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Measuring Magnetically-Tuned Ferroelectric Polarization in Liquid Crystals
Published on: August 15, 2018
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Chiral oily streaks in a smectic-A liquid crystal
Ian R Nemitz1, Andrew J Ferris, Emmanuelle Lacaze
1Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA. rosenblatt@case.edu.
Soft Matter
|July 19, 2016
Summary
Chiral dopants in liquid crystals create temperature-dependent stripe rotations. This phenomenon, observed in octylcyanobiphenyl (8CB) films, is linked to the surface electroclinic effect near the smectic-A phase transition.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Soft Matter Physics
Background:
- Liquid crystals exhibit complex structures influenced by boundary conditions.
- Octylcyanobiphenyl (8CB) in the smectic-A phase forms 'oily streaks' due to competing planar and vertical alignments.
- Chiral dopants can modify the optical and structural properties of liquid crystals.
Purpose of the Study:
- To investigate the effect of chiral doping on the stripe orientation in 8CB liquid crystal films.
- To explore the relationship between chiral concentration, temperature, and stripe rotation.
- To elucidate the underlying physical mechanism responsible for the observed stripe rotation.
Main Methods:
- Spin-coating of chiral-doped (CB15) and undoped 8CB onto treated substrates.
- Optical microscopy with crossed polarizers to observe stripe formation and orientation.
- Systematic variation of chiral dopant concentration (c) and temperature relative to the nematic-smectic-A transition (TNA).
Main Results:
- Competing boundary conditions induce flattened hemicylinder structures and 'oily streaks' in 8CB films.
- Chiral doping with CB15 causes a temperature-dependent rotation (φ) of the stripe orientation.
- Stripe rotation angle (φ) increases monotonically with CB15 concentration and is largest just below TNA.
- Observed rotation relaxes to near zero within ~1 °C of TNA as temperature decreases.
Conclusions:
- The stripe rotation in chiral-doped 8CB is attributed to the surface electroclinic effect.
- This effect arises from the interplay of weak smectic order, large director tilt susceptibility near TNA, and surface electric fields.
- The study highlights the sensitivity of liquid crystal structures to chiral doping and surface interactions.
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