Shear-banding in entangled xanthan solutions: tunable transition from sharp to broad shear-band interfaces

We report on the smooth transition between gradient-banded velocity profiles with a sharp interface and curved velocity profiles, both resulting from strong shear-thinning dispersions of concentrated xanthan (a highly charged poly-saccharide). Pronounced shear-banded flow, where two extended shear-bands are separated by a relatively sharp interface, is observed in a limited range of shear rates, at very low ionic strength and at a high concentration, using heterodyne light scattering to measure spatially resolved velocity profiles. The width of the interface between the coexisting shear-bands broadens to span a sizable fraction of the gap of the shear cell, either by changing the shear rate, by lowering the concentration, or by increasing the ionic strength. The broadening results in a smooth transition to highly curved velocity profiles and is connected to a disappearing flow birefringence. Thus, these experiments show that the classic shear-banding instability can give rise to highly curved velocity profiles, due to the existence of broad interfaces between the bands, with an extent of the order or larger than the cell gap width. This observation may aid to resolve the ongoing dispute concerning shear-banding of highly entangled polymeric systems, suggesting that the curved velocity profiles that have been observed in the past are in fact shear-banded states with an unusually broad interface.