Three-dimensional surfactant-covered flows of thin liquid films on rotating cylinders
Three-dimensional surfactant-covered flows of thin liquid films on rotating cylinders
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旋转圆柱体上表面活性剂覆盖的三维薄膜液流
DOI：
10.1017/jfm.2018.153
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发表时间：
2018
影响因子：
3.7
通讯作者：
Satish Kumar
中科院分区：
文献类型：
--
作者：
Weihua Li;Satish Kumar
离散物体的涂层是制造各种产品的重要但知之甚少的步骤。一个重要的模型问题是旋转圆柱体上的薄液膜的流动，其中可能出现不稳定性并损害涂层均匀性。在这项工作中，我们使用润滑理论和流动显示实验来研究表面活性剂对这些流动的影响。数值求解了描述膜厚和不溶性表面活性剂浓度随时间、角坐标和轴向坐标变化的耦合演化方程。结果表明，表面张力所产生的轴向和角度的变化，在角曲率驱动流在轴向方向上，往往平滑自由表面扰动，并导致一个稳定的速度窗口中，轴向扰动不增长。表面活性剂的存在导致Marangoni应力，其可以通过与稳定流相反的驱动流而导致稳定速度窗口消失。此外，马兰戈尼应力倾向于减小在低旋转速率下形成的液滴之间的间距，并且减小在高旋转速率下形成的环的生长速率。流动可视化实验产生的意见，定性与线性稳定性分析和模拟结果的预测一致。可视化还表明，表面活性剂倾向于抑制滴落，减缓自由表面扰动的发展，并减少环和液滴的移动和合并，从而在实际应用中有更多的时间用于固化涂层。
The coating of discrete objects is an important but poorly understood step in the manufacturing of a broad variety of products. An important model problem is the flow of a thin liquid film on a rotating cylinder, where instabilities can arise and compromise coating uniformity. In this work, we use lubrication theory and flow visualization experiments to study the influence of surfactant on these flows. Two coupled evolution equations describing the variation of film thickness and concentration of insoluble surfactant as a function of time, the angular coordinate and the axial coordinate are solved numerically. The results show that surface-tension forces arising from both axial and angular variations in the angular curvature drive flows in the axial direction that tend to smooth out free-surface perturbations and lead to a stable speed window in which axial perturbations do not grow. The presence of surfactant leads to Marangoni stresses that can cause the stable speed window to disappear by driving flow that opposes the stabilizing flow. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and reduce the growth rate of rings that form at high rotation rates. Flow visualization experiments yield observations that are qualitatively consistent with predictions from linear stability analysis and the simulation results. The visualizations also indicate that surfactants tend to suppress dripping, slow the development of free-surface perturbations, and reduce the shifting and merging of rings and droplets, allowing more time for solidifying coatings in practical applications.