Dynamical patterns in nematic active matter on a sphere.

Area of Science:

• Soft Matter Physics
• Non-equilibrium Statistical Mechanics
• Complex Systems Dynamics

Background:

• Active nematics, composed of self-propelled agents with orientational order, exhibit rich emergent behaviors.
• Confining active nematics to curved surfaces, like spherical vesicles, introduces geometric constraints that influence phase behavior.
• Previous studies on active microtubule bundles have observed diverse patterns on vesicle surfaces.

Purpose of the Study:

• To explore the dynamical phases of dense active nematics confined to a spherical surface using computational simulations.
• To map the non-equilibrium phase diagram as a function of key parameters: curvature, alignment strength, and activity.
• To understand the role of nematic persistence length in pattern formation on positively curved surfaces.

Main Methods:

• Utilized simulations of self-propelled agents incorporating short-range repulsion and nematic alignment interactions.
• Investigated systems confined to the surface of a sphere, varying parameters like alignment strength and activity.
• Analyzed the resulting phase transitions and topological defect structures.

Main Results:

• Identified distinct dynamical phases, including an equilibrium nematic ground state with four +1/2 defects at low activity.
• Observed a transition to a polar band phase with two +1 defects at the poles as activity increased.
• Further increases in activity led to band folding and a turbulent phase characterized by defect pair proliferation.

Conclusions:

• The study successfully reproduces experimental observations of active microtubule bundles on vesicles.
• Geometric confinement and activity are critical in dictating the emergent phases and defect dynamics of active nematics.
• Nematic persistence length plays a crucial role in pattern formation within these curved, confined active systems.