The Internal Flow Characteristics Of Droplets Containing Fluorescent Nanoparticles And The Eventual Deposition Patterns

Evaporation of sessile droplets can be seen everywhere in nature, and it is an important direction researched in heat and mass transfer field. After the evaporation of sessile droplets containing the particles on the substrate surface, they will leave a variety of deposition patterns. This phenomenon of particle deposition plays a significant role in the industrial production. For example, ink jet printing, thin-film coating, drug discovery, manufacture of novel optoelectronic components and so on.In this paper, we use an inverted microscope to observe the internal flow of droplets containing fluorescent particles in the evaporation process and final deposition patterns. Experimental studies observe that the evaporation time of water droplet is related to the particle concentration. There are three different styles of particles deposited from the outside to the center, and the overall deposition pattern of particles vary with increasing particle concentration. When the ethanol is added to the droplet, the surface tension of the droplet changes, thereby changing the radius and the evaporation time spreading droplets. Furthermore, the addition of ethanol will completely change the flow characteristics inside the droplet, and the final deposition pattern will change with the ethanol concentration showing a huge difference.We also studied the effect of mixing different size of the particles in the droplet. We found that although there is not much effect of mixing particles on the droplet evaporation and internal flow, but particles of different sizes can be well separated in the three-phase contact line position. Smaller diameter particles are deposited in the vicinity of the location of the three-phase contact line, while the larger particle diameters are deposited farther away from the line of contact. The study found, the distance of the position is not only related to the particle radius, but also influenced by the contact angle of a droplet.Finally, we analyze the desorption behavior of the contact line. The depinning of contact line is found to be related to the droplet surface free energy, and impacted by the energy barrier. The move of contact line can affect deposition pattern of particles. In general, contact line skip steps will be gradually reduced with the evaporation.