| Droplet evaporation is a common phenomenon in scientific research and industrial applications.It is widely used in inkjet printing,spray cooling,material fabrication,disease diagnostics etc.Current wide transmission of Covid-19 virus also depends on respiratory droplet evaporation.Droplet evaporation is a typical multi-physics coupling process.Current research mainly focuses on the influence of a limited number of parameters on the droplet evaporation process,and the results usually conflict with each other especially on the evaporation of sessile droplets on the substrate.In order to resolve these problems,this thesis focuses on the droplet evaporation on substrates with different properties,a theoretical model of multi-physics coupling is established for the sessile droplet evaporation,the mechanism is revealed on the heat and mass.The effect of the key parameters on the evaporation rate and evaporation lifetime is quantitatively analyzed.The studies provide guidance and theoretical basis for the numerical simulation and experimental study on the evaporation of sessile droplets on complex surfaces.The main findings of this thesis are as follows:(1)Firstly,the isothermal evaporation of droplet on a hydrophilic(contact angle θc≤90°)flat substrate is studied.According to the vapor diffusion model of isothermal droplet evaporation,and the existing correlation on evaporation flux distribution around droplet surface the theoretical model is established for droplet evaporation under the constant contact angle(CCA)mode,constant contact radius(CCR)mode and stick-slip(SS)mode,and the evolution of the contact angle,contact radius and droplet volume with time is studied.The effects of initial contact radius,initial contact angle and critical contact angle on evaporation rate and lifetime are provided.It is found that with fixed initial volume of the droplet,the low initial contact angle or the critical contact angle can result in the short evaporation lifetime.(2)Secondly,the non-isothermal evaporation of droplets is studied on flat substrate with any wettability(contact angle 0<θc<180°).The droplet surface is no longer isothermal due to the cooling effect caused by the endothermic evaporation.By virtue of the analogy between the vapor concentration field during vapor diffusion and the electrostatic field around the charge,the heat conduction inside the droplet and the vapor diffusion outside the droplet are coupled at the droplet surface in the axisymmetric toroidal coordinate.Temperature and vapor concentration fields are obtained for evaporating droplets on substrates at different temperatures,as well as the evaporation flux distribution around the droplet surface.Furthermore,the lifetime of evaporating droplet is also obtained under CCR and CCA modes.In order to quantify the difference between the actual non-isothermal evaporation and the ideal isothermal evaporation,a correction factor K is proposed to be the ratio of the actual non-isothermal evaporation rate over the isothermal evaporation rate.It is found that K only depends on the droplet contact angle and the evaporative cooling number.When the contact angle and evaporative cooling number are larger,the correction factor K is smaller,i.e.the inhibition effect of evaporative cooling on evaporation is larger.In addition,numerical simulation is carried out to simulate the multiphysics coupling process during droplet evaporation the theoretical results are compared with the numerical results and the available experimental results,and prove the reliability of our theoretical model.Furthermore the scope is clarified when our theoretical model can be applied.(3)The above theoretical model of non-isothermal droplet evaporation is extended from flat substrate to curved substrates,including the effect of thermal conductivity inside the substrate on droplet evaporation.Under the axisymmetric toroidal coordinate,the heat conduction inside the substrate and the droplet are coupled at the solid-liquid interface,the heat conduction inside the droplet and vapor diffusion are coupled at the gas-liquid interface,the theoretical model is established on the curved substrate.The temperature field inside the substrate and the droplet,the vapor concentration field outside the droplet are obtained as well as the evaporation flux distribution around the droplet surface.Thus the evaporation rates and droplet lifetime are obtained in the CCR mode.The effects of the relative thermal conductivity between the substrate and the droplet,the substrate shape,the contact angle of the droplet,and the evaporative cooling number on the evaporation rates are quantitatively analyzed.The results show that under the fixed droplet volume,the droplet evaporates rates will decrease while lifetimes will increase when the relative thermal conductivity between the substrate and the droplet decreases,the droplet evaporation cooling number increases,the substrate tangential angle increases,or the droplet contact angle increases,(4)The droplet evaporation on the surface of immiscible liquid is further studied.Droplet can form liquid lens on the surface of immiscible liquids,and evaporate in CCA mode.A theoretical model of liquid lens evaporation is established under the axisymmetric toroidal coordinate,the heat conduction inside the droplet and the vapor diffusion outside the droplet are coupled at the gas-liquid interface.The temperature field inside the droplet,the evaporation flux distribution around the droplet surface and the lifetime are obtained.The effects of the liquid lens volume,the ratio of the upper and lower contact angles,the evaporative cooling number and interface shape near the triple line on the evaporation law are quantitatively analyzed.It is found that due to the effect of the evaporation area and the evaporative cooling,the evaporation rates show three trends with the increasing angle ratio:1)When the lens volume or evaporative cooling number is small,with the increasing angle ratio,the evaporation rate of the liquid lens increases gradually,and finally approaches the asymptotic value.2)When the lens volume or evaporative cooling number is large,the evaporation rate decreases,and after reaching the minimum value,it starts to increase and finally approaches the asymptotic value.3)When the lens volume and evaporative cooling number are at certain values,the evaporation rate first increases,after reaching maximum value,it starts to decrease to minimum value,then increase again until approaching an asymptotic value.(5)Finally,sessile droplet evaporation is investigated under kinetics-controlled condition.For droplet evaporation under pure vapor environment or micro-or nano-scale the vapor transport outside droplet is not diffusion-controlled as in above models,instead it is kinetics-controlled at the gas-liquid interface.For droplet evaporation on hydrophilic substrate(θc≤40°),heat conduction from the substrate to the droplet surface can be assumed as one-dimensional.The evaporation kinetics at the gas-liquid interface is calculated by Hertz-Knudsen model,coupling with heat conduction inside the substrate and droplet.Based on the calculus theory the evaporation flux distribution is calculated as well as the evaporation lifetime of droplets in CCR and CCA modes.The effects of substrate bottom temperature,substrate thermal conductivity,droplet contact angle,ambient temperature and ambient pressure on droplet evaporation are quantitatively analyzed.It is found that the "2/3 law" of droplet volume over evaporation time under the vapor diffusion law in CCA mode,and no longer applies when the droplet evaporates in kinetics-controlled condition... |
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