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

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.

Through careful laboratory experimentation we aim at elucidating some of the physical phenomena underlying the breakup of liquid jets. Direct comparison with industrial systems will allow us to translate our finding directly to industry, via our partners. This will provide them with tools to improve the generation of drops for inkjet applications (Continuous Inkjet at Domino), and to explore the encapsulation of active/reactive inks, functional materials, and composites for additive manufacturing and even biotechnological applications (Trijet). We will also aim at producing short articles for non-specialists to disseminate our findings, and how these can be used by industry involved in digital printing, especially nowadays when 3D printing is one of the technologies expected to revolutionise the manufacturing industry.

Direct impact is expected to occur during the development of the project due to the following:
-Having industrial partners and collaborators supporting this proposal presents us with the potential of our results being directly applicable to their research and development. Therefore, we will work in close collaboration with our industrial supporters (Trijet and Domino) and promptly present our findings. Trijet and Domino are already looking forward to working with us as many parts of our research, such as the experimental setup (the visualisation rig in particular - Trijet) and feedback assembly (Domino) are of great interest to them.
-Moreover, we will continuously communicate our results to our network of industrial collaborators working on inkjet and deposition of small volume of biocompatible gels and complex inks (e.g. Biogelx, Inca Digital, and TTP).
-The group also maintains collaboration with MERCK, and they are becoming increasingly interested in novel methods for the precise deposition of novel material with optical properties which, unfortunately, cannot be dispensed with traditional methods - our single-drop technique could potentially solve some of these issues.

Other non-academic beneficiaries will include:
-Adhesive and paint developers (e.g. Magna International): there is a need to find new ways of depositing drops of glues and paints onto their novel substrates for the measurement of surface energy and wettability properties in order to test the quality of their products.
-The general public and schools: we plan to develop a dedicated experiment and provide online resources for the dissemination of the project's outcomes for non-specialist audiences.