De Dier Raf, Sempels Wouter, Hofkens Johan, Vermant Jan
Laboratory for Soft Materials, Department of Materials, ETH Zürich , Zürich, Switzerland.
Langmuir. 2014 Nov 11;30(44):13338-44. doi: 10.1021/la503655j. Epub 2014 Oct 28.
The drying of sessile droplets represents an intriguing problem, being a simple experiment to perform but displaying complexities that are archetypical for many free surface and coating flows. Drying can leave behind distinct deposits of initially well dispersed colloidal matter. For example, in the case of the coffee ring effect, particles are left in a well-defined macroscopic pattern with particles accumulating at the edge, controlled by the internal flow in the droplet. Recent studies indicate that the addition of surfactants strongly influences this internal flow field, even reversing it and suppressing the coffee ring effect. In this work, we explore the behavior of droplets at high surfactant loadings and observe unexpected outward fingering instabilities. The experiments start out with droplets with a pinned contact line, and fast confocal microscopy is used to quantify a radially outward surfactant-driven Marangoni flow, in line with earlier observations. However, the Marangoni flows are observed to become unstable, and local vortex cells are now observed in a direction along the contact line. The occurrence of these vortices cannot be explained on the basis of the effects of surfactants alone. Thermal imaging shows that thermocapillary effects are superimposed on the surfactant-driven flows. These local vortex cells acts as little pumps and push the fluid outward in a fingering instability, rather than an expected inward retraction of the drying droplet. This leads to a deposition of colloids in a macroscopical flower-shaped pattern. A scaling analysis is used to rationalize the observed wavelengths and velocities, and practical implications are briefly discussed.
固着液滴的干燥是一个引人入胜的问题，它虽是一个易于实施的简单实验，但却展现出许多自由表面流和涂层流所共有的典型复杂性。干燥过程可能会留下由最初分散良好的胶体物质形成的独特沉积物。例如，在咖啡环效应的情况下，颗粒会以明确的宏观模式沉积，颗粒在液滴边缘堆积，这是由液滴内部的流动控制的。最近的研究表明，添加表面活性剂会强烈影响这种内部流场，甚至使其反转并抑制咖啡环效应。在这项工作中，我们研究了高表面活性剂负载下液滴的行为，并观察到了意外的向外指进不稳定性。实验开始时使用具有固定接触线的液滴，并利用快速共聚焦显微镜来量化径向向外的表面活性剂驱动的马兰戈尼流，这与早期的观察结果一致。然而，观察到马兰戈尼流变得不稳定，并且现在在沿着接触线的方向上观察到了局部涡旋单元。这些涡旋的出现不能仅基于表面活性剂的作用来解释。热成像显示热毛细效应叠加在表面活性剂驱动的流动上。这些局部涡旋单元就像小泵一样，以指进不稳定性将流体向外推，而不是干燥液滴预期的向内收缩。这导致胶体以宏观的花形图案沉积。我们使用标度分析来解释观察到的波长和速度，并简要讨论了实际意义。
