Theory and computation of directional nematic phase ordering

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Physics

Background:

• Investigating morphological instabilities is crucial for understanding crystal growth and material properties.
• Liquid crystals exhibit complex phase transitions, such as the isotropic-nematic transition, which are sensitive to growth conditions.
• Previous studies have explored instabilities in simpler systems, but the unique anisotropic nature of liquid crystals requires specific models.

Purpose of the Study:

• To computationally study the morphological instabilities of a two-dimensional nematic front during directional growth.
• To model the first-order isotropic-nematic transition of 5CB (pentyl-cyanobiphenyl) using a Landau-de Gennes-type quadrupolar tensor order parameter.
• To analyze thermal instabilities in both linear and nonlinear regimes and compare with existing observations.

Main Methods:

• Utilized a Landau-de Gennes-type quadrupolar tensor order parameter model for simulations.
• Employed a previously derived energy balance, incorporating anisotropy, latent heat, and morphological gradients.
• Developed and used a sharp-interface model for linear instability analysis and compared it with full 2D simulations.

Main Results:

• Observed thermal instabilities in both linear and nonlinear regimes, consistent with prior experimental and theoretical work.
• Identified limitations of simplified sharp-interface models when applied to liquid crystal systems.
• Observed secondary instabilities in the nonlinear regime leading to defect formation, interfacial heterogeneities, and bulk texture dynamics.

Conclusions:

• The study provides insights into the complex behavior of nematic fronts under directional growth.
• The findings highlight the importance of anisotropic effects and the limitations of simplified models in liquid crystal systems.
• The observed secondary instabilities and resulting phenomena offer a deeper understanding of texture evolution and defect formation in liquid crystals.