Morphology of Silver Particles and Films Arising from Particle-Free Silver Ink Droplet Evaporation

The evaporation of particle-free silver ink droplets on heated substrates directly impacts the morphology of the resultant silver particles and electrically conductive inkjet-printed films. In this work, COMSOL Multiphysics simulations of the droplet evaporation process are used to help explain the fluidic processes responsible for the experimental observations. The results show that the silver particle morphology depends on its location within the evaporating droplet where large particles (3 to 5 mu m) accumulate at the center and smaller ones (1 to 2 mu m) in the area beyond the central region. The smallest particles (approximate to 500 nm) are near the contact line. Without the fluoro-surfactant FS-31, an inner ring with 3 mu m particles also appears within the evaporating droplet. The large particles at the central region originate from the fluid near the droplet's apex due to the accumulation of polyacrylamide in the concentrated ethylene glycol solvent near the flow-abundant apex. Furthermore, the capillary flow along the fluid-air interface transports silver ions and particles throughout the droplet, causing the larger particles to fall onto the substrate at the central vortex, forming the inner ring. The simulations also demonstrate that the evaporating droplet heterogeneously accumulates ethylene glycol in a sandhill formation at the bottom center. A comprehensive study that uses COMSOL Multiphysics simulations to help explain the fluidic flows responsible for the formation, transportation, and accumulation of electrically conductive particles during particle-free silver ink droplet evaporation. The results show that larger particles form at the droplet center while smaller particles are more sparsely distributed at the contact line forming the experimentally observed "coffee ring" effect.image