Modeling active nematics via the nematic locking principle

Area of Science:

• Soft Matter Physics
• Active Matter Systems
• Non-equilibrium Fluid Dynamics

Background:

• Active nematic systems comprise self-driven, rod-like units creating large-scale flows and active turbulence.
• Microtubule-kinesin systems are a key experimental model, but existing theories struggle to quantitatively match observations.
• Current models often fail to capture the collective behavior and emergent phenomena seen in experiments.

Purpose of the Study:

• To establish a fundamental principle, 'nematic locking,' governing the collective motion of subunits in active nematics.
• To derive a more accurate theoretical framework for modeling microtubule-based active nematics.
• To resolve discrepancies between existing models and experimental observations, particularly concerning fracturing.

Main Methods:

• Proposed the 'nematic locking principle' based on steric interactions in dense, elongated subunit systems.
• Derived a general nematic transport equation consistent with this principle and identified violating terms (fracturing).
• Modified the standard Beris-Edwards model to enforce nematic locking, particularly in low-density defect regions.

Main Results:

• The standard Beris-Edwards model was shown to violate nematic locking due to inherent fracturing.
• The modified model successfully enforces nematic locking throughout the bulk, allowing fracturing only at defect sites.
• Simulations using the modified model produced results consistent with experimental observations, including localized fracturing bands.

Conclusions:

• The nematic locking principle provides a more accurate foundation for modeling active nematics.
• The modified Beris-Edwards model accurately captures the collective dynamics and defect behavior of microtubule systems.
• Enforcing nematic locking eliminates non-physical steady states, improving theoretical predictions and experimental alignment.