Current-modulated Electrohydrodynamic (EHD) Jet Printing with Dual-channel Nozzles for Micro/Nano-Fabrication

喷墨打印作为一种增材制造方法已用于制造印刷电子产品、3D物体原型、太阳能电池和发光器件，以及应用于组织工程和其他生物和制药领域。与通常产生直径为10-50微米的单个液滴的更常见的压电喷墨印刷方法不同，电压调制电流体动力学（EHD）喷射印刷具有产生亚微米尺寸的液滴/纤维以用于制造纳米尺度的图案或特征的经证实的能力。然而，由于喷嘴堵塞、喷嘴出口处的油墨积聚和低打印频率（导致有限的生产率）的问题，EHD喷射打印尚未被认为是可行的制造工具。该项目对一种新形式的EHD喷射打印进行基础研究，使用新颖的双通道打印喷嘴和电流（而不是电压）调制。假设液体油墨在双通道中的循环将消除由于载体流体的蒸发或油墨的聚合引起的喷嘴堵塞，并且电流调制/控制将实现高速、按需滴液的EHD喷射印刷。这项研究也可能导致进一步改善目前的喷墨打印工艺或设备在工业和家庭环境。此外，该技术还将用于面向学生（各级）和先进制造业工人的教育和推广活动。本项目将研究新型电流调制、按需滴液EHD打印方法的基础科学，该方法采用新型双通道喷嘴，以高喷射频率（MHz水平）制造高分辨率的微/纳米图案。所提出的双通道打印喷嘴使用两个同心管，提供围绕内管的环形通道。在所提出的喷嘴构造中，一个通道提供新的油墨，另一个通道用于从喷嘴提取油墨，从而实现双管喷嘴内的流体循环。假设这种流体循环将消除或大大减少与喷嘴出口处的聚合或载体流体蒸发相关联的喷嘴堵塞和油墨积聚的问题。 该项目进一步假设液滴的喷射速率可以通过电流控制而不是通常使用的电压控制来增加。在所提出的EHD打印中，将需要对流体弯月面动力学和液滴生成的科学理解，以实现强大的电流控制，并且需要将流体再循环的影响纳入其中。所提出的EHD喷射印刷技术的新颖性是i）所提出的双通道喷嘴，其将解决在单毛细管喷墨印刷中经常遇到的技术问题;以及ii）用电流而不是电压调制频率，以便在高喷射频率下实现可靠的EHD喷射印刷。本计画的主要目的为：i）发展双通道喷嘴电压调变电流体喷射印刷过程中液体弯月面形成、喷射及液滴喷射的基础科学; ii）研究电流调变电流体喷射印刷过程中的基本喷射机制iii）对EHD喷射印刷技术中的喷射特性进行数字建模，以开发基础科学和辅助控制策略;以及iv）提供所提出的方法的概念验证，并且通过对所创建的微米/纳米尺寸图案的质量（即，尺寸、均匀性和分辨率）的参数调查来验证数值模型。

Inkjet printing as an additive fabrication method has been used in the manufacturing of printed electronics, 3-D object prototypes, solar cells, and light-emitting devices, as well as applications in tissue engineering and other biological and pharmaceutical fields. Unlike the more common method of piezo inkjet printing, which typically generates individual droplets of 10-50 micrometers in diameter, voltage-modulated electrohydrodynamic (EHD) jet printing has a demonstrated ability to produce sub-micrometer-sized droplets/fibers for the fabrication of patterns or features at nanometer scales. However, EHD jet printing has not been considered as a viable manufacturing tool because of the issues of nozzle clogging, ink accumulation at the nozzle exit, and low printing frequencies (resulting in a limited production rate). This project conducts fundamental research on a new form of EHD jet printing, using novel dual-channel printing nozzles and electrical current (instead of voltage) modulation. It is hypothesized that the circulation of liquid ink in the dual channels will eliminate nozzle clogging due to evaporation of carrier fluids or polymerization of ink, and the current modulation/control will enable high-speed, drop-on-demand EHD jet printing. This research could also lead to further improvements on current inkjet printing processes or devices in both industrial and household settings. Furthermore, the technique will be used in education and outreach activities geared toward students (at all levels) and workers in advanced manufacturing.This project will investigate the fundamental science involved in a new current-modulated, drop-on-demand EHD printing method with novel dual-channel nozzles for the fabrication of high-resolution micro/nano patterns at high jetting frequencies (on the level of MHz). The proposed dual-channel printing nozzles use two concentric tubes, providing an annular channel around the inner tube. In the proposed nozzle configuration, one channel provides new ink and the other for extracts ink from the nozzle, thereby achieving fluid circulation within the dual tube nozzle. It is hypothesized that this fluid circulation will eliminate or greatly reduce the issues of nozzle clogging and ink accumulation associated with polymerization or carrier fluid evaporation at the nozzle outlet.  This project further hypothesizes that the ejection rate of droplets can be increased through current control, instead of the voltage control commonly used. Scientific understanding of fluid meniscus dynamics and droplet generation in the proposed EHD printing will be required to achieve robust current control, and needs to incorporate effects of fluid recirculaiton. The novelties of the proposed EHD jet printing technique are i) the proposed dual-channel nozzles that will resolve the technical issues often encountered in single-capillary inkjet printing; and ii) the modulation of frequencies with current rather than voltage in order to achieve reliable EHD jet printing at high jetting frequencies. The specific aims of this project are to: i) develop the fundamental science involved in liquid meniscus formation, jetting, and droplet ejection in a voltage-modulated EHD jet printing process with the dual-channel nozzles; ii) investigate the fundamental jetting mechanisms in current-modulated EHD jet printing process (particularly at high frequencies); iii) numerically model the jetting characteristics in the EHD jet printing technique to develop the underlying fundamental science and aide control strategies; and iv) provide a proof-of-concept of the proposed approach and validate the numerical models through parametric investigations of the quality (i.e. the size, uniformity, and resolution) of micro/nano-sized patterns created.