Evaporation of multicomponent droplets has gained much attention nowadays because of their complex flow fields and various deposition patterns. Here we observe strong flows in evaporating sodium chloride saline droplets with suspended alumina oxide nanoparticles. The evolution of flow pattern was studied by tracking the trajectories of particles and the velocity field was investigated with Particle Image Velocimetry analysis. The non-uniform evaporation rate along the droplet surface leads to a concentration gradient which induces the convection flow. During the evaporation process before crystallization happens, evolution of the flow can be divided into two regimes. In Regime I, a centrosymmetric convection recirculation is formed gradually. In Regime II, the convection recirculation migrates to the droplet edge and evolves into several small vortexes. At the late evaporation stage, crystallization could induce strong convection flows. It is shown that the flow tends to become more chaotic with a lower salt concentration.

中文翻译：

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含悬浮纳米颗粒的盐水干燥过程中的水动力模式

如今，多组分液滴的蒸发由于其复杂的流场和各种沉积方式而备受关注。在这里，我们观察到悬浮的氧化铝纳米粒子蒸发氯化钠盐雾中的强劲流动。通过跟踪粒子的轨迹研究了流型的演变，并通过粒子图像测速分析研究了速度场。沿液滴表面的蒸发速率不均匀会导致浓度梯度，从而引起对流。在发生结晶之前的蒸发过程中，流动的演变可分为两种状态。在体制I中，逐渐形成了中心对称对流再循环。在制度II中，对流再循环迁移到液滴边缘并演变成几个小涡旋。在蒸发后期，结晶可能会引起强烈的对流。结果表明，盐浓度较低时，流动趋于变得更加混乱。