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Active Extensile Stress Promotes 3D Director Orientations and Flows
Mehrana R Nejad1, Julia M Yeomans1
1The Rudolf Peierls Centre for Theoretical Physics, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom.
Physical Review Letters
|February 11, 2022
Summary
Active nematic layers with extensile stress promote out-of-plane director orientations, driving three-dimensional layer formation. Contractile stress favors in-plane alignment and suppresses layer growth.
Area of Science:
- Soft Matter Physics
- Active Matter Systems
- Non-equilibrium Thermodynamics
Background:
- Active nematic materials exhibit spontaneous flows and orientational order.
- Understanding the role of director orientation in three-dimensional (3D) active nematic systems is crucial for their applications.
- Distinguishing between extensile and contractile active stresses is key to predicting material behavior.
Purpose of the Study:
- To investigate the influence of out-of-plane director orientations on active nematic layer dynamics.
- To differentiate the effects of extensile versus contractile stresses on active nematic behavior.
- To explore the mechanisms of layer formation and disclination line types in 3D active nematics.
Main Methods:
- Numerical simulations of active nematic layers.
- Linear stability analysis.
- Investigation of director orientation dynamics, including out-of-plane movement.
Main Results:
- Contractile stress suppresses perpendicular flows and favors in-plane director alignment.
- Extensile stress promotes instabilities leading to out-of-plane director orientations and 3D layer formation.
- Extensile activity causes planar drops to grow into 3D, while contractile activity keeps them 2D.
- Disclination lines in 3D active nematics are of twist type for extensile and wedge type for contractile materials.
Conclusions:
- Extensile forces are a likely mechanism for the initial stages of layer formation in biological systems.
- Director out-of-plane orientation is a critical factor in the transition of active nematic layers to three dimensions.
- The type of active stress (extensile or contractile) dictates the resulting morphology and defect structures in active nematics.
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