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The center between the voids rotates because the four ligaments (wide beams) connected to this center buckle when vacuum is applied to the elastomer
This and the next image are from:
Dian Yang (1,2), Bobak Mosadegh (1,3), Alar Ainla (1), Benjamin Lee (1), Fatemeh Khashai (1), Zhigang Suo (2,4), Katia Bertoldi (2,4) and George M. Whitesides (1,3,4)
(1) Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, USA
(2) School of Engineering and Applied Sciences, Harvard University
(3) Wyss Institute for Biologically Inspired Engineering, Harvard University
(4) Kavli Institute for Bionano Science & Technology, Harvard University,
“Buckling of elastomeric beams enables actuation of soft machines”, Advanced Materials, 2015,
ABSTRACT: This article demonstrates the utility of a special type of non-linear behavior-the reversible, cooperative torsion, and collapse of a set of elastomeric beams (fabricated as one connected piece) under pressure. Understanding the motions exhibited in these systems started with observations and analyses by Boyce and co-workers of cooperative transformations in the shapes of patterns of through-holes cut into elastomeric slabs, on applying external pressure to these structures in the plane of the slab. Our work extends these studies, and greatly increases the ability of this kind of system, by using negative pressure (e.g., vacuum) applied to an elastomeric structure containing a number of elastic beams and interconnected, deformable cavities sealed within a thin elastomeric membrane. When negative pressure is applied, cooperative interactions among the components of the structure cause its elements (beams) to bend and buckle in ways that produce a range of useful motions. Buckling of materials is ordinarily considered an undesired mode of mechanical and structural failure, as it often causes permanent damage to structural components (e.g., metal frames, concrete pillars). The buckling of elastomeric materials, however, is reversible, and can provide useful new functions when designed properly. One such useful function is a rotary motion that provides a torque.
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