Cooperative ordering at liquid crystal interfaces and its role in orientational memory

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Orientational memory in liquid crystal films, or the surface memory effect, describes the return to an initial ordered texture upon cooling after heating above the isotropic transition temperature.
• The precise origins of this phenomenon, observed for over 80 years, remain incompletely understood, with surface interactions previously considered the primary driver.

Purpose of the Study:

• To investigate the origins of orientational memory in thermotropic liquid crystal films.
• To elucidate the roles of molecule-surface interactions and cooperative effects in maintaining orientational memory.

Main Methods:

• Utilized scanning tunneling microscopy and polarizing optical microscopy to study 4'-octyl-4-cyanobiphenyl liquid crystal films on graphite.
• Prepared cells in a defined initial orientation and measured bulk and surface order parameters during thermal disordering.
• Employed a magnetic field to partially decouple surface and bulk layers.

Main Results:

• Observed strong orientational memory despite weak molecule-surface interactions, challenging previous assumptions.
• Demonstrated that cooperative interactions between bulk and surface layers are essential for retaining long-range order.
• Found that decoupling the surface and bulk layers with a magnetic field significantly diminished surface layer orientational memory.

Conclusions:

• Reinterprets the origins of orientational memory in liquid crystal films, highlighting the critical role of cooperativity.
• Underscores the importance of bulk-interfacial liquid crystal interactions and cooperative phenomena in surface memory effects.
• Suggests that cooperative interactions, not solely surface forces, drive the recovery of orientational order.