Fig. 2.

Energy condensation in the perversion of the bistrip system. (A) The formation and evolution of one perversion with the reduction of the strip length L, $w=5\hspace{0.5em}\text{mm}$, $h=3\hspace{0.5em}\text{mm}$, $L=30\hspace{0.5em}\text{cm}$, and $L\prime =L/3.5$ following the notation labeled in Fig. 1B. The stress state of the bistrip is characterized by the distribution of the scalar von Mises stress which is originally proposed to analyze the yielding of materials (30). (B) The scaled strain energy profile of the strips in A. $\tilde{E}\left(n\right)=E\left(n\right)/{\text{min}}_{\left\{n\right\}}\left\{E\left(n\right)\right\}-1$, where n labels the slab of elements as shown (Inset), and $E\left(n\right)$ is the total strain energy of all these elements in the nth slab. (C–F) The designed bistrip system made of polyethylene and prestretched rubber strip to confirm the numerically observed energy condensation over the perversions. (C and D) Shape of the bistrip before and after buckling. (C, Inset) Featureless birefringence pattern of the initial bistrip. The dashed red lines in E and F indicate the interface of the two strips. The different birefringence patterns in the perversion (E) and the helical (F) regions reflect the distinct strain energy distributions.