Doctor of Philosophy
Mechanical and Materials Engineering
Knopf, George K.
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.
Summary for Lay Audience
The evaporation of particle-free silver ink droplets on heated substrates directly impacts the size and shape of the resultant silver particles and, thereby, the functional performance of printed electronic films and circuitry. In this thesis, physics-based computer simulations of the droplet evaporation process explain the impact of internal microfluidic flows, chemical concentration changes, and substrate heat distributions on the experimentally observed changes to the particle-free ink droplet as it dries. Reactive silver inks often incorporate a surfactant and liquid polymer to suppress the accumulation of silver particles along the droplet edge during drying. The uniform distribution of silver particles is critical for achieving inkjet-printed films with high electrical conductivity. However, experiments show that the ink droplet evaporation produces a non-uniform distribution of silver particles with varied shapes and sizes. The size of the particle is dependent, in part, on the location within the droplet. Particles are largest at the droplet center and become smaller toward the edges. Research shows that water concentration, temperature, the surface area of the droplet, and capillary flow all affect the droplet evaporation rate. However, the capillary flows dominate the internal flows within the evaporating droplet, collecting the smaller particles at the edge to form a coffee-ring effect. The simulation studies examine the internal flows generated by the evaporating water-ethylene glycol solvent and the role of the evaporating solvent in transporting the particles formed by the chemical reactions. A “piling sandhill” model explains the accumulation of ethylene glycol at the bottom center of the evaporating droplet by circulating capillary flows. The simulations also show that the robust capillary flow near the top of the droplet transports silver ions and particles throughout the fluid as it dries. As a result, the larger silver particles settle onto the substrate near the center, forming an inner ring. Furthermore, the unequal evaporation at the droplet edge collects ethylene glycol, producing internal flows that push the fluid outwards splitting the droplet as it dries. This investigative study into particle-free silver ink droplet evaporation will lead to advances in reactive ink preparation, substrate material design, and the production of electrically conductive films.
Zhang, Weipeng, "Experimental and Simulation Study of Reactive Silver Ink Droplet Evaporation" (2023). Electronic Thesis and Dissertation Repository. 9758.
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