Auxetic Bending Response in Crumpled Membranes of Magnetic Nanoparticles

Soft quasi-2D materials are frequently studied as mechanical systems due to their bending response which can allow substantial elastic deformation. Thin, soft materials wrinkle and crumple easily in practice. And although macroscale crumpled sheets can display an auxetic-like elastic response, such effects have not been well-studied in crumpled nano-films. We will show that crumpled nano-films can have an auxetic bending response. Using freely suspended ~10nm thick membranes made of self-assembled superparamagnetic nanoparticles, we can manipulate thin, crumpled sheets with applied magnetic fields and track the micron-scale deflections in 3D with high-resolution confocal microscopy. By analyzing the curvatures induced by the bending, we demonstrate an auxetic bending response due to the geometric structure of the crumples, in contrast with the standard linear-elastic bending response of uncrumpled films. The ability to design mechanical elements with an auxetic bending response due to crumpling, even with non-auxetic materials, should open new design pathways for mechanical micro-structures.