Experimental Research On Internal Flow And Deposition Patterns During Droplet Evaporation Of Multicomponent Fluids

Droplet evaporation is a common physical phenomenon in nature and human society.As a basis for understanding the mechanism of evaporation process,the evaporation of sessile droplets has been widely researched.The heat and mass transfer inside the droplet,especially in the thin film region,is a mesoscale problem processing with unsteady state and multi-physical field coupling,and there is not a theory which can fully reveal its mechanism.The internal flow of an evaporating droplet may be influenced by various factors,including the wettability of substrate surfaces,the species and physical properties of nanoparticles included,the temperature,humidity and pressure of the environment,the external electric,magnetic and sound field,composition of multiple components etc.The use of self-rewetting fluids as a method to enhance heat and mass transfer of heat pipes has been researched for many years.But there are few experiments on the influence of self-rewetting fluids on droplet evaporation.Because of the limited conditions of traditional observation methods,there is a lack of valid visualization to observe the heat and mass transfer in the thin film region.Experiment rig based on the total internal reflection fluorescence microscope is assembled and the evanescent wave and ordinary laser are used as light sources,which can observe and measure the internal flows from the bottom of droplets.The multilayer nano-particle image velocimetry(MnPIV)technique is used to process the images based on MATLAB software.The temporal and spatial evolution of velocities and thicknesses in the thin films near the triple lines during droplet evaporation were obtained by using a proposed sub-region method.The experimental working fluids are deionized water,low carbon alcohol solution and high carbon alcohol solution,which represent conventional fluids and self-rewetting fluids,respectively.The effects of the diameters and concentrations of the fluorescent particles on the evaporation of droplets was studied first.The experimental results show that the changes of particle diameter and particle concentration influence the evaporation mode of droplets,and the depinning time of contact lines decreases with the increase of particle concentrations gradually.After selecting the suitable fluorescent particles and concentration,the internal flows inside the droplets containing self-rewetting fluids and conventional fluids,especially near the three-phase contact line region.It shows that the self-rewetting fluids form a vortex due to Marangoni convection at the beginning of evaporation which differs from that of the conventional-fluids which results in the difference in the expressions of the near wall velocities of the droplets.The velocity corrections show that the average velocities of the conventional fluids are related to the depinning time of contact lines,and for the self-rewetting fluids,the velocities are related to the depinning time and vortex time.The temporal and spatial evolutions of the thin films are different for self-rewetting fluids and conventional fluids droplets due to the difference of internal flows.The experimental results show that the critical liquid film height increases with the increase of the number of carbon atoms in the solute molecular formula and the capability of reducing the energy barrier for the self-rewetting fluids is weaker than that of the conventional fluids.The changes of ambient temperature also affect the evaporation behavior of the self-rewetting fluids droplets and the conventional liquid droplets.The relationships between the average velocities of the droplets and the ambient temperature are corrected by the experiments and the corrections are different for the two kinds of fluids.The change rates of liquid films of the conventional liquid droplets gradually increase with the increase of ambient temperatures,and which is contrary to self-rewetting fluids.As the environment temperatures decrease,the difference between the energy barrier between the conventional and the self-rewetting fluids gradually decreases.