| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 106 === The resolution of flexographic printing process determines the quality of the printed electronic devices. The fundamental issues are to determine the quantity of the transferred ink and to produce the colloidal stabilized conductive ink. A commonly used ink material, the single-walled carbon nanotubes (SWCNTs), was chosen to be investigated. Therefore, a computational framework combining the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with the coarse-grained molecular dynamics (CG-MD) simulations is developed to predict the stability of the SWCNTs in aqueous solutions. The dispersion of SWCNTs in aqueous solutions is an important issue. However, it is a challenging task since it requires the understanding of both macroscopic properties of the solution and the colloidal mechanism at the molecular level.
The various concentration of surfactant, sodium dodecyl sulfate (SDS), is considered to each size of the SWCNTs, including (6, 6), (12, 12) and (18, 18). The Langmuir isotherm model is used to find the relationship between the amount of the adsorbed SDS and the bulk SDS concentration. With the increasing number of the SDS covered on SWCNTs, the surface charge density is also enhanced, and thus, greater electrical double layer repulsion is achieved to prevent the aggregation of the SWCNTs. The potential energy barrier as a function of the radius of the SWCNT and the SDS concentration can then be obtained by using DLVO theory so that the dispersion of the SWCNTs in the solution can be predicted. The results suggest an optimal surfactant concentration which can stabilize the SWCNTs in the solution. Also, we calculate the zeta potential for SWCNTs with the radius of 0.35~0.7 nm which has a good agreement with the experimental results when the concentration of SDS is within 30~80 mM. The current framework is expected to provide the guidance for the design of the concentration of SDS surfactants and the radius of SWCNTs in dispersion experiments.
Furthermore, to study another issue of flexographic printing processes, it is necessary to consider multi-physics mechanics such as fluid mechanics and the interface evolution between liquid and air. In the current work, a multi-physics model was developed by coupling Navier–Stokes equations and phase field method to analyze the behavior of the droplets and to model the air-liquid interface. We proposed a plate-to-roll model to calculate the ink transfer ratio by changing the contact angle, the radius of the roller, and the viscosity of the ink, which affect the flexographic printing. We simulated the effects of these different parameters and compared the results with the literature. Modeling and simulations proposed herein should be useful to improve designs of the geometry of roller, properties of the ink, and printing accuracy.
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