Nanoscale 3D Printing with Focused Electrojetting

Abstract

In the present thesis, we have fabricated nanoscale 3D structure with nanofibers which are generated by specilized electrohydrodynamic jets that are invented in this work, focused electrospinning method and electomechanical pulling method. To control the chaotic motion of the nanofiber in conventioanl process condition, we completely change the process for stable jetting and fine printing resolution. The behavior of the nanofiber significantly relies on the distance between the tip and the collector, we reduced the distance greatly in to few millimeter scale and few tens of micrometer scale according to the collecting plate shape and the pattern. When we use the sharp tip and the micropattened metal collecting substrate with the distance tip and the collector in few millimeter scale, the electrical field strongly focuses the nanofiber into the center of the metal part, thus it can form automatically the hollow cylindrical structure on the tip and the straight wall-like structure on the micro-metal line. When we reduce the distance between the tip and the plate in a few tens of micrometer scale, the jet is pulled by the movement of the collecting plate as the fiber attached on the surface by the electrostatic attractive force. Using this slow and stable method of fiber generation, we can construct 3D structure with nanofiber. And also we have shown that simply theoretical analasys of the moment and the force applied on the nanofiber can estimate the resultant dimension of the stacking structure of nanofiber.

First we have fabricated hollow cylindical structure on a sharp electrode tip by reducing the tip and the plate distance in the order of few millimeter scale and stabilizing the jet in a controlled fashion using pendant droplet at the end of metal nozzle. The sharp tip induces strongly focused electric field in the direction of the tip center which confines the charged nanofiber in center. The pendant drop forms a natural micro-sized capillary hole which eject slow nanojet by electrostatic repulsive force. When the nozzle situated on top of the sharp electrode tip, the nanofiber spontaneously forms a cylindrical wall with a uniform radius. A scaling law is given based on the balance of the electrostatic compression force and the elastic resistance to predict the coil radius and frequency as the functions of relevant physical parameters. The structures formed by the nanofibers can be used in diverse fields of nanotechnology, for example, as nanomagnets, bioscaffolds, and nanochannels. Next, we show that the electrified nanojet, which tends to become unstable as traveling in the air due to the Coulombic repulsion, can be stably focused onto a microline of metal electrode. To control the whipping instability of the electrified nanojets, we use a conducting microline on an insulating plate as a ground that focuses the electrical field. On the conducting line, the polymer nanojet is spontaneously stacked successively to form a wall-like structure. We rationalize the length of the wall by balancing the tension in the polymer fiber with the electrostatic interaction of the fiber with the metal ground. We also show that the length of nanowalls can be controlled by translating the substrate. Besides, we have tested the possibility of fabrication of aany desired shape we have tested with various metalic micro-patterned shapes, which are circular, wavy, and letter-like structure. These results shows the pros and cons of this fabrication method utilizing the microscale pattern. This novel three-dimensional printing scheme can be applied for the development of various three-dimensional nanoscale objects including bioscaffolds, nanofilters, nanorobots, and nanoelectrodes.

Next, we have shown that it is possible to print nanoscale fiber directly on the substrate by utilizing the combination of mechanical pulling force at the bottom and electrostatic repulsion at the end of the nozzle. This electro-mechanical pulling method could be realized by reducing the distance between the tip and the plate greatly to the few tens of micrometer scale, using nonmetal glass plate as a collecting surface with weak negative voltage, and applying no pneumatic pressure to supply the polymer solution. Thus the resultant nanofiber is pulled from the tip of the nozzle according to the speed of the plate. Using this printing method, we have constructed 3D square and Letter-like structures SNUwhich show the possibility of direct 3D printing of nanofibers. These results can be applied to maskless fabrication of nano/micro structures, cost-effective direct mask printing in the photolithography, and printing electrical circuit and connections using conductive material.

This thesis provided three novel methods to fabricate 3D structure with nanofiber in a controlled fashion. To realize each method for producing an ordered 3D structure, we have found many useful information about the focused electrojetting process. Thus this thesis will not only helpful in understanding of near-field electrohydrodynamic jet process, but also provide practical technique to fabricate directly nano/micro scale structure using nanofibers.