用纤维素纳米晶绘制泰勒涡：超临界光谱动力学
Painting Taylor vortices with cellulose nanocrystals: supercritical spectral dynamics
AI总结：
本研究利用纤维素纳米晶的剪切诱导双折射可视化泰勒-库埃特流，发现其流动稳定性主要由剪切变稀特性主导，并揭示了不同旋转组合下从轴对称到螺旋模式的转变规律。
摘要：
我们首次利用纤维素纳米晶（CNC）悬浮液固有的剪切诱导双折射和液晶特性进行流动可视化，在定制的泰勒-库埃特流动池中研究了纤维素纳米晶悬浮液的流动稳定性及时空光谱动力学。分析在独立旋转圆柱的恒定加速速度输入下进行，涵盖了从仅内圆柱或仅外圆柱旋转到三种反向旋转情形的多个案例。所有CNC悬浮液均具有可测量的弹性和剪切变稀特性，且两者均随CNC浓度增加而增强。我们表明，记录的流动图案本质上是类牛顿型的，非牛顿效应表现为波数减小以及改变不稳定模式起始的临界参数。外圆柱旋转流动情形在所有浓度下均保持稳定，而内圆柱旋转流动情形则转变为轴对称和方位角周期性的二次流。然而，不稳定的反向旋转情形变得对非对称螺旋模式不稳定。随着CNC浓度的增加，发现一种反向旋转情形中方位角周期性波状模式转变为非对称螺旋模式。与具有类似低至中等弹性和剪切变稀特性的聚合物溶液相比，CNC悬浮液的剪切变稀区域预计会导致手性向列相破坏，其弹性常数构成主要的结构弹性机制。因此，我们将CNC悬浮液的泰勒-库埃特稳定性解释为由其剪切变稀特性主导，这是由于在非线性流动条件下预期会丧失弹性。
英文摘要：
We study the flow stability and spatio-temporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor-Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elastic and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, unstable counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. In contrast to polymeric solutions of similar low to moderate elasticity and shear thinning, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor-Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions.
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