Structural order and pair formation in a two-dimensional colony of hydrodynamically interacting pushers

Area of Science:

• Soft matter physics
• Hydrodynamics
• Statistical mechanics

Background:

• Mean-field kinetic theory predicts orientational instability in spheroidal pusher systems above a critical density.
• Instability arises at the smallest available length scale for homogeneous and isotropic distributions.

Purpose of the Study:

• To investigate the structural properties of hydrodynamically interacting spheroidal pushers using a particle-based approach.
• To determine how pusher body elongation influences emergent order.
• To compare particle-based findings with mean-field theory predictions.

Main Methods:

• Particle-based simulations of spheroidal pushers in a 2D sheet within a 3D fluid.
• Analysis of positional and orientational order as a function of pusher elongation and density.
• Examination of stable bonding pair formation and their characteristics.

Main Results:

• A well-defined positional and orientational order emerges below the critical density predicted by mean-field theory.
• Stable bonding pairs, acting as rotational attractors, are identified as the source of this order.
• Sharp peaks in the angular position-orientation pair distribution function indicate bonding at low densities.

Conclusions:

• The particle-based approach reveals emergent order in spheroidal pusher systems not captured by mean-field theory.
• Bonding pair size is linked to the balance between hydrodynamic flow and self-propulsion speed.
• Steric repulsion does not disrupt orientational order if it's smaller than the bonding pair size.