Active Matter

Active matter describes systems whose constituent elements consume energy and are thus out-of-equilibrium. Examples include flocks or herds of animals, collections of cells, and components of the cellular cytoskeleton.

When these objects interact with each other, collective behavior can emerge that is unlike anything possible with an equilibrium system. The types of behaviors and the factors that control them however, remain incompletely understood for most systems. Our group has worked to develop and apply theoretical and computational descriptions of several model active systems.

Example Active Matter Systems

Starlings and gray suspensions

Fig. 1. Left: A flock of starlings exhibits collective motion. Center: A bacterial suspension exhibits collective flow and characteristic defect patterns. Right: A suspension of microtubules studied in the Dogic Lab at Brandeis.

Fig. 2: The system behavior for an active nematic at high activity. (left) The velocity field (arrows) is superimposed on a plot of the concentration of active nematogens (green=large concentration, red=small concentration). (right) A plot of the nematic order parameter, S,  (blue=large S, brown=small S) is superimposed on a plot of the nematic director (arrows). The flow under high activity is characterized by large vortices that span lengths of the order of the system size and the director field is organized in grains. Figure is from [2]

Fig. 3: Mixtures of active and passive rods segregate [3]. Pictured rods are identical except that red rods are self-propelled while green rods undergo only passive Brownian motion. Snapshots from simulations with different densities and self propulsion speeds are shown, which illustrate three different system behaviors. Movies illustrating the dynamics are available.

  1. Giomi, L., L. Mahadevan, B. Chakraborty, and M.F. Hagan, Banding, Intermittency and Chaos in Active Nematic Suspensions. arXiv:1110.4338, 2012.
  2. Giomi, L., L. Mahadevan, B. Chakraborty, and M.F. Hagan, Excitable Patterns in Active Nematics. Physical Review Letters, 2011. 106(21).
  3. McCandlish, S.R., A. Baskaran, and M.F. Hagan, Spontaneous Segregation of Self-Propelled Particles with Different Motilities. Soft Matter, 2012. in press.