  Many living systems use assemblies of soft and slender structures whose
deflections allow them to mechanically probe their immediate environment. In
this work, we study the collective response of artificial soft hair assemblies
to a shear flow by imaging their deflections. At all hair densities, the
deflection is found to be proportional to the local shear stress with a
proportionality factor that decreases with density. The measured collective
stiffening of hairs is modeled both with a microscopic elastohydrodynamic model
that takes into account long range hydrodynamic hair-hair interactions and a
phenomenological model that treats the hair assemblies as an effective porous
medium. While the microscopic model is in reasonable agreement with the
experiments at low hair density, the phenomenological model is found to be
predictive across the entire density range.
