A flock of birds flows like a liquid, but in one respect it acts more like a solid, according to a new computer simulation (above). Flocking birds can fly together as an impressive fluidlike mass, and a team of physicists wanted to know whether a flock possesses a cohesion similar to surface tension in a real liquid. So they used a computer simulation to fire into a wall a flock of virtual birds, each programmed with a bird brain that keys off the direction and speed of its neighbors much as an actual bird would. In the simulation, the left panel shows a side view of the blue birds as they impact the red wall, whereas the right side displays a top-down perspective. As the flock smacked against the wall, it shattered into uneven clusters as a brittle solid would, rather than splashing into equal-sized drops as a liquid would, the team reported this month in Physical Review E. The result suggests that, like a solid, a bird flock has no apparent surface tension even though interactions between the birds promote the overall cohesion of the flock. The work could help scientists decipher the dynamics of animal herds and other "materials" made up of active agents.
(Video credit: Pearson Miller and Nicholas Ouellette)