低表面张力液滴类超疏悬浮状态实验

Experiment on the quasi-superhydrophobic levitation state of droplets with low surface tension

低表面能工质被广泛应用于各种工业和先进技术领域,如制冷剂喷雾冷却和微流控系统等,其润湿性和动力学行为的有效调控对提高分析过程的效率具有非常重要的作用.本文利用表面规则微孔的气流作用,调控低表面张力液滴的润湿行为,形成类超疏悬浮状态,并对该现象中液滴运动行为及润湿性变化规律开展研究.类超疏悬浮状态中,液滴的运动行为演变可以分为悬停、振荡、弹跳、脱离4个阶段,本文探究了弹跳阶段液滴重力势能与表面能之间的转换关系.类比Young’s方程,提出了类超疏悬浮状态中的类接触角概念,用于研究液滴不同阶段的润湿行为变化规律,分析了在惯性力和表面张力影响下液滴类接触角与韦伯数的关系.

Droplet wettability and dynamic behavior of working fluids with low surface energy are widely used in various industrial and advanced technology fields,such as refrigerant spray cooling and microfluidic chip.Their wettability and dynamic behavior play a critical role in improving the efficiency of industrial processes.Some scholars have realized the wettability control of droplets of low surface energy fluids by changing the characteristics of micro-nano structures on solid surfaces.In addition,many external incentive ways,such as electrowetting,magnetic field,thermal stimulation,and supersonic vibration,have been proposed to control the droplet wetting state and motion behavior.However,such methods have limitations and no significant effect on low surface energy working fluids.Inspired by the above methods and the Leidenfrost phenomenon,this study made use of the action of the microporous airflow,which can regulate the wettability and motion behavior of low surface tension droplets without adding additional components to the droplets and without special requirements for the droplets themselves.A visual experimental system for wetting and dynamic behavior of droplets regulated by the microporous airflow was set up,and the experimental section of the regular array micropore was machined by laser.The momentum force generated by the microporous flow was used to balance droplet gravity,which overcomes the wetting behavior of the low surface tension droplets FC3283 and FC40 and the realizes the quasi-superhydrophobic levitation state of droplets with low surface tension.This study aimed to explore the regulatory mechanism of the microporous flow additional momentum force on the wettability and dynamic behavior of droplets with low surface energy.Experimental results showed that the motion behavior of droplets in the quasi-superhydrophobic levitation state could be divided into four stages(i.e.,hovering,bobbing,bouncing,and detaching)with the decrease in droplet radius.The motion behavior of droplets was related to the dynamic balance of gravity,microporous flow momentum force,inertia force,and surface tension of droplets.In the bouncing stage,mutual conversion of energy was observed in the process of droplet falling and rising.A positive correlation was found between gravitational potential energy and surface energy.This kind of energy conversion made droplets oscillate periodically at a stable frequency.In addition,the deformation degree and amplitude of droplets with two kinds of low surface energy working fluids during the bouncing stage were compared,and the effect of viscosity on the droplet energy conversion process was analyzed.For wettability research,analogous to Young’s equation,combined with the force analysis of droplets at the inflection point during maximum deformation,the concept of pseudo contact angle was proposed to describe the wetting behavior of droplets.The pseudo contact angle of droplets in different oscillation stages was analyzed.In the hovering stage,the pseudo contact angle of droplets reached153.6°.In the bouncing stage,as the droplet radius and falling velocity increased,the inertia of droplets increased and the surface tension decreased.Therefore,with the increase in Weber number,the pseudo contact angle of droplets decreased gradually.This method can supplement the external field force droplet manipulation method,and it provides a new method for potential applications in low surface tension droplet manipulation,lossless rapid directional transport,and microfluidic system design.