Research on the phenomenon and mechanism of rheological instability of PEO water-based solution

During study of non-Newtonian properties of PEO water solutions by using a flat plate rheometer, an unusual oscillation has been found out. In this paper, the reason of the oscillation occurring at the high shear rates is investigated and discussed. After excluding possibilities of turbulence, oil film oscillation and secondary fluid, the cause of the new oscillation has been analyzed in-depth, combining with the non-Newtonian fluid constitutive equations with the limit shear stress. The mechanism of the oscillation is that slippage occurs between the fluid and solid interface because its shear stress has reached the limit. The slippage presents as stick-slip which is a randomized phenomenon with a significant fluctuation property. As the slip region increases, the shear stress oscillation will exacerbate. When the average shear stress is close to the limit shear stress, that is, the slippage occurs on most of the surface, the constitutive curve no longer increases, but may start to decline on the contrary, showing the buckling characteristic. The decrease in shear stress will bring about a decrease of the load-carrying capacity of the lubricant film so as to lead to lubrication failure finally.