Dr. Nir Emuna
Geometric incompatibilities, and the resulting residual stresses, are often regarded as defects that should be avoided in synthetic materials. Natural structures, however, seem to exploit such mechanical features to facilitate optimized functionality. Several examples include the control of homeostatic stresses in arterial walls, folds formation in the brain cortex, morphogenesis of the gut, opening of seed pods, and structural coloration in petals, processes in which geometric incompatibilities play a major role. In my research, I extended these observations to explore possible bio-inspired applications in synthetic systems such as medical devices, soft actuators, and artificial muscles. This talk will focus on the induced twist effect in tubes, namely, twist motion generated by non-torque loading. Interestingly, the twist direction and even its handedness can change along the loading path. These unintuitive phenomena are observed in natural structures (e.g., DNA molecule, spider draglines, and phycomyces fungus) and stem from material anisotropy. Via mathematical modeling of non-linear soft solids, I will show that similar behavior can be achieved with bi-layer tubes comprising two isotropic layers that are twisted in opposing directions and adhered together, forming a twist incompatible tube. Controlling the level of the twist incompatibility, harnessing the strain-stiffening effect characterizing soft materials, and modulating the relative thickness and stiffness ratios of the layers allow to manipulate a variety of induced twist responses.