Ring-shaped deposits can be often found after a droplet evaporates on a substrate. If the fluid in the droplet is a pure liquid and its contact line remains pinned during the process, the mechanism behind such ring-shaped deposition is the well-known coffee-stain effect. However, adding small amounts of salt to such a droplet can change the internal flow dramatically and {consequently} change the deposition mechanism. Due to an increase of surface tension in the contact line region, a Marangoni flow arises which is directed from the apex of the droplet towards the contact line. As a result, particles arrive at the contact line following the liquid-air interface of the droplet. Interestingly, the deposit is also ring-shaped, as in the classical coffee-stain effect, but with a radically different morphology: particles form a monolayer along the liquid-air interface of the droplet, instead of a compact three-dimensional deposit. Using confocal microscopy, we study particle-per-particle how the assembly of the colloidal monolayer occurs during the evaporation of droplets for different initial concentration of sodium chloride and initial particle dilution. Our results are compared with classical diffusion-limited deposition models and open up an interesting scenario of deposits via interfacial particle assembly, which can easily yield homogeneous depositions by manipulating the initial salt and particle concentration in the droplet.

中文翻译：

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蒸发胶体液滴中的颗粒单层组件

液滴在基材上蒸发后通常会发现环形沉积物。如果液滴中的流体是纯液体，并且在此过程中其接触线保持固定，则这种环形沉积的机制就是众所周知的咖啡污渍效应。但是，向这种液滴中添加少量盐会极大地改变内部流动，从而（因此）改变沉积机理。由于在接触线区域中表面张力的增加，出现了从液滴的顶部朝向接触线引导的马兰戈尼流。结果，颗粒沿着液滴的液-气界面到达接触线。有趣的是，沉积物也呈环形，就像经典的咖啡色效果一样，但形态却截然不同：颗粒沿着液滴的液-气界面形成单层，而不是紧凑的三维沉积物。使用共聚焦显微镜，我们研究了在不同的氯化钠初始浓度和初始稀释度下，液滴蒸发过程中胶体单层组装如何发生。我们的结果与经典的扩散受限沉积模型进行了比较，并通过界面粒子组装打开了一个有趣的沉积场景，通过操纵液滴中的初始盐和粒子浓度，可以轻松地产生均匀沉积。我们研究了在不同的氯化钠初始浓度和初始颗粒稀释度下，液滴蒸发过程中胶体单层组装如何发生。我们的结果与经典的扩散受限沉积模型进行了比较，并通过界面粒子组装打开了一个有趣的沉积场景，通过操纵液滴中的初始盐和粒子浓度，可以轻松地产生均匀沉积。我们研究了在不同的氯化钠初始浓度和初始颗粒稀释度下，液滴蒸发过程中胶体单层组装如何发生。我们的结果与经典的扩散受限沉积模型进行了比较，并通过界面粒子组装打开了一个有趣的沉积场景，通过操纵液滴中的初始盐和粒子浓度，可以轻松地产生均匀沉积。