Breakdown of continuum models of fluid structure interaction in a nanoporous biological material*

Determining how porous materials interact with permeating fluids is important for understanding their mechanical properties. Existing approaches often treat the permeating fluid as a continuous medium, but for many materials the pore dimension can be on the order of a nanometer, close to the size of a single water molecule. It is not clear how the discreteness of the permeating fluid affects the macroscopic mechanical properties of these materials. Here we show the bacterial spore, a dormant biological nanoporous structure, exhibits mechanical properties that challenge theoretical models based on continuum treatment of water. We found a statistical mechanical treatment of the confined water correctly predicts a range of equilibrium and dynamic properties of the spore, including an extreme slowdown of relaxation kinetics and a highly nonlinear mechanical response. Because the underlying assumptions of this approach are based on the geometry and not specific to the spore chemistry, these findings could also be applicable to other nanoporous materials.