Spatial Confined Microfluid Flow Involved Model For Micro Droplet Crystallization Regulation

Evaporative crystallization is not only a widely used crystallization method(such as inorganic salt,food,petrochemical,medicine and other fields),but also common in the gas-liquid solid multiphase separation process(such as desalination of seawater,flue gas desulfurization and other fields).The droplet evaporation is one of the most common natural phenomena in nature.The droplets containing nonvolatile materials will leave a variety of deposition patterns after evaporation,which makes the droplet evaporation crystallization process can be used in new fields such as particle self-assembly,biomass inspection and so on.However,the crystal structure and distribution are determined by many factors,such as contact line movement,droplet internal flow,phase transformation and so on.The complex crystal growth and movement process makes the regulation mechanism of droplet evaporation crystallization process still unclear.In the process of droplet evaporation,there are two kinds of circulation mechanisms: evaporation compensation flow(caused by uneven distribution of evaporation flux on the droplet surface,from low flux to high flux)and Marangoni flow(caused by surface tension gradient caused by uneven temperature distribution on the droplet surface,from low surface tension to high surface tension).In order to explore the movement and growth mechanism of crystal in droplet during evaporation,a mathematical model of the evaporation crystallization process of sodium chloride droplet was established,and the finite element calculation was carried out by using the software of ANSYS.Coupled with the VOF model,and through the Clapeyron Clausius equation to define the saturation temperature,change the substrate temperature and hydrophobicity of the evaporation plate,simulated the evaporation process of sodium chloride droplets with different concentrations.The model was tested by comparing the experimental and calculated values of the temperature distribution at the gas-liquid interface with the infrared thermal imager.The conclusion is as follows: the contact angle has a great influence on the circulation pattern,the larger the contact angle is,the greater the circulation intensity from the substrate to the gas-liquid interface is;the higher the substrate temperature is,the intensity of Marangoni flow and evaporation compensation flow in the droplet increases at the same time,but because the two flow directions are opposite,the circulation intensity first decreases and then increases(Marangoni flow dominates at low temperature,compensation flow dominates at high temperature).when the concentration of sodium chloride increases,the evaporation rate becomes smaller,but the surface tension gradient becomes higher,the circulation intensity first increases and then decreases,and the circulation pattern remains unchanged.In addition,the optical observation system and infrared thermal imaging observation system were established to observe the crystal movement and growth process during the crystallization of sodium chloride water droplet,as well as the temperature distribution at the gas-liquid interface in real time.The temperature distribution was used to predict the flow direction of Marangoni flow.The conclusion is as follows: in the process of droplet crystallization,the crystal flows from the bottom center of the droplet to the top of the droplet,and then extends the gas-liquid interface from the top of the droplet to the edge of the droplet;in the process of droplet evaporation,the Marangoni flow at the gas-liquid interface points from the edge of the droplet to the top of the droplet,and the Marangoni flow inside the droplet flows from the substrate to the gas-liquid interface,circulation intensity gradually weakened during evaporation;when the contact angle increases,the probability of the crystal ring distribution increases;when the substrate temperature decreases,the probability of the crystal ring distribution increases,and the crystal size distribution becomes more uniform;when the concentration of sodium chloride increases,the crystal size increases,the probability of the ring distribution decreases.In addition,the change of morphology parameters in the process of evaporation crystallization of sodium chloride droplets was compared with the classical formula of volume change of single component droplets in the process of droplet evaporation.It was found that the volume change rate formula of single component droplets was also applicable to for sodium chloride droplets evaporation crystallization.Finally,this paper used the needle tube to stretch the droplet,and enhanced the circulation by changing the morphology of the droplets.The circulation pattern and strength of the droplets were simulated by CFD.After stretching,the circulation pattern of the droplets changed and the average velocity increased.By changing the ratio of length to diameter of droplets,the circulation flow inside the droplets can be regulated.It is expected to be used in the design and development of new micro-scale crystallizers.