Droplet Evaporation Deposition Morphology Study Of Particle Suspensions Under Marangoni Flow

Since the recent years,a lot of researchers have discovered in daily life that,as coffee,blood or milk is dripped on the tabletop,the deposition morphology of comparatively shallow color in the middle,comparatively deep color in the edge is formed,this phenomenon is called as the coffee ring effect(CRE).Essentially it belongs to the evaporation behavior of particle suspension droplet on the solid substrate of droplet with granule.However,because of its technic applications on the fields such as biologic technique,medical treatment inspect,electron manufacture and spray-black print etc.and the expansive research foreground,it has attracted the attention of more and more researchers to explore deeply.Therefore,it is of comparatively great significance and practical value to research the influence of droplet evaporation on the deposition morphology.The influence of particle suspension droplet evaporation on the deposition morphology generated under the Marangoni flow was researched.In this thesis,the Marangoni flow generated under the influence of surface tension gradient of droplet,and the capillary flow of droplet evaporation as well as the influence of the mixing effect of the two flows on particle movement and deposition morphology in droplet were mainly investigated.By means of the theoretical analysis,numerical simulation and experimental research,the movement and action force of particles while droplet evaporation were researched,and the deposition morphology of particles in the edge was also investigated.Moreover,through combining theoretical analyses,numerical simulation and experimental researches,the evaporation process and deposition morphology of droplet under different conditions were also researched.The main research contents and achievements are as follows:1.By applying the theory of hydromechanics,thermodynamics and surface chemistry,the surface tension of droplet,the influence of the surfactant on the surface tension,the Marangoni flow generated by the change of surface tension in the part,flow field and action force instance in droplet during its evaporation process were analyzed.(1)In view of that surface tension is a physical property of droplet,due to changing the conditions of the solution,the surface tension gradient is produced,resulting in the Marangoni flow from the edge of the droplet to the center of droplet.The capillary flow generated by droplet evaporation is from the center of droplet to the edge of droplet contact line.Since the capillary flow and the Marangoni flow can be canceled out each other,ultimately the movement of particles in droplet and the deposition morphology after droplet evaporation are affected.(2)By the theoretical analysis of droplet evaporation,the two evaporation models of droplet(i.e.the constant contact angle model and constant contact radius model of droplet)were derived.The flow conditions of the two evaporation modes inside droplet were different.The internal flow field in the evaporated droplet was theoretically analyzed.The theoretical analysis results showed that,the evaporation process of particle suspension droplet is not steady-state,and there is a process from the constant contact angle evaporation to the constant contact radius evaporation;Due to the uneven evaporation flux of the droplet,the flow velocity inside the droplet is smaller at the middle of the droplet and larger at the edge;Considering the Marangoni flow generated by the addition of the surfactant,the flow velocity of the Marangoni flow in the droplet was researched,and the magnitude of Ma number was used to indicate the intensity of the Marangoni flow;With the help of Matlab sofeware,the capillary flow and Marangoni flow inside the droplet evaporation process were simulated numerically to research the capillary flow at different locations and the intensity of Marangoni flow in different Ma numbers.2.Based on the theoretically analysis of evaporation process of particle suspension droplet,the experimental research in allusion to the influence of droplet evaporation on the coffee ring effect(CRE)was made and the experimental results were analyzed.(1)During the whole experimental research,polystyrene particle of the suitable concentration was selected as coffee particle,the evaporation process of the particle suspension droplet and the deposition morphology after its evaporation were investigated.Moreover,SDS(sodium dodecyl sulfate)was used as the surfactant to change the surface tension of the droplet,and the Marangoni flow was generated in the droplet,which would lead to the change in the deposition morphology.(2)The first one is using Nikon-Eclipse TE2000 U fluorescence inversion microscope,Nikon-SMZ745 T body type microscope with Pixe Link camera lens and Phantom high speed vidicon directly linked with the computer,the evaporation process of the droplet was shot and recorded.The second one is that,the deposition after particle suspension droplet evaporation was observed by using microscope,the contrary deduction of the evaporation was made,and the observed experimental results were analyzed.(3)The experimental results showed that,the evaporation process of particle suspension droplet is not steady-state,the evaporation went on continually through the change of the contact line and angle of the droplet,especially after adding the surfactant with different concentration the contact angle of the droplet,the contact line was enlarged outwards to generate the spreading of droplet;The time of the spreading of droplet was very short,and about 1/4～1/3 of the time of the whole evaporation process,after the end of spreading of droplet the contact line was pinned to form the coffee ring of droplet;After adding the surfactant with different concentration,the deposition morphology after its evaporation occurred to obvious change,especially the ring width of the coffee ring after evaporation was changed obviously.As the concentration of the surfactant was higher,its ring width was broader,and the percentage of the ring deposition area in the whole evaporation deposition area would be also increased.The research achievements of the influence of particle suspension droplet evaporation on deposition morphology generated under the Marangoni flow in this thesis will lay a good foundation for the further research and application of droplet evaporation.