Macroscopic traveling packet and soliton states of quasi-one-dimensional flocks

Using a continuum model for inhomogeneous flocks, we show that a finite but arbitrarily large moving "packet" of active particles (e.g., moving creatures) can form in a background of a lower density disordered phase of these particles, like a liquid drop surrounded by vapor. The "vapor density" of the disordered background can be made arbitrarily low. We find three basic types of quasi-one-dimensional states: "longitudinal", "transverse", and "oblique" states, with their internal velocity fields, respectively, parallel, perpendicular, and oblique to the interface. The transitions between these states are also studied. © 2014 American Physical Society.