Liquid crystals: new phases and topological defects

5 problems

• Dynamics of disclination loops in three-dimensional active nematics

  Open

  In three-dimensional gels driven by molecular motors, the chaotic flow is organized by closed loops of misalignment (defect loops) that appear, grow and vanish. What sets their birth rate, size and number is not understood.

• Molecular origin of the ferroelectric nematic phase

  Open

  Some recently made liquid crystals line up with their electric dipoles all pointing the same way, which makes the fluid strongly polar, like a liquid ferroelectric. Which molecular features cause this is not understood.

• True critical behaviour of the nematic to smectic-A transition

  Open

  When a nematic liquid crystal forms layers it becomes a smectic-A, and this transition was predicted to behave like the superconducting transition. Measured critical exponents instead vary from material to material and differ along and across the layers, and the true behaviour has not been settled for about fifty years.

• Does a thermotropic biaxial nematic phase exist?

  Contested

  Theory predicts a liquid crystal in which molecules align along three perpendicular directions instead of one, called a biaxial nematic. It has been seen in mixtures with solvent, but claims of it in pure single-component liquid crystals remain disputed.

• What stabilizes the twist-bend nematic phase?

  Contested

  Bent-shaped molecules can form a nematic whose alignment spirals with a pitch of only about 100 Angstrom, the twist-bend nematic. Whether it forms because bending becomes energetically favourable or because of hidden polar order is disputed.