Directed aging and memory: Teaching an old foam new tricks

As a material ages, its physical properties change. Under an applied stress, it plastically deforms in order to relieve the internal stress in incremental steps. At each instant, it lowers the stress in the most effective way. Thus, over long times, the final state of the material depends on the external stresses it was exposed to during the aging process. A material thus has a memory of the stresses to which it was exposed during the aging process. We exploit this property and direct the aging process with specific protocols in such a way that our material reaches a distinct, final state with a prescribed and desired functionality. In order to demonstrate this behavior, we use sheets of foam that we cut with a laser cutter and place under stress in such a way that the material develops unusual elastic properties. To accelerate the aging process, we apply heat to the sample. We have been able to modify the Poisson’s ratio of our system considerably; we can make a sample that was initially nearly incompressible and make it auxetic (negative Poisson’s ratio). We can likewise take an auxetic sample and make it incompressible. We have also been able to train local behavior so that a sample responds with a prescribed local deformation in response to a global perturbation.