Hygroelastic Transition of Hydration Solids

In an experiment designed to probe transport kinetics of water in the nanoscale pores of hygroscopic spores of a common soil bacterium, we observed transport behavior many orders of magnitude slower than what is expected using conventional models. We hypothesized that the slow kinetics are due to hydration forces, interfacial forces that arise between surfaces at the nanoscale. A prediction of this hypothesis is a jamming-like transition in the mechanical properties of spores that should be observed at short timescales, where the critical timescale is informed by the kinetics of hydration and dehydration. Using dynamic atomic force microscopy measurements, we probed the elastic modulus of spores over the span of five frequency decades and observed an order-of-magnitude increase in elastic modulus at short timescales, consistent with our prediction (1). After accounting for the length scale effects, the observed timescale is also found to be consistent with the theoretical predictions. The findings reveal an interesting nonequilibrium phenomenon that differs from glassy and poroelastic behaviors, which we termed the hygroelastic transition. Together with other unusual mechanical properties, these materials represent a fundamentally different class of solids called hydration solids (1).

[1] Harrellson, S.G. et al. Hydration solids. Nature 619, 500–505 (2023)