Swimmer Suspensions on Substrates: Anomalous Stability and Long-Range Order

Area of Science:

• Physics
• Soft Matter Physics
• Active Matter Physics

Background:

• Active matter systems exhibit complex dynamics and emergent behaviors.
• Understanding orientational order and fluctuations is key to characterizing active suspensions.
• Previous work suggested giant number fluctuations and bend instabilities in polar active systems.

Purpose of the Study:

• To develop a comprehensive theory for the dynamics and fluctuations of 2D polar active particle suspensions.
• To investigate the stability of polar orientational order and its impact on fluctuations.
• To reconcile theoretical predictions with experimental observations regarding number fluctuations and instabilities.

Main Methods:

• Theoretical modeling of a two-dimensional suspension of polar active particles in an incompressible fluid.
• Analysis of angular fluctuations and their relaxation/growth rates at zero wave number.
• Extension of the theory to confined 3D thin-film suspensions and dense compressible active polar rods.

Main Results:

• Polar orientational order can be anomalously stable or unstable, determined by a single parameter.
• Screening of the broken-symmetry mode leads to conventional, not giant, number fluctuations.
• The predicted bend instability in splay-stable flocks does not exist; polar phases exhibit long-range order in 2D.
• A flocking transition without banding instability is predicted for dense compressible active polar rods.

Conclusions:

• The developed theory provides a unified framework for understanding active polar systems.
• It corrects previous interpretations of fluctuation types and instabilities in these systems.
• The theory predicts novel phenomena, including long-range order in 2D and flocking without banding.