Particle Deposition Patterns from the Evaporation of a Sessile Droplet Containing Bi-Dispersed Colloidal Particles

We study the particle deposition patterns from the evaporation of a sessile droplet containing bi-dispersed fluorescent microspheres of different sizes on non-heated and heated substrates with different droplet contact angles below 90 degrees. The temporal variation of the droplet shape and particle deposition at the contact line region are simultaneously recorded using a high-speed camera and a confocal microscope, respectively. In all tested cases, the particles separate near the contact line with smaller size particles depositing closer to the contact line as compared to the larger size particles; this is due to the contact angle and the curvature of the liquid-gas interface. On non-heated substrates, ring-like patterns form and within the ring, the separation of particles improves with a decrease in contact angle. On heated substrates, a thick inner deposit with mixed particles and a thin outer ring with separated particles form. The inner deposit is caused by the Marangoni flow towards apex inside the droplet. Our measurements help to understand the coupled effects of contact angle, substrate heating and the particle size combination in the bi-dispersed colloidal particles to achieve a particle sorting behavior near the contact line inside evaporating droplets.