Synopsis

Polar Swarms

Physics 7, s42
A new theory can explain the formation of swarming patterns observed in ensembles of self-propelled polar particles.
J.-B. Caussin et al., Phys. Rev. Lett. (2014)

How do individual animals form swarms, schools, and flocks? In the 1990s, physicists modeled collections of self-propelled particles (so-called “active matter”) and could simulate the ordering that occurs in animal flocks.  Theoretical models have reproduced many aspects of this collective behavior, but a number of questions have persisted. One concerns the observation that in polar, active matter—think of a collection of small, mutually interacting swimming arrows—the particles organize themselves into three possible pattern classes: density waves, solitary waves (solitons), and traveling “droplets.”

No single theory has been able to explain the formation and diversity of these patterns. However, in a paper in Physical Review Letters, Jean-Baptiste Caussin and collaborators from institutes in France, Germany, and the Netherlands, have solved a hydrodynamic model of polar active particles and have accounted for the origin and variety of these propagating swarm structures.  

The authors described a polar fluid’s motion governed by a density field (capturing the distribution of particles) and a polarization field (capturing the polar interactions determined by the direction in which each particle is pointing).   By including an effective mean-field potential and frictional forces, the model could reproduce all of the commonly observed patterns. The authors suggest their model provides a “unified theory” of flock patterns—one that allows patterns to emerge as a general feature of the dynamics of polar active fluids, independently of specific model details (e.g., the functional form of the hydrodynamic coefficients). – David Voss


Subject Areas

Biological PhysicsComplex Systems

Related Articles

Insect Larvae Inspire Rolling Robots
Mechanics

Insect Larvae Inspire Rolling Robots

A study of the rolling motion of fruit-fly larvae has enabled researchers to create a soft robot that can rotate by itself. Read More »

Roses Offer Mechanical Clues for Shape-Shifting Materials
Biological Physics

Roses Offer Mechanical Clues for Shape-Shifting Materials

Understanding the mechanical mechanisms that alter the shape of rose petals as they grow could inspire new types of self-morphing materials and structures. Read More »

Embryo Cells Sort by Softness
Biological Physics

Embryo Cells Sort by Softness

The segregation of two cell types at the earliest stages of embryo development may be enabled by differences in their stiffness. Read More »

More Articles