Experimental Study On Evaporation Of Droplets On The Surface Of Immiscible Liquids

Understanding the evaporation of liquid droplets on the surface of an immiscible liquid is of practical importance in the petroleum industry and pollution control.Compared with traditional solid surfaces,liquid substrates can eliminate the effects of unavoidable roughness and chemical heterogeneity on droplet evaporation,and can therefore minimize contact angle hysteresis(CAH).What's more,the bending deformation of the liquid surface of the substrate causes droplet evaporation to be unique.In this paper,the study of evaporation process of n-hexane droplets on the surface of various immiscible liquids is experimentally and theoretically studied.The research results help to understand the heat and mass transfer laws of droplet evaporation,and provide a new method for controlling droplet evaporation.In the experimental research,an experimental system composed of a temperature control system,a mass weighing system,and a photography system is designed and built.The n-hexane droplets were placed on the surface of deionized water and ionic liquid(1-butyl-3-methylimidazole hexafluorophosphate,[BMIM]PF6)respectively.The evolution of n-hexane droplet morphology during the entire evaporation process was captured from the horizontal direction and directly above using high-speed cameras,so as to obtain the change of contact angle and contact diameter with time,and the change of the droplet mass was recorded using an electronic balance.When a hexane droplet was brought into contact with the water surface,a train of interfacial capillary waves were generated by the disturbance of liquid-liquid interface.The capillary waves propagated in two directions:the propagation in the vertical direction caused the deformation of the droplet,and that in the radial direction created concentric rings ahead of the droplet.The wetting mode of n-hexane droplets on the surface of deionized water is pseudo-partial wetting.The entire evaporation process of n-hexane droplets undergoes two stages of spreading and shrinking,which are dominated by spreading and evaporation,respectively.The former stage accounts for about 6%of the droplet lifetime.During the spreading phase,the contact angle of the n-hexane droplets decreases rapidly.For most time of the receding stage,the contact angle remains constant and the square of the dimensionless contact radius(r/r_max)² is linearly related to time.The geometric shape of the contact edge between the n-hexane droplet and the deionized water is circular for most of the time.During the final rapid evaporation stage(about 2%of the droplet lifetime),the shape of the hexane droplet changed and the droplet shrank rapidly until it disappeared.When the liquid substrate is an ionic liquid([BMIM]PF6),the spreading coefficient of the system is negative,and the wetting mode is partial wetting.The evaporation process of droplets also undergoes two stages o f spreading and shrinking.However,the spreading stage is significantly shortened,which only accounts fo r 2%of the life of the droplet.And the contour of the contact line is always round without significant deformation.The turbulence of the n-hexane droplets in contact with the ionic liquid also generates capillary waves.However,the low surface tension and high viscosity of the ionic liquid cause the capillary waves to propagate mainly in the vertical direction,causing only significant deformation of the droplet.Compared with the situation of deionized water surface,when the n-hexane droplet is placed on the surface of the ionic liquid,the surface of the liquid substrate is significantly bent,and the evolution of the substrate surface was recorded by a high-speed camera system,so the contact line,the lowest point position of the liquid substrate surface,and the change in contact angle with time were obtained.The results show that the n-hexane droplet sink after being placed on the surface of the ionic liquid,accompanied by the downward movement of the contact line,and then it is maintained in specific plane to shrink until completely evaporated.The change of the contact angle of n-hexane droplets goes through three stages.First,it decreases rapidly during the spreading phase,and then the constant contact angle remains constant until the contact diameter shrinks to a certain value(about 3 mm).Finally,the contact angle decreases rapidly again during the rapid contraction phase of the droplet.In the theoretical research,a theoretical model based on diffusion-controlled evaporation under the constant contact angle mode was developed to describe the evaporation of a hexane lens on the surface of an immiscible liquid.In terms of geometry,the model assumes that a lens is composed of upper and lower spherical caps,and the apparent contact angle is defined based on the intersection of the two caps.The results calculated using the model were found to be in good agreement with the experimental data,and the model is applicable to two liquid-liquid systems:n-hexane-water and n-hexane-ionic liquid.Finally,the effects of initial lens volume,water temperature,and water surface deformation on lens evaporation were discussed through calculations.The results showed that increase in the base liquid temperature and deformation of the base surface accelerated the evaporation process.