表面活性剂液滴过渡沸腾的Marangoni效应与二次液滴形成

Marangoni effect of surfactant droplet in transition boiling and formation of secondary droplet

表面活性剂液滴撞击不同温度基底的动力学过程,在传热、冷却和打印等领域均有广泛涉及.本文利用高速摄影技术对表面活性剂SDS,CTAB和Triton X-100的水溶液液滴撞击热铝板的过程进行观测,研究不同表面活性剂液滴撞击热铝板动力学过程.实验发现,处于过渡沸腾的表面活性剂液滴,在蒸发的最后阶段会形成一个处于非浸润状态的二次液滴.分析表明,液滴撞击基底后,液滴的三相接触线和液滴顶部产生温度梯度,三相接触线附近的表面活性剂分子浓度显著大于液滴顶部.由浓度梯度驱动的Marangoni效应使上层液体得以保持,并在蒸发的最终阶段,逐渐收缩为球形,形成二次液滴,在底部气泡爆炸产生的冲击下脱离基底并起跳.二次液滴的半径随初始液滴浓度升高而增大,最终达到饱和半径.该项工作阐明了二次液滴形成过程中表面活性剂的作用,为理解Leidenfrost效应的物理机制以及调控沸腾传热提供了参考.

The dynamic processes of surfactant droplets impacting onto substrates of varied temperatures have been widely studied in heat transfer,cooling and printing.In this work,we observe the impacting process of aqueous droplets of surfactants SDS,CTAB,and Triton X-100 on a hot aluminum plate via a high-speed camera,in order to study the dynamics of different surfactant droplets impacting on a hot aluminum substrate.Experimentally,it is discovered that the surfactant droplets in transition boiling produce a secondary droplet of non-wetting state in the final stage of evaporation.The analysis demonstrates that after the droplet impacts the substrate,a temperature gradient is created between the top of the droplet and the triple-phase contact line,increasing the surfactant concentration near the triple-phase contact line as compared with that of the top.The top liquid is maintained by the Marangoni effect,which is caused by the concentration gradient.In the final stage of the evaporation process,the residual droplet gradually shrinks into a sphere.It is detached from the substrate and taken off under the impulse force of the bubble explosion at the bottom,generating the secondary droplet.The radius of the secondary drop increases with the raising of initial concentration of the drop,but ultimately reaches the saturation size.This work explains the role of surfactants in forming secondary droplets.Additionally,this work provides a reference for understanding the physical mechanism of Leidenfrost effect and the controlling of boiling heat transmission.