Spatiotemporally Programmable Surfaces via Viscoelastic Shell Snapping

Abstract

Many species can dynamically alter their skin textures to enhance their motility and survivability. Despite the enormous efforts on designing bio‐inspired materials with tunable surface textures, developing spatiotemporally programmable and reconfigurable textural morphing without complex control remains challenging. Herein, a design strategy is proposed to achieve surfaces with such properties. The surfaces comprise an array of unit cells with broadly tailored temporal responses. By arranging the unit cells differently, the surfaces can exhibit various spatiotemporal responses, which can be easily reconfigured by disassembling and rearranging the unit cells. Specifically, viscoelastic shells as the unit cells is adopted, which can be pneumatically actuated to a concave state, and recover the initial convex state sometime after the load is removed. It is shown computationally and experimentally that the recovery time can be widely tuned by the geometry and material viscoelasticity of the shells. By assembling such shells with different recovery times, surfaces with pre‐programmed spatiotemporal textural morphing under simple pneumatic actuation is built, and temporal evolution of patterns, such as digit numbers and emoji, and spatiotemporal control of friction are demonstrated. This work opens up new avenues in designing spatiotemporal morphing surfaces that could be employed for programming mechanical, optical, and electrical properties. A preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.164020946.62560710.

Pseudo-bistability of viscoelastic shells

“…We find that a deeper monostable shell with higher b / a , which is closer to that of bistable shells, leads to a more significant delay time with longer t rec / τ r . The effects of t creep / τ r , b / a and E rel on t rec / τ r mentioned above are consistent with the FEA simulations and experimental observations reported in literature [ 16 , 18 , 21 , 22 ].

Figure 11 Dependence of the critical creeping time on relative modulus of relaxation E rel and minor-to-major-length ratios b / a .

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“…After this snapping deformation, the shell gradually recovers its undeformed shape, with Δ V / V 0 slowly decreasing to zero. The characteristics of this observed deformation history agree with those of FEA simulations and experiments reported in literature [ 16 , 21 , 22 ], indicating that the proposed viscoelastic shell model can capture the pseudo-bistability exhibited in viscoelastic shells.

Figure 5 Pseudo-bistable behaviour of a viscoelastic shell.

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“…b αβ and b α β are the covariant and the mixed components of the second fundamental form of the mid-surface, yielding the following non-zero components 22)…”

“…This argument is not entirely convincing, since it is unclear how the ratio of bending to stretching energy evolves in viscoelastic shells, and how it is related to the stability. Using finite-element simulations together with experiments, researchers capture the pseudo-bistable behaviour of viscoelastic shells [ 16 , 21 , 22 ]. Although the numerical simulations can accurately predict the responses of viscoelastic shells, they provide limited information on their stability.…”

“…The recovery time is governed by the viscous time scale of the material and is much longer than that for elastic snap-through buckling. This pseudo-bistable behaviour has been harnessed to achieve spatio-temporal control of morphing structures [16,[22][23][24].…”