几何形状变化的流动聚焦通道中胶体线的形成
Formation of colloidal threads in geometrically varying flow-focusing channels
摘要：
当两种可混溶的流体相互接触时，浓度梯度引起应力。这些被称为Korteweg应力，并且类似于两种不混溶流体之间的界面张力，从而在不均匀混溶系统中充当有效界面张力（EIT）。EIT控制两种可混溶流体流动聚焦中粘性线的形成。为了进一步研究其意义，我们研究了由其自身溶剂聚焦的胶体分散体的线形成。实验与三维数值模型相结合，利用不同的流动聚焦通道设置系统地扩展以前的知识。在参考设置中，鞘流正交地撞击芯流，而在四个其它通道设置中，鞘流以相对于参考鞘方向为正和负的斜角撞击芯流。作为EIT的初始估计，我们将参考设置中的实验确定的螺纹形状拟合到通过有效毛细管数依赖于EIT的主曲线。通过数值再现这些实验结果，得出的结论是，估计的EIT是在25%的最佳EIT值，可以推导出通过迭代拟合的数值结果的实验测量。无论通道设置，使用最佳EIT从参考设置评估进行进一步的数值计算显示良好的协议与实验结果的螺纹形状，润湿区域的形态，和速度流场。数值和实验结果的一对一比较揭示了EIT对螺纹从通道顶壁和底壁分离的关键作用，为理解具有高粘度对比度的混溶系统中涉及的物理现象带来了有用的见解。
英文摘要：
When two miscible fluids are brought into contact with each other, the concentration gradients induce stresses. These are referred to as Korteweg stresses and are analogous to interfacial tension between two immiscible fluids, thereby acting as an effective interfacial tension (EIT) in inhomogeneous miscible systems. EIT governs the formation of a viscous thread in flow-focusing of two miscible fluids. To further investigate its significance, we have studied thread formation of a colloidal dispersion focused by its own solvent. Experiments are combined with three-dimensional numerical models to systematically expand previous knowledge utilising different flow-focusing channel setups. In the reference setup, the sheath flows impinge the core flow orthogonally while in four other channel setups, the sheath flows impinge the core flow at an oblique angle that is both positive and negative with respect to the reference sheath direction. As an initial estimate of the EIT, we fit the experimentally determined thread shape in the reference setup to a master curve that depends on EIT through an effective capillary number. By numerically reproducing these experimental results, it is concluded that the estimated EIT is within 25% of the optimal EIT value that can be deduced by iteratively fitting the numerical results to the experimental measurements. Regardless of channel setups, further numerical calculations performed using the optimal EIT evaluated from the reference setup show good agreement with the experimental findings in terms of thread shapes, wetted region morphologies, and velocity flow fields. The one-to-one comparison of numerical and experimental findings unveil the crucial role of EIT on the thread detachment from the top and bottom walls of the channel, bringing useful insights to understand the physical phenomenons involved in miscible systems with a high-viscosity contrast.
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