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.
Solvent Casting Process for the Manufacturing of Microstructures or Coating Layers:
The evaporation of a thin fluid film is a common process in industry and nature. In recent years, the evaporation of a solvent from a polymer solution inside a micron scale cavity
and the manipulation of particle-deposits, referred as “solvent casting”, has been of high interest for the research community. It is a cost-effective technique with applications in
fabrication of micro-needles, surface coatings and electronic-chips. Solvent casting is a highly transient process which mainly depends on fluid viscosity, surface tension forces,
wetting forces, height of the cavity, environmental conditions and gravity. The proposed project involves solvent evaporation from polymer solutions in cavities resulting in a polymeric coating on the walls of the cavity to form a desired polymeric microstructure or coating layers.