Cholesteric pitch transitions induced by mechanical strain.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Physical Chemistry

Background:

• Cholesteric liquid crystals exhibit unique optical properties dependent on their helical pitch.
• Surface anchoring conditions significantly influence the bulk behavior of liquid crystal phases.
• Understanding pitch transitions is crucial for developing advanced liquid crystal displays and devices.

Purpose of the Study:

• To investigate the influence of layer thickness and surface anchoring strength on pitch transitions in planar cholesteric liquid crystals.
• To analyze the cholesteric-nematic transition under varying surface conditions.
• To determine critical parameters governing pitch transitions and multistability.

Main Methods:

• Theoretical modeling of a planar cholesteric liquid crystal layer with mixed (strong/weak) anchoring.
• Application of parabolic and Rapini-Papoular potentials to describe surface anchoring energy.
• Analysis of bulk deformation and surface twist angle variations under strain.

Main Results:

• All pitch transitions occur at a critical thickness equal to one-quarter of the natural cholesteric pitch, irrespective of anchoring strength or material properties.
• The twist angle at the weakly anchoring surface exhibits strain-dependent behavior (slipping or discontinuous changes).
• The bifurcation point's position depends on the ratio of extrapolation length to layer thickness, but its value is model-dependent.

Conclusions:

• Layer thickness is a primary determinant of pitch transitions in planar cholesteric liquid crystals.
• Surface anchoring properties modulate the manner of twist angle variation and multistability.
• The study provides a fundamental understanding of pitch transitions, crucial for designing novel liquid crystal applications.