  The motion of soft-glassy materials (SGM) in a confined geometry is strongly
impacted by surface roughness. However, the effect of the spatial distribution
of the roughness remains poorly understood from a more quantitative viewpoint.
Here we present a comprehensive study of concentrated emulsions flowing in
microfluidic channels, one wall of which is patterned with micron-size equally
spaced grooves oriented perpendicularly to the flow direction. We show that
roughness-induced fluidization can be quantitatively tailored by systematically
changing both the width and separation of the grooves. We find that a simple
scaling law describes such fluidization as a function of the density of
grooves, suggesting common scenarios for droplet trapping and release.
Numerical simulations confirm these views and are used to elucidate the
relation between fluidization and the rate of plastic rearrangements.
