Evaporation kinetics of pure water drops: Thermal patterns, Marangoni flow, and interfacial temperature difference
Evaporation kinetics of pure water drops: Thermal patterns, Marangoni flow, and interfacial temperature difference
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DOI：
10.1103/physreve.98.052804
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发表时间：
2018-11
影响因子：
2.4
通讯作者：
Tejaswi Josyula;Zhenying Wang;Alexandros Askounis;D. Orejón;S. Harish;Y. Takata;P. S. Mahapatra;A. Pattamatta
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文献类型：
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作者：
Tejaswi Josyula;Zhenying Wang;Alexandros Askounis;D. Orejón;S. Harish;Y. Takata;P. S. Mahapatra;A. Pattamatta
我们系统地研究了Marangoni对流在纯水滴蒸发动力学中的作用，考虑了加热制度和表面润湿性的影响。Marangoni流是通过恒定壁温(均匀加热)和恒定热流(局部加热)下的加热来诱导的。为了可视化蒸发过程中出现的热图案，我们使用了红外热像仪，并用CCD相机捕捉到了液滴轮廓的演变，以跟踪每个液滴的蒸发动力学。我们观察到，在不同的蒸发模式(即，恒定半径、角度或粘滑)下，液滴内部的温差与液滴形状的演变之间存在很强的相关性，从而导致不同的Marangoni流型。在均匀加热条件下，由于Marangoni对流引起的稳定的再环流涡旋在高温时出现，但在蒸发过程的后期逐渐消失。另一方面，在局部加热的情况下，恒定的热通量导致降温内的温差迅速增加，从而能够在整个蒸发过程中维持Marangoni流动。表面润湿性也在Marangoni流的出现和蒸发动力学中发挥作用。特别是，对于均匀加热和局部加热，疏水性表面上的液滴中的再循环流动比亲水性表面上的流动更强。为了量化加热模式的影响和Marangoni流的重要性，我们计算了每种情况下的蒸发通量，发现在局域加热情况下蒸发通量要高得多。蒸发通量取决于蒸气相向环境的扩散和自然对流。因此，我们估计了每种情况下的Grashof数，并发现气相中的自然对流与加热制度或液相中的Marangoni对流之间有很强的关系。随后，我们证明了以前报道的纯扩散模型在描述加热液滴蒸发时的局限性。
We report a systematic study of the role of Marangoni convection in the evaporation kinetics of pure water drops, considering the influence of the heating regime and surface wettability. Marangoni flows were induced via heating under constant wall temperature (uniform heating) and constant heat flux (local heating) regimes below the drops. To visualize the thermal patterns emerging during the evaporation, we employed infrared thermography and we captured the evolution of the drop profile with a CCD camera to follow the evaporation kinetics of each drop. We observed a strong correlation between the temperature difference within the drop and the evolution of the drop shape during different modes of evaporation (i.e., constant radius, angle, or stick-slip) resulting in different Marangoni flow patterns. Under uniform heating, stable recirculatory vortices due to Marangoni convection emerged at high temperature, but they faded at later stages of the evaporation process. On the other hand, in the localized heating case, the constant heat flux resulted in a rapid increase in the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found also to play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows in drops on hydrophobic surfaces were stronger when compared to flows on hydrophilic surfaces for both uniform and local heating. To quantify the effect of the heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found it to be much higher in the localized heating case. Evaporative flux depends on both diffusion and natural convection of the vapor phase to the ambient. Hence, we estimated the Grashof number for each case and found a strong relation between natural convection in the vapor phase and heating regime or Marangoni convection in the liquid phase. Subsequently, we demonstrate the limitation of the previously reported diffusion-only model in describing the evaporation of heated drops.