  We discuss the microscopic origin of dynamical instabilities and segregation
patterns discovered in granular mixtures under oscillating horizontal shear, by
investigating, via molecular dynamics simulations, the effective interaction
between like-particles. This turns out to be attractive at short distances and
strongly anisotropic, with a longer range repulsive shoulder along the
direction of oscillation. This features explain the system rich phenomenology,
including segregation and stripe pattern formation. Finally, we show that a
modified Cahn-Hilliard equation, taking into account the characteristics of the
effective interaction, is capable of describing the dynamics of the mixture.
