Study On The Evaporation Mechanism Of Droplets On Immiscible Liquid Substrate Surfaces

The droplet evaporation process is a ubiquitous physical phenomenon in nature,and it also widely exists in many engineering applications,including DNA-chip manufacturing,inkjet printing,spray cooling,and particle deposition applications,etc.Compared with the solid substrate surface,the liquid substrate surface is more suitable for the self-assembly of particles due to its deformability and flexibility.In this paper,experimental and theoretical studies on the droplet evaporation process on the immiscible liquid substrate surface are carried out,which will help to deeply understand the heat and mass transfer laws in the droplet evaporation process and provide a theoretical basis for realizing the self-assembly of ordered microstructures on the liquid substrate surface.An experimental system was designed and built to record the droplet evaporation process on the immiscible liquid substrate surface.The ionic liquid(1-butyl-3-methylimidazolium hexafluorophosphate)was used as the substrate liquid to study the evaporation process of a hexane droplet on its surface.The experimental study shows that there are a spreading stage in which the contact line gradually increases and an evaporation stage in which the contact line gradually decreases when the hexane droplet evaporation on the surface of an ionic liquid with a depth of 4 mm.In addition,both the maximum contact diameter and the droplet lifetime decrease with decreasing initial volume of the hexane droplet,or with increasing temperature of the ionic liquid.A theoretical model was established to describe the morphology evolution of a droplet evaporation process on the liquid substrate surface.The theoretical model considers the effect of the droplet evaporation on its morphology evolution.The axisymmetric profile at a certain moment in the droplet evaporation process can be obtained by numerically solving the Young-Laplace equation for each interface with coupled boundary conditions at the contact line.The calculation results show that the contact angles remain almost unchanged during most of the droplet lifetime,that is,the evaporation process of the hexane droplet on the ionic liquid surface in this stage follows the constant contact angle evaporation mode.However,in the later stage of evaporation,the contact angles gradually decrease and the contact line rapidly shrinks with the evaporation time.In addition,the dynamic contact angle decreases with increasing initial volume of the hexane droplet,or with decreasing temperature of the ionic liquid.And the variation law of the dynamic contact angle with the dimensionless evaporation time is consistent.Consider the effect of the interfacial evaporative cooling during droplet evaporation,and the analytical solutions for the temperature distribution inside the droplet and the vapor concentration distribution around it are derived with coupled boundary conditions at the droplet/air interface established by the heat and mass transfer relationship.The calculation results show that the temperature distribution inside the hexane droplet is also inhomogeneous when it evaporate on the ionic liquid,and the lowest temperature appears in the center of the hexane droplet surface.The excess temperature,heat flux and evaporation flux at the droplet/air interface all increase monotonically along the droplet surface from the center to the contact line.The excess temperature decreases while both the heat flux and evaporation flux increase with increasing temperature of the ionic liquid.In addition,the effects of density ratio and evaporative cooling number on the droplet evaporation process are analyzed.Consider the effect of the evaporation flux distribution on the droplet surface and the contact line shrinkage on its internal flow field.The self-similar solution for the flow field near the contact line inside the droplet is derived when it evaporate on the liquid substrate surface.The calculation results show that within a certain range from the contact line,the uneven evaporation flux will cause an outward flow inside the hexane droplet,while the contact line shrinkage will cause an inward flow.The outward flow caused by the uneven evaporation flux inside the hexane droplet is partially offset by the inward flow caused by the contact line shrinkage,and the flow velocity is quite small at positions far from the contact line.There is a counterclockwise flow vortex near the contact line inside the hexane droplet,that is,the outward flow along the droplet/air interface,and the inward flow along the droplet/substrate liquid interface.In addition,the effect of viscosity ratio on the flow field near the contact line inside the droplet is analyzed.