Temperature Dependence of the Acoustic Properties of a Natural Slug Mucus by Brillouin Light Scattering Spectroscopy

Brillouin light scattering spectroscopy was used to probe the acoustic properties of a natural gastropod mucus, which consists primarily of water and glycoproteins, over the range -11 ^oC ≤ T ≤ 52 ^oC. Two peaks were observed in the spectra: one at a frequency shift of ~ 8.0 GHz present throughout the full temperature range and assigned to the longitudinal acoustic mode of the liquid component of mucus, and another at a shift of ~ 18.5 GHz observed for temperatures ≤ -2.5 ^oC assigned to the longitudinal acoustic mode of polycrystalline ice Ih. Discontinuous changes in the temperature dependence of the frequency shift, linewidth, and intensity of these peaks and, consequently, in the hypersound velocity and sound absorption occur at T = -2.5 ^oC. These changes are accompanied by the appearance in the Brillouin spectrum of a spectral peak due to ice for T ≤-2.5 ^oC and together suggest that the mucus undergoes a phase transition from a viscous liquid state to one in which liquid mucus and solid ice phases coexist. The freezing point hysteresis relative to that of water and failure of the liquid-solid transition to proceed to completion is due to inhibition of ice growth caused by glycoprotein adsorption and incorporation. Furthermore, analysis on Brillouin linewidth (FWHM) and consequently the viscosity provide information on the activation energy of the gastropod mucus. The lack of measurable differences in sound absorption for samples containing various amounts of liquid mucus and ice suggests that increased viscosity and the fact that these glycoproteins crosslink to form a polymer network, rather than an increase in the quantity and/or size of ice crystallites in the liquid phase, is the dominant mechanism for increased damping of hypersound with decreasing temperature.

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