquid crystals, revealing new insights into their structural and optical behavior under electric fields.

Contribution

It provides a detailed experimental and theoretical analysis of MWCNT aggregation stages, percolation thresholds, and the influence on liquid crystal structures, including the first observation of inversion walls.

Findings

Aggregation involves two stages: loose and compacted aggregates.

Percolation thresholds for different MWCNT types were identified.

Inversion walls form between MWCNT aggregates under electric fields.

Abstract

The nonocomposites on the base of long (5-10 $\mu$m, ${o}$-MWCNTs) and short (2 $\mu$m, ${m}$-MWCNTs) multi-walled carbon nanotubes (MWCNTs) hosted by nematic 5CB were investigated in details by means of polarizing microscopy, studies of electrical conductivity and electro-optical behaviour. The spontaneous self-organization of MWCNTs was observed and investigated both theoretically and experimentally. The simple Smoluchowski approach was used for estimation of the half-time of aggregation. It was shown that aggregation process includes two different stages: fast, resulting in formation of loose aggregates (${L}$-aggregates) and slow, resulting in formation of compacted aggregates (${C}$-aggregates). Both ${L}$- and ${C}$- aggregates possess extremely ramified fractal borders. Formation of the percolation structures was observed for ${o}$-MWCNTs at $C=C_p\approx$ 0.0025-0.05 % wt and for $m$-MWCNTs at $C=C_p\approx$ 0.005-0.25 % wt. A physical model describing formation of $C$-aggregates with captured 5CB molecules inside was proposed. It shows good agreement with experimentally measured characteristics. It was shown that MWCNTs strongly affect the structural organization of LC molecules captured inside the MWCNT skeleton and of interfacial LC layers in the vicinity of aggregate borders. Moreover, the structure of the interfacial layer, as well as its birefringence, drastically changed when the applied electric voltage exceeded the Freedericksz threshold. Finally, formation of the inversion walls between branches of the neighbouring MWCNT aggregates was observed and discussed for the first time.