Influence Factors Of Droplet Evaporation Particles Deposition Morphology

The evaporation of droplet and the particle deposition are often observed innature and human activities, and have a wide application in biological fieldand industrial production, such as: self-assembly of macromolecular andparticle, printing, coating materials and particles, biology detection and designof microelectronic device, etc. Particularly, in the biology field, assembly ofbiological macromolecular material, such as proteins and DNA, can be got byusing the principle of droplet evaporation and colloids deposition. Therefore,further understanding mechanism of droplet evaporation and particledeposition, is useful to control process of particle deposition and to obtain therequired deposit morphology.In this paper, particle deposition morphology was observed by usingmicroscope after droplet evaporates freely on the hydrophilic glass substrate.Silica nanoparticles were dispersed in droplet evenly. By observing the results,it was found that suspended particles were carried to three-phase contact lineto leave dense, ring-like deposit on the glass substrate under the action ofradial compensation flow, at the end of the evaporation. Meanwhile, arelatively low concentration particle deposition can also be found near thecontact line, and a few of particles existed between the two rings, and noparticle deposition was found in the inner region. The equivalent volume of mixing droplet, which was obtained by adding a certain amount of ethanol topure water, evaporate in the glass substrate surface freely. It was found thatevaporation model of mixing droplet was the model of constant contact area,by observing droplet evaporation process with microscope. In addition, twodistinct rings formed in the contact line and near the contact line afterevaporation, but the particle concentration of mixing droplet was bigger thanthe concentration of the pure water on the internal ring. There were a largenumber of particles and the distance between the two rings also increased.By comparing the deposition morphology of different volume fractionmixed droplet evaporation, it was found that particles concentration on theinternal ring， particle deposition and distance between the two ring increasedgradually with the increase of ethanol volume fraction. When the volumefraction of ethanol was30%,the distance between two rings increases largely.Because the volatileness of the ethanol is very strong, the buoyancy convectionformed inside mixing droplet by adding ethanol at the early stage of dropletevaporation. Movement of suspended nanoparticles was affected together bythe radial compensation flow and buoyancy convection. As the dropletevaporates, ethanol content reduces quickly inside the mixing droplet; thesubsequent evaporation process is similar to evaporation of pure water droplet.Movement of suspended particle was only affected by the radial compensationflow and then particle was carried to the perimeter.In addition, the shape of the suspended particle inside droplet can affect theparticle deposition morphology. The main attraction force between suspendedparticles is van der Waals force according to the DLVO theory; we studied vander Waals force between the spherical particles and between the ellipsoidalparticles in the air by Derjaguin approximation. The force between particleswas translated into the relationship of van der Waals force per unit areabetween two tablets. The forces between spherical particles and ellipsoid particles were compared in condensed media, according to McLachlan theory.Through theoretical derivation and analysis, it was found that the vdW forcebetween ellipsoid particles is larger than the force between spherical particleswhen they have the same volume, and the force between ellipsoid particlesgradually increases when aspect ratio of ellipsoid particles increases.