Suppression of purely elastic instabilities in the torsional flow of viscoelastic fluid past a soft solid

Abstract

Experiments are performed to explore the role of a soft, deformable solid layer on the purely elastic instability in the torsional flow of polymer solutions between two circular discs. The gel layer is placed on the stationary bottom plate of a rheometer, and the polymer solution is placed between the gel and the rotating top disc. The observed variation of viscosity with shear rate (or shear stress) is correlated with the presence or absence of purely elastic instability in the viscometric flow. Earlier work has shown that with increase in shear rate, the torsional flow of a polymer solution between rigid discs undergoes transition from the simple viscometric flow state to elastic turbulence via a sequence of instability modes. We combine rheological observations and flow visualization to show that the deformable solid has a profound effect on the stability of the torsional flow. In marked contrast to flow between rigid plates (where the fluid shows apparent shear-thickening at the onset of instability), the apparent viscosity continues to decrease up to a much larger value of shear rate with the presence of a soft gel. At a fixed shear rate, for flow past a soft gel, the measured stress does not exhibit marked temporal fluctuations that would otherwise be present without the soft gel. Using flow visualization, we show that secondary flow patterns that form after the instability for a rigid surface disappear for flow on soft gel surfaces. In the case of rigid surfaces, the instability is sub-critical and exhibits hysteresis behavior, which again is absent when the flow occurs past a soft solid layer. Our results show that the role of the soft deformable solid is to suppress the purely elastic instability in torsional flows of polymeric liquids for intermediate shear rates. While it is known that soft deformable solids destabilize the flow of Newtonian liquids in the absence of inertial effects, our study shows that the effect of deformability can be opposite in the torsional flow of viscoelastic liquids.