Elastic and Shear thinning instabilities for the flow of polymer solutions through microtubes

It is well known that the flow of dilute polymer solution through tubes undergoes a non-linear subcritical transition at Re slightly higher than 2000. However, when the polymer concentration is increased beyond a threshold concentration, the instability occurs at a Re much lower than 2000. The instability occurring at Re lower than 2000 at higher polymer concentration (hence higher elasticity) is a linear instability. A recent article by Poole, 2016, shows that if the polymer solution prepared is sufficiently shear thinning in nature, an instability occurs at very low inertia (Re\textasciitilde 50). We explore the possibility of very low Re instability for the flow of concentrated polymer solutions through micro-tubes. High concentration of polymer solution coupled with small tube diameters enable us to reach high elasticity numbers. We observe that the polymer solution destabilize at very low Re (Re\textasciitilde 10). The nature of instability is also observed to be very different as compared to elasto-inertial instability as observed from the scaling relationships.