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The active particles achieve self-propulsion converting stored
or ambient free energy into systematic movement. Unlike traditional
non-equilibrium systems, the energy input that drives active particles
out of equilibrium is local, rather than at the boundaries as in a shear
flow. They show collective behavior different from individuals,
non-equilibrium order-disorder transitions, strong boundary
affinitiy, giant fluctuations, and pattern formation in the mesoscopic
scales. We shall illustrate these properties using models of active
Brownian particles. For example, the collective properties of
repulsively interacting active polar particles that align their active
velocities nematically, are controlled by the amount of active speed and
the orientational noise. The nematic-isotropic phase transition of
structural liquid remains well separated from an active phase separation
observed at a lower noise. With increasing activity, the system
undergoes a re-entrant fluid- hexatic- fluid melting. The melting of
hexatic clusters is associated with proliferation of topological
defects. The collective dynamics in the system progresses with sliding
clusters, jamming and lane formation. |