Electro-Collapse Jetting: Towards the Next Generation of Printing Technologies

以射流或液滴形式产生的小尺寸液体在许多层面上对我们的日常生活产生重大影响。当电场施加到喷嘴外形成的液体弯月面时，电荷在液体表面上积累，产生应力。这种电驱动的应力使弯月面变形为称为泰勒锥的圆锥形状，并且由于顶点处的奇异性，产生了尺寸比喷嘴小得多的精细射流（电喷射）。然后，由于瑞利不稳定性，该射流破碎成液滴。了解这种现象的物理机制一直是科学家和工程师的重点，因为它在各种技术应用中的使用，如电喷雾质谱和电流体动力印刷。自由液体表面上的空腔的塌陷是另一个有趣的现象，其中诸如动量聚焦之类的效应可以导致产生微小的液滴和气溶胶。这种现象已被应用于废水处理、微流体药物输送、作物喷洒和喷墨打印等领域。虽然上述两种现象都产生小液滴，但这些现象中的每一种都具有限制，这些限制阻止其产生具有高粘度和密度的复杂流体的亚微米液滴。然后，我们的提案旨在首次全面研究空腔塌陷喷射和电喷射的行为，以深入了解这两种现象相结合时出现的微液滴的动力学。这将使我们能够根据我们在整个研究中获得的知识开发一种新的打印技术。我们还将根据操作条件和液体的物理性质开发液滴尺寸及其速度的预测理论模型。该项目的最终目标是使用所提出的印刷方法制造高性能压电器件，以证明该技术的适用性和有效性。目前可用的液滴生成技术可以产生与喷嘴尺寸相当的液滴。小而薄的喷嘴更容易堵塞和破裂，并且更难以制造。这阻碍了这些技术在各种应用中的实施，其中需要高颗粒负载油墨（>5000 cP）的高分辨率印刷。该项目旨在通过提出一种能够打印小尺寸（< 1微米）的高粘性功能材料的新技术来解决这个问题，超越了市场上当前打印系统提供的尺寸和材料范围。初步数据显示，新技术可以产生比喷嘴尺寸小100倍的射流（不需要小喷嘴），打印频率比传统的自然电喷射脉动技术（快速打印）高一个数量级。所提出的系统还提供了一种解决方案，在非导电表面上的电喷射的问题。在非导电表面上沉积具有相同极性的后续带电液滴是有问题的，因为这在液滴之间产生排斥力，导致飞溅并因此导致不良打印。这是因为非导电表面不允许液滴内的电荷消散。然而，所提出的系统的灵活性可以使我们能够中和随后的液滴的电荷，这将解决这个问题，并确保即使在非导电表面上也能进行高分辨率打印。这将推动高分辨率印刷电子产品、通过沉积液晶微/纳米液滴制造微透镜等应用的实现，以及依赖于以高分辨率印刷复杂流体和活性材料的许多其他应用，如组织和器官的增材制造。

The generation of small sizes of liquids in forms of jets or droplets has a significant impact on our daily life in many levels. When an electric field is applied to a liquid meniscus formed out of a nozzle, electric charges are accumulated on the liquid surface producing stress. This electrically-driven stress deforms the meniscus into a cone shape known as Taylor cone and due to the singularity at the apex, a fine jet, much smaller than the nozzle in size is produced (electrojetting). This jet then breaks up into droplets due to Rayleigh instability. Understanding the physical mechanisms of this phenomenon has been the focus of scientists and engineers due to its use in a variety of technical applications, such as electrospray mass spectrometry and electro-hydrodynamic printing. The collapse of cavities on free liquid surfaces is another interesting phenomenon, in which effects such as momentum focusing can lead to the production of diminutive droplets and aerosols. This phenomenon has been exploited in applications such as wastewater treatment, drug delivery in microfluidics, crop spraying and inkjet printing. While both phenomena described above produce small droplets, each one of these has limitations that prevent it from producing submicron droplets of complex fluids with high viscosity and density. Our proposal then aims to comprehensively study, for the first time, the behavior of both cavity collapse jetting and electrojetting to provide deep insights into the dynamics of the micro-droplets emerged when both phenomena are combined. This would then allow us to develop a novel printing technique based on the knowledge acquired throughout our study. We will also develop a predictive theoretical model for the droplet size and its speed based on the operation conditions and the physical properties of the liquids. The ultimate goal of the project is to use the proposed printing method to fabricate high performance piezoelectric devices as evidence of the applicability and the effectiveness of the technique.The current available droplets generation techniques can produce droplets comparable to the nozzle size. Small and thin nozzles are more prone to clogging and breaking and more difficult to manufacture. This has hindered the implementation of these technologies in a variety of applications, in which the high-resolution printing of highly particle-loaded inks (>5000 cP) is required. This project aims to solve this problem by proposing a novel technique that capable of printing highly viscous functional materials with small sizes (< 1 micron), surpassing the range of sizes and materials offered by the current printing systems in the market. A preliminary data shows that the new technique can produce jets that are up to 100 times smaller than the nozzle in size (no need for small nozzles) and printing frequency that is one order of magnitude higher than the traditional natural electrojetting pulsation technique (fast printing). The proposed system offers also a solution to the problem of electrojetting on non-conductive surfaces. Depositing subsequent charged drops with the same polarity on nonconductive surfaces is problematic because this creates a repulsion force between the droplets leading to splashing and hence poor printing. This is because the nonconductive surface does not permit the charges within the drops to dissipate. However, the flexibility of the proposed system could allow us to neutralize the charges of the subsequent droplets, which will solve the problem and ensure high-resolution printing even on non-conductive surfaces. This will push forward the implementation on applications such as high-resolution printed electronics, manufacturing microlenses by depositing liquid crystals micro/nano droplets and many other applications that depends on printing complex fluids and active materials with high resolution such as additive manufacturing of tissues and organs.