Electrohydrodynamically-Driven Micro-Additive Manufacturing Processes: Characterization and Control

Martinez Prieto, Nicolas

doi:https://doi.org/10.21985/n2-kx3r-nw72

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Electrohydrodynamically-Driven Micro-Additive Manufacturing Processes: Characterization and Control

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Two major areas of research in the field of manufacturing are miniaturization and 3D printing. The trend towards miniaturization stems from the desire and need for additional functionality and improved performance for products with ever-smaller footprints. Miniaturization opens manufacturing to new applications such as tissue engineering and surface texturing. Meanwhile, the main appeal of 3D printing is its flexibility and low cost. Micro-additive manufacturing (µAM) is a field that is being developed to leverage the potential of 3D printing techniques and apply them at the micron and submicron scales. Among µAM technologies, electrohydrodynamic methods, where electric fields are used to drive the flow, are promising as they can overcome the limitations of traditional pressure-driven technologies. Near-field electrospinning (NFES), an electrohydrodynamic technique, is capable of continuously printing by utilizing nanofibers and microbeads. This process, first proposed in 2006, must first overcome limitations in accuracy, repeatability, and flexibility, before it can truly become a freeform printing technique with micron to submicron resolution. The work described on this thesis is aimed to overcome two major limitations of the near-field electrospinning process: (1) the lack of accuracy and reproducibility, and (2) the presence of residual charges which inhibit fiber stacking and limit how close to each other fibers can be printed. Overcoming these limitations will enable a more flexible and reliable process with higher resolution. To address the first limitation, the effect of ink properties and process variables on the deposition morphology was experimentally explored. From a material perspective, viscoelasticity and surface tension were shown to play a major role in morphology. On one hand, an increase in viscosity is correlated with the transition from the deposition of beads to the deposition of fibers. On the other, a decrease in surface tension is linked to the transition from fibers to ribbons (fibers with a flat cross-section). From a process variables perspective, changes in nozzle size, stage speed, and mass flow rate had little effect in morphology. However, these process parameters had a major effect on the diameters of beads and fibers and on bead spacing when beads were present. The results highlight the importance of tuning the inks to obtain the desired deposition morphology while the process parameters, which can be changed on the fly, are used to adjust the size and spacing of the printed patterns. To improve process control and repeatability novel control strategies to overcome the undesired effects on deposition accuracy by jet bending, which occurs due to the viscoelastic nature of the jet and the mismatch between jet and collector speeds, were developed. Two open-loop control approaches to regulate jet bending were proposed and implemented. In the first approach, auxiliary electric fields that apply time-varying horizontal forces on the charged jet were used. This approach enables the production of micro-features (~50 µm) by actuating the jet and improves the repeatability of printing by a factor of three by constraining the jet. In the second, motion compensation, by adding dwell times at corners during printing, was implemented. This enables the jet to return to its neutral straight position, guaranteeing that the jet is at the desired position. Although both approaches were successful, they lack generality and require tuning for each geometry to be printed. To design a more reliable system a closed-loop controller was proposed. As a first step toward closed-loop control, a machine vision system was developed. The algorithm was capable of measuring the jet diameter and jet bending at 50 frames per second. Overcoming the adverse effects caused by the presence of residual charges, the second limitation of NFES, is essential to achieve multilayer structures and the patterning of non-conductive substrates. A self-focusing mechanism that can circumvent this limitation was discovered. This mechanism promotes attraction of the jet to previously deposited layers allowing the deposition of additional material on previously deposited structures. Experimental measurements of the electric current flowing through the system and a finite element model of the electric fields were used to determine the nature of the mechanism. The results show that self-focusing relies on two mechanisms, i.e., the discharge of existing charges due to the liquid nature of the ink followed by the polarization of the remaining ink due to its dielectric nature. By taking advantage of the self-focusing mechanism and by tuning ink and process parameters multilayer bead arrays were printed. Besides the self-focusing mechanism, it has been shown that residual charges can also be overcome by printing with alternating current (AC) fields instead of the traditional direct current (DC) fields. NFES using AC fields was demonstrated and it was shown that the printed fibers do not retain net charges. The absence of charges enables the direct patterning of non-conductive collectors such as polyimides and elastomers. This has only been possible before by patterning conductive regions on insulating collectors. Overall, the work presented shows an advancement in the understanding, control, and flexibility of NFES. Future research directions are proposed to further improve the knowledge base of the process, improve the theoretical models, increase the number of applications, and the throughput of the process.

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