Transient grating spectroscopy nondestructively characterizes the mechanics of rubbery polymers and soft gels

Designing efficient mechanisms for moving mechanical assemblies requires the use of materials with well-defined mechanical responses. Appropriate methods are needed to characterize these mechanical responses. Mechanical characterization of soft materials is critical in the high strain rate regime where intuition from manipulating a material at low rates fails to translate to applications including impact protection, tire traction, and sound damping. Here, transient grating spectroscopy (TGS) is used to measure wave propagation for soft elastomers and hydrogels. TGS is a non-destructive and non-contact optoacoustic technique that enables high strain rate measurements of the bulk modulus, a measure of a material's resistance to changing volume. The bulk modulus of elastomers and hydrogels is measured using TGS and its conversion to Young's modulus is discussed. This data is used to resolve values of Poisson's ratio in nearly incompressible gels to high degrees of precision.