  To elucidate mechanisms of mesoscopic turbulence exhibited by active
particles, we experimentally study turbulent states of non-living
self-propelled particles. We realize an experimental system with dense
suspensions of asymmetrical colloidal particles (Janus particles)
self-propelling on a two-dimensional surface under an AC electric field.
Velocity fields of the Janus particles in the crowded situation can be regarded
as a sort of turbulence because it contains many vortices and their velocities
change abruptly. Correlation functions of their velocity field reveal the
coexistence of polar alignment and anti-parallel alignment interactions, which
is considered to trigger mesoscopic turbulence. Probability distributions of
local order parameters for polar and nematic orders indicate the formation of
local clusters with particles moving in the same direction. A broad peak in the
energy spectrum of the velocity field appears at the spatial scales where the
polar alignment and the cluster formation are observed. Energy is injected at
the particle scale and such conserved quantity as energy could be cascading
toward the larger clusters.
