## Abstract

An experimental study has been made of the elastohydrodynamic behaviour of a range of fluids, including some of widely different pressure coefficient of viscosity. It is shown that the complex pattern of elastohydrodynamic behaviour observed at high pressure is due predominantly to the interplay between two non-Newtonian effects. The first is viscoelasticity. This has the effect that, at small degrees of shear, all the fluids behave as viscous liquids at low pressure and as elastic solids at high. The transition occurs when the viscosity attains $10^{5}$ Pa s. The second is the nonlinearity at high degrees of shear between the shear stress and the rate of shear. The consequence of this is that at sufficiently high rates of shear the fluid loses any elastic character and behaves as a non-Newtonian liquid. It is shown that the behaviour of the real system resembles that of a Maxwell viscoelastic model into which Eyring's elementary expression for viscosity is incorporated to describe the non-Newtonian character of the liquid component. At high shear stresses the value of the traction coefficient (T/W) is given by the expression $\frac{T}{W}=\overline{\alpha}\tau _{0}-\frac{\tau _{0}}{p}$ln $\left(\frac{\tau _{0}}{2\eta _{0}\dot{s}}\right)$, where $\overline{\alpha}$ is the pressure coefficient of viscosity, $\tau _{0}$ a characteristic shear stress, p the pressure, $\eta _{0}$ the viscosity at atmospheric pressure and $\dot{s}$ the rate of shear. The values of $\overline{\alpha}$ which are appropriate relate to the actual pressures applied. These pressures are considerably in excess of those obtainable in conventional high pressure viscometry but it is shown that the appropriate value of $\overline{\alpha}$ can be derived satisfactorily from the elastohydrodynamic experiments themselves. The fluid giving the highest value of the traction coefficient proved to be one whose pressure coefficient of viscosity was considerably greater at high pressure than at low. The results suggest that with high polymers the structural unit responsible for flow is considerably smaller than the polymer molecule itself. In contrast, certain types of molecules having branch chains appear to become entangled making the unit for flow appreciably larger than the individual molecule.