Near-Field Electrohydrodynamic Writing Of Polystyrene Based Micro-Nano Structures

With the development of functional devices,the manufacturing cost of micro/nano devices is increasing exponentially.Besides,high-resolution and low-cost manufacturing processes are the key to the development of next-generation micro devices.Compared to other micro/nano processing technologies,Electrohydrodynamic(EHD)direct printing technology is a new micro/nano scale additive manufacturing technology with high printing resolution,low manufacturing cost and non-mask.However,there are still many problems with EHD direct printing technology.Polymers are commonly used for EHD direct printing,but it is difficult to form three-dimensional structures because the polymer solution tends to collapse and spread on the substrate during the printing process.At the same time,the existing EHD direct printing substrates are mostly flat substrates,and the process research based on curved substrates needs to be developed urgently.In the thesis,we study the process of forming three-dimensional structures of polystyrene(PS)materials on flat and curved substrates via EHD direct printing technology.Based on the theory of EHD,the effects of different parameters on dimensions of the printed dots and filaments are investigated.The three dimensional micro/nano structures are successfully printed with the addition of the heating substrate and improvement of solution volatility.The effects on the electric field and jet are investigated by adjusting the relative position of the nozzle and substrate.A mesh 3D structure is successfully deposited on the cylindrical substrate by adding a rotational axis to the motion platform.The main research contents of this thesis are as follows:(1)The process of depositing 3D structures in Cone-jet mode is investigated,and the effects of voltage,air pressure and printing speed on filament width are analyzed by controlled variable methods.With the addition of heating substrate,the fidelity of printed filament is significantly improved.Based on a 60°C substrate,a 3D "wall" structure with a thickness of less than 6μm,a height of more than 50μm and an aspect ratio of 9 is successfully printed by adding a positive and negative voltage alternating printing process.(2)In Micro-dripping mode,the PS concentration is too small to be stacked into threedimensional structures.By adjusting the solution volatility,the feasibility of printing 3D structure by dissolving low molecular weight polystyrene with cyclopentanone is determined.Under Micro-dripping mode,the droplet with small size is obtained by reducing the air pressure and voltage as much as possible.The effects of solution concentration,molecular weight,and capillary tip diameter on the morphology of the 3D structure are analyzed.We conclude that a smooth 3D column structure with a diameter of 5 μm can be printed stably using a PS material with a molecular weight of 190,000,and a micro/nano column array with a diameter of less than 700 nm and an aspect ratio of about 3 can be printed stably using a PS material with a molecular weight of 35,000.(3)The electric field distribution between the nozzle and substrate at different distances and angles is simulated with comsol multiphysics simulation software.Combining the experimental and simulation,the jet can be deposited stably when the electrode distance is in the ranging of 200-600 μm;the jet mode is more likely to change when the substrate tilt angle increases.The rotary axis is added to the motion platform to ensure constant relative position of the nozzle and the cylindrical substrate.And the speed of rotary axis rotation is controlled to match the printing speed.Finally,a 3D grid structure with a width of 22 μm is successfully printed on a 20 mm diameter cylindrical substrate by using a 60 μm diameter nozzle.