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，已被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，以及组织工程和其他生物和制药领域的应用。与更常见的压电喷墨打印方法不同，压电喷墨打印通常会产生直径为10-50微米的单个液滴，而电压调制电流体动力（EHD）喷墨打印已被证明能够产生亚微米大小的液滴/纤维，用于制造纳米尺度的图案或特征。然而，EHD喷射打印还没有被认为是一种可行的制造工具，因为喷嘴堵塞、喷嘴出口的油墨积聚和低打印频率（导致有限的生产率）等问题。本项目对一种新型EHD喷射打印形式进行基础研究，采用新型双通道打印喷嘴和电流（而不是电压）调制。据推测，液体油墨在双通道中的循环将消除由于载体流体蒸发或油墨聚合而导致的喷嘴堵塞，并且电流调制/控制将实现高速，按需滴式EHD喷射打印。这项研究还可以进一步改进目前工业和家庭环境中的喷墨打印工艺或设备。此外，该技术将用于面向学生（各级）和先进制造业工人的教育和推广活动。该项目将研究一种新的电流调制、按需滴式EHD打印方法的基础科学，该方法采用新型双通道喷嘴，用于在高喷射频率（兆赫兹水平）下制造高分辨率微/纳米图案。所提出的双通道印刷喷嘴使用两个同心管，在内管周围提供环形通道。在所提出的喷嘴配置中，一个通道提供新墨水，另一个通道用于从喷嘴中提取墨水，从而实现双管喷嘴内的流体循环。据推测，这种流体循环将消除或大大减少与喷嘴出口聚合或载液蒸发相关的喷嘴堵塞和油墨积聚问题。本项目进一步假设可以通过电流控制来提高液滴的喷射速率，而不是通常使用的电压控制。在提出的EHD打印中，需要科学地理解流体半月板动力学和液滴的产生，以实现强大的电流控制，并且需要考虑流体再循环的影响。提出的EHD喷墨打印技术的新颖之处在于：(1)提出的双通道喷嘴将解决单毛细管喷墨打印中经常遇到的技术问题；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喷射打印。该项目的具体目标是：i)在双通道喷嘴的电压调制EHD喷射打印过程中，发展涉及液体半月板形成、喷射和液滴喷射的基础科学；ii)研究电流调制EHD喷射打印过程的基本喷射机制（特别是在高频下）；iii)对EHD喷射打印技术中的喷射特性进行数值模拟，以开发潜在的基础科学和辅助控制策略；iv)对所提出的方法进行概念验证，并通过对所创建的微/纳米尺寸图案的质量（即尺寸、均匀性和分辨率）进行参数化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.