  In a viscoelastic flow over a microfluidic canopy of polymeric pillars, we
report the spontaneous emergence of waves in the form of propagating regions of
low flow velocity compared to the surrounding flow. The occurrence of the wave
is chaotic and shows characteristics of elastic turbulence. We systematically
study the coupling between the low velocity wave and the microfluidic canopy by
combining flow velocimetry experiments and high speed tracking of the pillars.
The waves form an angle $\beta$ with the primary flow direction that depends on
the geometry of the pillar array. If the canopy is composed of flexible
structures, the passage of a wave deflects the structures locally in a manner
reminiscent of the emergence of the Monami waves observed in inertial
turbulence over canopies of vegetation. Due to the analogies with classical
(inertial) canopy turbulence, we name our newly-observed phenomenon as canopy
elastic turbulence.
