Hydration solids and the hygroelastic transition: Unusual fluid dynamics in a porous biological material

A large fraction of biological matter on Earth is hygroscopic and metabolically inert. Wood, bamboo, pollen grains, pinecones, silk, wool, bacterial and fungal spores are among these materials. They have Angstrom-scale pores filled with water. How should we think about these materials? Using experiments on hygroscopic spores, we show that hydration forces govern their equilibrium, nonequilibrium, and water-responsive characteristics. A simple microscopic theory quantitatively predicts how the material changes size with changing relative humidity, how it deforms under external forces, and how the kinetics of hydration and dehydration are related to short timescale nonequilibrium mechanical properties. Importantly, the new theory led to the discovery of strong nonlinear elastic behavior and a marked transition in mechanical properties that we named the hygroelastic transition. The hygroelastic transition is an unusual transition in porous media originating from jamming of water molecules in Angstrom-scale pores. It offers the possibility to create materials with tailored frequency responses, materials that simultaneously exhibit high rigidity and damping, materials that convert energy with high power densities. Our findings point to a class of solid matter, called the hydration solids, with highly unusual nanoscale fluid dynamics.