  In this work, we report observations of phase transitions in 2D multistable
mechanical metamaterials that are initiated by collisions of soliton-like
pulses in the metamaterial. Analogous to first-order phase transitions in
crystalline solids, we experimentally and numerically observe that the
multistable metamaterials support phase transitions if the new phase meets or
exceeds a critical nucleus size. If this criterion is met, the new phase
subsequently propagates in the form of transition waves, converting the rest of
the metamaterial to the new phase. More interestingly, we observe that the
critical nucleus can be formed via collisions of soliton-like pulses. Moreover,
the rich direction-dependent behavior of the nonlinear pulses enables control
of the location of nucleation and the spatio-temporal shape of the growing
phase.
