Study On The Flow And Evolution Of Three Phase Lines During The Evaporation Process Of Droplets On The Liquid Surface

Droplets are the most common way of liquid existence in nature,and therefore,scientific research and technological development in many fields involve this phenomenon,such as combustion in internal combustion engines,phase change enhancement of heat exchangers,etc.Mastering the geometric evolution laws and heat and mass exchange characteristics of droplets has important application value.At present,the main research direction of domestic scholars is focused on the relevant mechanisms of droplet evaporation on solid surfaces and in gases,while research on droplets on flexible material surfaces and liquid surfaces is relatively insufficient.In this paper,aiming at three different droplet sizes(1μl,2μl,3μl)and three different base liquid temperatures(10 ℃,30 ℃,50 ℃),the whole process of droplet evaporation on the liquid surface was photographed by using an infrared thermal imager,and its surface temperature distribution characteristics and internal flow pattern evolution process were obtained;The actual imaging of droplet evaporation on the liquid surface was obtained by using a 3D video microscope,and the changes in its geometric morphology and the process of three phase lines were analyzed;By using a three-dimensional surface measurement instrument to capture microscopic images of small droplets on the three phase line during evaporation on the liquid surface,the evolution process of the three phase line of small droplets and the mechanism of shrinkage,bending,and fracture of the three-phase contact line were analyzed.The following conclusions were finally obtained:(1)The surface temperature of n-hexane droplets on the liquid surface can be divided into two or three parts based on the temperature of deionized water.The fluidity and uneven temperature distribution of deionized water will affect the temperature distribution and internal flow of hexane lens.The interaction among air,water and n-hexane makes the surface temperature distribution and internal flow of n-hexane droplets special.The increase in temperature of deionized water gradually changes the vertical and radial temperature gradients within n-hexane droplets,and changes the importance of buoyancy convection and thermocapillary flow within n-hexane droplets.Their competition determines the flow mode within n-hexane droplets.(2)Under the combined action of circumferential surface tension gradient and radial "surface tension barrier",n-hexane droplets at the strong evaporation edge generate circumferential flow,and then form a low-temperature zone at the edge,forming a "low-temperature channel" between the central low-temperature zone and the strong evaporation edge.(3)We found that the change in the diameter of n-hexane droplets can be divided into three stages: 1)diffusion stage;2)Contraction stage;3)Instantaneous disappearance stage.The initial and maximum diameters of n-hexane droplets begin to increase with the increase of deionized water temperature,and gradually decrease with the increase of alkali solution temperature after reaching a certain peak.When n-hexane droplets are added to the base solution,the evaporation rate of the entire solution increases.When n-hexane droplets evaporate,the solution returns to the original evaporation rate of the base solution.