Evidence for the Predicted Behavior of Strong Nonlinear Elasticity in Hydration Solids

Recent experiments with the hygroscopic spores of a common soil bacterium revealed a set of unusual equilibrium and nonequilibrium properties (1). Among these is a strong nonlinear elastic behavior that is predicted to depend on the characteristic molecular size scale, the decay length of hydration forces, and the relative humidity of the environment. Although some key features of the predicted strong nonlinear elastic behavior are demonstrated experimentally, the theory makes additional specific predictions regarding the quantitative and qualitative aspects of the observed nonlinear elasticity. Here we present quantitative evidence for the predicted scaling behavior of elastic modulus and nonlinear elasticity with relative humidity. Atomic force microscopy measurements on bacterial spores at different relative humidity levels show (i) a logarithmic dependence of elastic modulus on relative humidity and (ii) a strong nonlinear elasticity present at a wide range of relative humidity values. The quantitative results are also in agreement with the equations based on the theory. Findings highlight the unusual mechanics of hydration solids and strengthen the support for the hygroelastic theory (1).