isometric nature of the deformations (i.e., length invariant, as measured along the central axes of the ribbons) associated with formation of the 3D structures.
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Fig. 2Experimental and computational studies of various 3D mesostructures and classification according to their modes of deformation.
(A) Average curvature components and mode ratio of a 3D mesostructure (3D wavy ribbon) that involves only bending, as a function of prestrain in the stretched assembly platform. (B) Similar results for a 3D mesostructure (3D single-helical coil) that involves both bending and twisting. Dots represent FEA results; solid lines represent the scaling law . The colors in the 3D FEA correspond to the maximum principal strains. (C and D) 2D precursors, mode ratios, optical micrographs, and FEA predictions for 18 3D mesostructures that exhibit bending-dominated modes (C) and bending-twisting mixed modes (D). Scale bars, 200 μm.
Buckling always involves considerable bending, whereas the amount of twisting depends strongly on the 2D structural details. One means of classification relies on a quantity, R, defined by the ratio of the average twisting curvature (κtwist) to the average bending curvature (κbend), which can be determined by FEA (33). A given 3D mesostructure belongs to the bending-dominated mode when R, referred to as the mode ratio, is smaller than a critical value (e.g., 0.2 for the present purposes); otherwise, it belongs to the bending-twisting mixed mode. Representative examples presented in Fig. 2, A and B, fall into these two different regimes: a 3D wavy ribbon (R= 0) and a 3D helical coil (R = 0.82). The magnitudes of both κtwist and κbend increase with compressive strain (εcompr) applied to the 2D precursor,