SELF-STIMULATION AND SINGLE DROPLET/PARTICLE ENCAPSULATION IN THE CONTROLLED BREAKUP OF LIQUID JETS

两个多世纪以来，液体射流分解成液滴一直是研究的焦点。微射流和微液滴的快速生产在其纯粹的科学兴趣之外获得了额外的重要性，其动机是它们在微流体设备和一些现代数字技术中的应用，如2D和3D-Print。目前对这一主题的大多数研究都旨在改善对液滴及其卫星的位置、数量和方向性的控制。这个项目的目标有两个：(I)我们将调查和利用液体射流的自激发(共振)，以更好地控制破裂频率和长度；(Ii)一旦我们能够提取最不稳定(最有效)的频率，我们将研究通过间歇压力脉冲从连续液体射流中产生单滴。由于表面张力的作用，液体射流/柱会分解成液滴。在连续喷墨应用中，通过压电元件对流体的压力(或速度)施加外部扰动来诱导和控制墨水喷射(或柱)的破裂。如果这些扰动的频率和幅度在所谓的“最不稳定模式”范围内，则可以得到尺寸均匀的液滴。虽然这些频率大致由Rayleigh/Weber方程预测，但在实践中仍需要大量调整和微调；这种微调是一个经验过程，当使用不同的液体或油墨时必须重复，这既有限又耗时。我们建议检测和利用系统通过反馈将自身调谐到其最不稳定频率的自激模式。从定义上讲，这是最有效的分手方式。在该项目的这一部分，将研究自我激励的机制。这种方法的明显优点是不需要微调，而且可以很容易地找到各种流体的破碎频率(在合理的操作范围内)。项目的第二部分，将研究从未受干扰的射流中产生单个液滴的情况。这些单滴可用于按需精确沉积各种应用(如喷墨打印)的少量液体。此外，预计在这些液滴中，单个颗粒(或细胞，或乳状液中的其他不相容的液体等)。可以被实时捕获，并有选择地发送到特定目标。这些“颗粒”可以是功能材料、化学反应物、细胞等，它们通常故意分散在载体流体中(例如，具有维持生命所需的正确营养的流体，或处于“潜伏”模式的功能材料)或无意和不需要的(例如，固体污染物)。这两项重叠和相辅相成的研究将提高液滴速度和体积的可预测性和再现性，因此，这些研究将提高印刷技术的可靠性、效率和质量。

The breakup of liquid jets into droplets has been the focus of study for more than two centuries. The fast production of microjets and microdroplets has gained additional importance beyond its pure scientific interest motivated by their application in microfluidics devices and in some modern digital technologies, such as 2D and 3D-Printing. Most current studies of this topic aim to improve the control over the position, number and directionality of droplets and their satellites. The objective of this project is two-fold: (i) we will investigate and exploit self-stimulation (resonance) of liquid jets for a better control of the breakup frequency and length; and (ii) once we are able to extract the most unstable (most efficient) frequency we will study the generation of single drops from a continuous liquid jet by means of intermittent pressure pulses. A liquid jet/column will break up into droplets due to the action of surface tension. In continuous inkjet applications the breakup of a jet (or column) of ink is induced and controlled by applying external perturbations in the pressure (or velocity) of the fluid via piezoelectric elements. If the frequency and amplitude of these perturbations are within the so-called 'most unstable modes' range, droplets of uniform size will be obtained. Although these frequencies are roughly predicted by the Rayleigh/Weber equations, in practice this still requires much adjustment and fine tuning; this fine tuning is an empirical process that has to be repeated when different fluids, or inks, are used, which is both limiting and time consuming. We propose to detect and exploit self-stimulated modes in which the system tunes itself to its most unstable frequency by means of feedback. This, by definition, is the most efficient breakup. In this part of the project, mechanisms for self-stimulation will be investigated. The clear advantage of this approach is that the fine tuning is not needed and the breakup frequency can be readily found for a wide range of fluids (within a reasonable operating regime).The second part of the project, the generation of single drops from an otherwise unperturbed jet will be investigated. These single drops could be used for precise deposition, on demand, of small volumes of fluids for a variety of applications (e.g. Inkjet Printing). Moreover, it is envisaged that within these drops single particles (or cells, or other immiscible liquids in emulsion, etc.) can be trapped in real time and selectively delivered to a specific target. These 'particles' may be functional materials, chemical reactants, cells, etc. which are normally dispersed in a carrier fluid on purpose (e.g. fluids with the correct nutrients to sustain life, or functional materials in 'latent' mode) or unintentional and undesired (e.g. solid pollutants). These two overlapping and complementing studies would increase the predictability and reproducibility of the velocity and volume of droplets, and as a consequence these would increase reliability, efficiency and quality of printing technologies.