Brillouin spectroscopy of collagen hydrogels and glycerol solutions: effect of hydration

Brillouin spectroscopy is rapidly gaining popularity in the study of biological objects. However, modeling the results of Brillouin experiments in such materials is a complex task, especially to take into account the contribution of water. Here, we investigated the Brillouin spectra of a collagen hydrogel, which mimics real tissues, over the entire range of water concentrations. The Brillouin spectra of glycerol solutions with concentrations ranging from pure water to pure glycerol as an example of a simple aqueous binary system were examined. It was found that a combination of the Reuss and Voigt models can describe the elastic properties of both of these binary aqueous compounds over the entire concentration range. Orientational relaxation spectra were measured using depolarized light scattering to shed light on the behavior of the Brillouin linewidth. Unlike glycerol solutions, the concentration dependence of the Brillouin linewidth in collagen hydrogels was not reflected in the dependence of the orientational susceptibility. This points to the peculiarity of longitudinal relaxation in hydrogels.