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Numerical Modelling and Experimental Validation of Nano Colloid Droplet Formation in the Electrohydrodynamic 3D Printer

Mohammadi, Kaivan | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53404 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Movahhedy, Mohammad Reza; Khodaygan, Saeed; Simchi, Abdolreza; Mofrad, Mohammad Reza
  7. Abstract:
  8. The electrohydrodynamic (EHD) 3D printing technology is an Additive Manufacturing (AM) method that can be used in the manufacture of submicron-size structures. Due to the multiphysics dynamics and the multiphase nature of the microdroplet formation in the EHD printers, modeling of this phenomenon is complicated. In this thesis, the formation of a droplet in an EHD printer—under a pulsed electrical field—is simulated using a new numerical model which couples the fluid flow, the electric field distribution and the movement of the electric charges under dynamic and transient conditions. The level-set method is applied to the entire multiphysics domain in order to study the formation of the droplet. The presented model is verified by comparing to the existing experimental results in the literature. In order to find the effective parameters with significant impact, at 95% confidence level, a sensitivity analysis of the process parameters is carried out based on the Plackett-Burman design of experiment approach. It is found that the voltage, the permittivity of ink, the nozzle height above substrate, the density and the conductivity of ink are the most effective parameters. In this process, which is based on the formation of Taylor jet from a metallic ink, the investigation of particles reaction and aggregate formation in ink during printing is of great importance. In this thesis, a new Finite Element model based on particle tracing in fluids is presented that couples the different physics that govern an EHD phenomenon. Vortex flow caused by boundary shear stress was observed in the ink jet; which was more like the Marangoni phenomenon in a stationary drop. By experimentally validating the presented model with the help of PIV technique, the effects of important parameters (volume fraction, number and size of particles, zeta potential, viscosity, voltage application rate) on aggregate formation were studied based on the DLVO theory. In general, any change of parameters that leads to the increase of volume fraction also boosts the amount of formed aggregates. No interaction effect was observed between the parameters of volume fraction, particle radius and number of particles. It was demonstrated that the DLVO theory by itself is not sufficient for determining colloidal ink stability, and that the parameters of volume fraction, radius and number of particles should also be investigated for this purpose. Considering the conditions of EHD printers, the amount of zeta potential needed for colloid stability increases significantly; and a minimum zeta potential of 110 mV is required for colloid stability. A too-low or too-high value of viscosity (3 mPa.s>μ>500 mPa.s) and a low rate of applied voltage also lead to reduced aggregation.
  9. Keywords:
  10. Electrohydrodynamic Printer ; Three Dimensional Printer ; Sensitivity Analysis ; Colloidal Ink ; Particle Image Velocimetery (PIV) ; Additive Manufacturing ; Multiphysics Particle Tracing Model ; Droplet Formation

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