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

两个多世纪以来，液体射流破碎成液滴一直是研究的焦点。微射流和微滴的快速生产已经获得了额外的重要性，超出了其在微流体设备和一些现代数字技术（如2D和3D打印）中的应用所激发的纯科学兴趣。目前对这一主题的研究大多旨在改善对液滴及其卫星的位置、数量和方向性的控制。该项目的目标是双重的：（i）我们将研究和利用液体射流的自激励（共振），以更好地控制破碎频率和长度;以及（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/柱将破碎成液滴。在连续喷墨应用中，通过经由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的射流（或柱）的分裂。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，则将获得均匀尺寸的液滴。虽然这些频率是由瑞利/韦伯方程粗略预测的，但在实践中，这仍然需要大量的调整和微调;这种微调是一种经验过程，当使用不同的流体或油墨时，必须重复进行，这既有限制又耗时。我们建议检测和利用自激励模式，其中系统通过反馈将自己调谐到最不稳定的频率。从定义上讲，这是最有效的分手方式。在本项目的这一部分，将研究自我刺激的机制。这种方法的明显优点是，微调是不需要的，破碎频率可以很容易地发现了广泛的流体（在一个合理的操作制度）。该项目的第二部分，从一个，否则不受干扰的射流产生单滴将进行调查。这些单滴可用于根据需要精确沉积小体积流体，用于各种应用（例如喷墨打印）。此外，设想在这些液滴内，单个颗粒（或细胞，或乳液中的其他不混溶液体等）可以在这些液滴中被释放。可以在真实的时间内被捕获并选择性地递送到特定目标。这些“颗粒”可以是功能性材料、化学反应物、细胞等，其通常有意地（例如，具有维持生命的正确营养物的流体，或处于“潜伏”模式的功能性材料）或无意地和不期望地（例如，固体污染物）分散在载体流体中。这两项重叠和互补的研究将提高液滴速度和体积的可预测性和可再现性，从而提高打印技术的可靠性、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.