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Updated: Mar 25, 2026
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High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
Published on: October 31, 2019
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Flow coupling alters topological phase transition in nematic liquid crystals
Jayeeta Chattopadhyay1, Simon Guldager Andersen1, Kristian Thijssen1
1Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, Copenhagen, Denmark.
Summary
Fluid flow significantly alters defect behavior in nematic materials. Coupling to flow prevents defect recombination, changing phase transitions away from the standard Berezinskii-Kosterlitz-Thouless (BKT) scenario.
Area of Science:
- Soft Matter Physics
- Fluid Dynamics
- Materials Science
Background:
- Defect-mediated transitions are crucial in 2D systems.
- The Berezinskii-Kosterlitz-Thouless (BKT) scenario describes these transitions in passive systems.
- Understanding flow effects on these transitions is key for active and passive nematics.
Purpose of the Study:
- To investigate the influence of fluid flow coupling on defect-mediated transitions in 2D passive and active nematic fluids.
- To determine how flow alignment parameters affect the BKT scenario.
- To explore the role of self-generated flows in active nematics.
Main Methods:
- Fluctuating nematohydrodynamic simulations were employed.
- The system was driven by tuning fluctuation strength through a defect creation threshold.
- Simulations considered both forward and backward protocols for increasing/decreasing fluctuations.
Main Results:
- In the absence of flow, the transition follows the BKT scenario with reversible defect pair binding/unbinding.
- Flow coupling, via strain-rate alignment ($\\lambda \\neq 0$), leads to bend-splay walls, lowering the nucleation threshold.
- Flow coupling prevents sustained defect recombination; defects remain unbound across all fluctuation strengths.
- Active nematics with self-generated flows exhibit the same unbound defect behavior.
Conclusions:
- Coupling to fluid flow fundamentally alters defect-mediated phase transitions in 2D nematics.
- The canonical BKT transition is observed only in the absence of flow coupling.
- Flow alignment and activity-induced flows disrupt defect recombination, leading to persistent unbound defects.
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