蒸发多组分微液滴中由分凝引起的瑞利-泰勒不稳定性
Rayleigh-Taylor instability by segregation in an evaporating multi-component microdroplet
摘要：
多组分液滴的蒸发与各种应用相关，但由于复杂的物理化学动力学而具有挑战性。最近，Li（2018）报告了1，2-己二醇-水二元液滴中的蒸发触发分离。在本工作中，我们向1，2-己二醇-水二元溶液中加入0.5wt%的硅油。这一微小的硅油浓度极大地改变了蒸发过程，因为它触发了从混合物中提前提取1，2-己二醇。令人惊讶的是，我们观察到1，2-己二醇的分离形成羽流，在液滴蒸发期间从固着液滴的边缘向顶点上升。通过将液滴倒置，即，通过研究悬垂液滴，没有羽流表明流动结构是由浮力引起的，浮力驱动瑞利-泰勒不稳定性（即，由密度差和重力加速度驱动）。从micro-PIV测量，我们进一步证明，隔离的非挥发性组分（1，2-己二醇）阻碍了蒸发附近的接触线，从而导致抑制在该区域的Marangoni流。因此，在长时间尺度上，重力效应在流动结构中起主导作用，而不是马兰戈尼流动。我们将蒸发速率的测量与Popov（2005）的扩散模型，结合拉乌尔定律和活度系数进行了比较。该比较确实证实了硅油引发的非挥发性1，2-己二醇的分离显著延迟了蒸发。采用扩展的扩散模型，考虑了偏析的影响，可以很好地描述蒸发过程。
英文摘要：
The evaporation of multi-component droplets is relevant to various applications but challenging to study due to the complex physicochemical dynamics. Recently, Li (2018) reported evaporation-triggered segregation in 1,2-hexanediol-water binary droplets. In this present work, we added 0.5 wt% silicone oil into the 1,2-hexanediol-water binary solution. This minute silicone oil concentration dramatically modifies the evaporation process as it triggers an early extraction of the 1,2-hexanediol from the mixture. Surprisingly, we observe that the segregation of 1,2-hexanediol forms plumes, rising up from the rim of the sessile droplet towards the apex during the droplet evaporation. By orientating the droplet upside down, i.e., by studying a pendant droplet, the absence of the plumes indicates that the flow structure is induced by buoyancy, which drives a Rayleigh-Taylor instability (i.e., driven by density differences & gravitational acceleration). From micro-PIV measurement, we further prove that the segregation of the non-volatile component (1,2-hexanediol) hinders the evaporation near the contact line, which leads to a suppression of the Marangoni flow in this region. Hence, on long time scales, gravitational effects play the dominant role in the flow structure, rather than Marangoni flows. We compare the measurement of the evaporation rate with the diffusion model of Popov (2005), coupled with Raoult's law and the activity coefficient. This comparison indeed confirms that the silicone-oil-triggered segregation of the non-volatile 1,2-hexanediol significantly delays the evaporation. With an extended diffusion model, in which the influence of the segregation has been implemented, the evaporation can be well described.
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