Foreshadowing elastic instabilities by negative group velocity in soft composites

We reveal the existence of a state in soft composites, characterized by the omni-directional negative group velocity in the vicinity of elastic instability. We show that the appearance of the negative group velocity in layered and fibrous composites foreshadows microscopic loss of the stability. In contrast with classical instability-induced pattern transformations, the transition between states with positive and negative group velocities is not accompanied by geometrical rearrangements and can be triggered by very fine variation of the compressive deformation in stable composites. Finally, we analyze the effect of the geometrical characteristics and elastic moduli of the constituents on the strain range for induced state with negative group velocities.