  In this article, we calculate the friction between two counter-flowing
bosonic and fermionic super-fluids. In the limit where the boson-boson and
boson-fermion interactions can be treated within the mean-field approximation,
we show that the force can be related to the dynamical structure factor of the
fermionic component. Finally, we provide asymptotic expressions for weakly and
strongly attractive fermions and show that the damping rate obeys simple
scaling laws close to the critical velocity.
