  The feedback forces exerted by particles suspended in a turbulent flow is
shown to lead to a new scaling law for velocity fluctuations associated to a
power-spectra $\propto k^{-2}$. The mechanism at play relies on a direct
transfer of kinetic energy to small scales through Kelvin--Helmholtz
instabilities occurring in regions of high particle density contrast. This
finding is confirmed by two-dimensional direct numerical simulations.
