The effect of pressure on the viscoelastic properties of liquids

The viscoelastic behaviour of di(2-ethylhexyl) phthalate and bis(m-(m-phenoxy phenoxy)-phenyl) ether has been determined as a function of pressure under conditions of alternating shear. The shear mechanical resistance has been measured at frequencies of 10 and 30 MHz. For di(2-ethylhexyl) phthalate the pressure range of measurement extended to 1400 MN/m$^{2}$ and to 300 MN/m$^{2}$ for bis(m-(m-phenoxy phenoxy)phenyl) ether. Associated measurements have also been made of the density, $\rho $, and steady-flow viscosity, $\eta $, of each liquid as a function of pressure, the viscosity results being confined to the range 0.001 to 300 N s/m$^{2}$. The limiting shear modulus, $G_{\infty}$, of each liquid has been found to vary linearly with pressure whilst, within experimental error, the viscoelastic relaxation can be represented by an equation put forward by Barlow, Erginsav & Lamb (1967 a), $J^{\ast}(\text{j}\omega)=J_{\infty}\left[1+\frac{1}{\text{j}\omega \tau _{\text{m}}}\right]+\frac{2J_{\infty}}{(\text{j}\omega \tau _{\text{m}})^{\frac{1}{2}}}$. (1) $J^{\ast}(\text{j}\omega)$ is the complex compliance measured at angular frequency $\omega,J_{\infty}(=1/G_{\infty})$ is the limiting high-frequency shear compliance and $\tau _{\text{m}}$ is the 'Maxwell relaxation time' equal to $\eta J_{\infty}$. This equation gives an adequate description of the relaxational behaviour when due account is taken of the variation of viscosity, density and shear modulus with pressure and temperature.