Spectrum of light scattered from a viscoelastic gel

TLDR: In this paper, the spectrum of light scattered from thermally excited displacement fluctuations in polyacrylamide gels was measured using optical mixing spectroscopy, and the correlation function for the displacements having wave vector q is predicted for these gels to have the form of an exponential decay: exp(− Γt).

Abstract: We report measurements of the spectrum of light scattered from thermally excited displacement fluctuations in polyacrylamide gels. These measurements have been carried out on the polarized scattered light as a function of scattering angle and temperature for 5% and 2.5% polyacrylamide gels using the methods of optical mixing spectroscopy. We also present a theory for the amplitude and time dependence of the thermally excited longitudinal and transverse displacements of the gel fiber network. These displacements are responsible, respectively, for the polarized and depolarized scattered light. The correlation function for the displacements having wave vector q is predicted for these gels to have the form of an exponential decay: exp(− Γt). The decay rate is given by Γ = Glq2/f or Gtq2/f, where f is the frictional force per unit volume on the fiber network as it moves with unit velocity relative to the gel liquid. Gl is the longitudinal compressional modulus for longitudinal displacements and Gt is the shear...