Study Of The Evaporation Characteristics Of Volatile Droplets On A Liquid Substrate

Evaporation is widely present in various fields of natural and industrial production,such as MEMS refrigeration technology,inkjet printing technology,distillation process,etc.When there is a surface tension gradient in volatile liquids and basal liquids,the evaporation of volatile droplets often causes convection at the interface,the Marangoni effect.It is of great engineering value and academic significance to change the heat and mass transfer law in the liquid-liquid system by controlling the Marangoni effect to improve production efficiency.In this project,the experimental study of the evaporation characteristics of volatile droplets on the liquid substrate is carried out,and the interfacial dynamics and thermal mass transport laws of evaporation of volatile droplets on the liquid substrate are explored from the perspectives of unheated and horizontal heating.Theoretical research:Based on the kinetic principle of evaporation of volatile droplets on the liquid substrate,the Marangoni effect caused by the concentration gradient and temperature gradient was analyzed,and the variation relationship between the droplet spread radius,the spreading time with the thickness of the substrate,the mass fraction of IPA and the initial volume of the droplet was derived.The results show that the spread radius and the initial volume and substrate thickness of the droplets are R_max∝Ω₀^1/4,R_max∝H^1/4 power functions,respectively.At the same time,the droplet spread time and the initial volume and substrate thickness of the droplets are t∝Ω₀^1/2,t∝H^-1/2 power function relationship,respectively.The theory analyzes two kinds of instabilities in which droplets spread on the liquid substrate:Marangoni convective instability occurs when the temperature gradient is small,and annular R-P instability occurs when the temperature gradient is large.In terms of experimental research,an experimental platform for droplets to evaporate on the liquid substrate was built,and the characteristics of volatile droplets spreading on the isothermal and heated liquid substrate were carried out.The dynamic mechanism of the spread of volatile droplets on the liquid substrate was explained experimentally,and the balance of viscous stress and Marangoni tangential stress was the main cause of spreading and contraction.In terms of droplet spread characteristics,with the increase of mass fraction,the spread inner diameter and spread time of droplets gradually increase.Considering the effect of volume change,the maximum value of the spread diameter increases as the initial droplet volume increases as the power function increases,R_max=5.81Ω₀^1/4.The spread time increases as the initial droplet volume increases as the power function increases,t=1.87Ω₀^1/2.Considering the influence of substrate thickness change,the maximum value of the spread diameter increases as the substrate thickness increases exponentiated,R_max=6.5H^1/4.The spread time decreases as the substrate thickness increases exponentially,t=32.77H^-1/2.The experiment found that when the volatile droplets spread on the liquid substrate of different temperature gradients,three different spreading phenomena appeared in the volatile droplets:Marangoni convection instability,ring R-P instability and droplet non-diffusion.With the increase of the mass fraction of IPA in the droplets,the critical temperature gradient values of the three phenomena increased,withΔT₁ increasing from 7.08°C to 12.66°C andΔT₂ increasing from 9.38°C to 19°C.For different initial volumes,ΔT₁ changed from7.49°C to 7.91°C andΔT₂ increased from 9.5°C to 11.58°C.The droplet spreading rate decreased significantly during marangoni convection instability,and the spreading speed did not change much when transforming from Marangoni convection instability to ring R-P instability.