Coexisting nematic and smectic-A phases in a twisted liquid-crystal cell

Area of Science:

• Materials Science
• Condensed Matter Physics
• Liquid Crystal Physics

Background:

• Nematic liquid crystals (NLCs) are known for their anisotropic optical and electrical properties.
• Phase transitions in liquid crystals are crucial for display technologies and optical devices.
• Planar-aligned NLC cells typically exhibit specific defect structures upon cooling into smectic phases.

Purpose of the Study:

• To investigate the structural behavior of twisted homogeneously planar-aligned nematic liquid-crystal cells upon cooling into the smectic-A phase.
• To understand the optical-guiding characteristics of these cells despite the absence of expected defect structures.
• To explore the phase separation phenomena within the liquid crystal cell.

Main Methods:

• Cooling twisted nematic liquid-crystal cells into the smectic-A phase.
• Optical examination of cell characteristics.
• Utilizing a half-leaky guided-mode arrangement for detailed exploration.
• Developing a theoretical model to explain observed phenomena.

Main Results:

• The expected defective structure did not form upon cooling.
• Cells maintained good optical-guiding characteristics.
• Phase separation into three or more distinct regions was observed.
• Regions of highly twisted nematic liquid crystal were found adjacent to the cell boundaries.
• Central regions of homogeneous smectic-A phase with layers normal to the cell surfaces were identified, separated by twisted nematic regions.
• Smectic-A regions grew thicker, and nematic regions thinned with increased cooling, accompanied by a higher twist gradient.

Conclusions:

• The cooling of twisted nematic liquid crystals into the smectic-A phase results in an unexpected phase-separated structure.
• This novel structure preserves optical-guiding properties, deviating from typical defect formation.
• The observed phase separation and structural evolution are explained by a presented theoretical model.