Exploring phase transitions and thermal dynamics in nanoconfined liquid crystals using liquid-phase TEM

TL;DR

This study uses liquid-phase STEM to observe phase transitions and thermal effects in nanoconfined liquid crystals, revealing heat-induced phase changes and defect formations, and highlighting the technique's potential for advanced LC research.

Contribution

First detailed use of LP-STEM to investigate phase transitions in nanoconfined liquid crystals, demonstrating thermal effects and dynamic behavior at the nanoscale.

Findings

Electron beam induces phase changes from smectic A to nematic and isotropic.

Higher dose rates accelerate phase transition kinetics due to increased heat.

Nanoconfined 8CB exhibits different thermal properties compared to bulk material.

Abstract

Nanoconfined liquid crystals (LCs) and their nanocomposites are driving the next generation of photonic applications. Consequently, deepening our understanding of mesophase stability, defect topology, and the dynamic response of LCs at the nanoscale requires the development of novel characterization approaches. This motivates us to perform in situ observations on model 4'-octyl-4-cyanobiphenyl (8CB) LC using liquid-phase scanning transmission electron microscopy (LP-STEM). We find that the electron beam induced consecutive phase changes from smectic A to nematic (SmA-N) and from nematic to isotropic (N-I). The kinetic dependence of the phase transition on dose rate shows that the time between SmA-N and N-I shortens with increasing rate, revealing the hypothesis that a higher electron dose rate increases the energy dissipation rate, leading to substantial heat generation in the sample. We report on the spontaneous formation of disclinations, ordering effects, and complete process reversibility. Radiolytic effects of the electron beam are discussed in detail, and additional experiments with external heating indicate that the observed phenomena are mainly thermal in nature. The results are supported by calculations of heat diffusion, suggesting the nanoconfined 8CB differs significantly in thermal properties compared to the bulk one. This is the first detailed study of LC phase transitions using LP-STEM, which paves the way for further studies of nanoconfined LCs and for the development of the technique for advanced LC materials research.