Experimental and Simulation Study of Reactive Silver Ink Droplet Evaporation
Abstract
The evaporation of particle-free silver ink droplets on heated substrates directly impacts the morphology of the resultant silver particles and films. In this thesis, COMSOL Multiphysics simulations of the solvent (water-ethylene glycol mixture) droplet evaporation process are used to explain the microflows, mass transfers, and heat distribution responsible for the experimental observations. The reactive ink incorporates fluoro-surfactant FS-31 and poly (acrylamide) (PAM) to suppress the coffee-ring effect that negatively impacts the electrical conductivity. Experiments show that the droplet evaporation process results in varied silver particle morphology, depending on the locations within the droplet, leading to uneven surfaces. Large particles (3 to 5 μm) originate from the flow-abundant apex, while 1 to 2 μm and 500 to 700 nm particles appear in the spreading area and at the contact line, respectively. Water concentration, temperature, surface tension, and capillary flow all affect the droplet evaporation rate. However, the capillary flows dominate the internal flows within the evaporating droplet, collecting the particles at the contact line to form a coffee ring. Multiphysics simulations are used to provide theoretical insight into the formation, transportation, and aggregation of the silver particles created during the ink droplet evaporation process. The simulation studies closely examine the microflows generated by the evaporating water-ethylene glycol solvent and the role of the evaporating solvent in transporting the particles from the thermally driven silver reactions. A “piling sandhill” model explains the accumulation of ethylene glycol at the bottom center by the circulating capillary flow during evaporation. The simulations also show that the robust capillary flow near the apex helps to transport silver ions and particles throughout the droplet. The larger particles tend to settle onto the substrate at the central vortex, producing an inner ring. Furthermore, the uneven evaporation at the contact line collects ethylene glycol like a stripe at the center, lowering the local surface tension and producing internal microflows that push the fluid outwards splitting the droplet. This investigative study into the microflow mechanisms arising during particle-free silver ink droplet evaporation will lead to advances in reactive ink synthesis, substrate material design, and the fabrication of electrically conductive films.