For periodic bar-and-joint frameworks, a one-parameter deformation is called expansive when distances between all pairs of vertices increase or remain unchanged. Expansive motions are auxetic, since auxetic behavior requires expansion only within periodicity orbits. In dimension two, expansive motions are explained through the possibility of framework refinements to periodic pseudo-triangulations. For illustrations, we revisit planar frameworks considered in the previous talk of Ciprian Borcea on "auxetic deployments", pointing out that auxetic behavior may persist even though expansion has ceased.

This is joint work with Ciprian Borcea.