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.

中文翻译：

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纠缠的黄原胶溶液中的剪切带：从尖锐的剪切带到宽剪切带的界面的可调过渡†

我们报告了陡峭的带状速度分布曲线和弯曲的速度分布曲线之间的平滑过渡，这两者都是由于浓缩的黄原胶（一种高度带电的多糖）的剪切稀疏性很强而导致的。使用外差光散射测量空间分辨速度时，在有限的剪切速率范围内，非常低的离子强度和高浓度下，观察到明显的剪切带流动，其中两个扩展的剪切带被相对尖锐的界面分开。个人资料。通过改变剪切速率，降低浓度或增加离子强度，共存的剪切带之间的界面宽度加宽以覆盖剪切单元间隙的相当一部分。加宽导致平滑过渡到高度弯曲的速度曲线，并与消失的流双折射有关。因此，这些实验表明，经典的剪切带不稳定性会导致高度弯曲的速度分布，这是由于带之间存在较宽的界面，其程度约为或大于单元间隙宽度。该观察结果可能有助于解决有关高度纠缠的聚合物体系的剪切带的争论，这表明过去观察到的弯曲速度分布实际上是具有异常宽界面的剪切带状态。由于带之间存在较宽的界面，其程度等于或大于单元间隙宽度。该观察结果可能有助于解决有关高度纠缠的聚合物体系的剪切带的争论，这表明过去观察到的弯曲速度分布实际上是具有异常宽界面的剪切带状态。由于带之间存在较宽的界面，其程度等于或大于单元间隙宽度。该观察结果可能有助于解决有关高度纠缠的聚合物体系的剪切带的争论，这表明过去观察到的弯曲速度分布实际上是具有异常宽界面的剪切带状态。