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

喷墨打印作为一种添加剂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，以及在组织工程和其他生物和制药领域的应用。与更常见的压电式喷墨打印方法不同，后者通常会产生直径为10-50微米的单个液滴，而电压调制电流体动力(EHD)喷墨打印具有证明的能力，可以产生亚微米大小的液滴/纤维，用于制造纳米级的图案或特征。然而，由于喷嘴堵塞、喷嘴出口处油墨积累和打印频率低(导致生产率有限)的问题，EHD喷墨打印一直不被认为是一种可行的制造工具。本项目对一种新型的EHD喷墨打印进行了基础研究，该打印喷嘴使用了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而导致的喷嘴堵塞，并且电流调制/控制将使高速、按需滴入的EHD喷墨打印成为可能。这项研究还可能导致工业和家庭环境中当前喷墨打印工艺或设备的进一步改进。此外，这项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。本项目将研究一种新的电流调制、按需液滴EHD打印方法所涉及的基础科学，该方法具有新颖的双通道喷嘴，用于在高喷射频率(在MHz水平上)制造高分辨率微/纳米图案。建议的双通道打印喷嘴使用两个同心管，在内管周围提供环形通道。在所提出的喷嘴结构中，一个通道提供新的墨水，另一个通道从喷嘴中提取墨水，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极大地减少与聚合或喷嘴出口处的载液蒸发相关的喷嘴堵塞和油墨积累的问题。该项目进一步假设，可以通过电流控制来提高液滴的喷射率，而不是通常使用的电压控制。为了实现稳健的电流控制，需要对拟议的EHD打印中的流体半月面动力学和液滴生成进行科学理解，并需要纳入流体再循环的影响。拟议的EHD喷墨打印技术的创新之处在于：i)拟议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；以及ii)以电流而不是电压来调制频率，以便在高喷射频率下实现可靠的EHD喷墨打印。该项目的具体目标是：i)发展与电压调制EHD喷射打印过程中的液体半月面形成、喷射和液滴喷射有关的基础科学；ii)研究电流调制EHD喷射打印过程中的基本喷射机制(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，以制定基本的基础科学和辅助控制策略；以及iv)提供所建议方法的概念验证，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)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.