Drop-jet collisions for advanced fiber production

This project entitled Drop-Jet Collisions for Advanced Fiber Production is at the junction between the fundamental study of a physical process, namely the collisions between a drop stream and a jet of immiscible liquid, and its exploitation to develop and establish a new production process for microcapsules of controlled sizes and shapes and fibers with advanced properties. Due to their common natural occurrence and their wide industrial spreading, drop collisions and liquid jet fragmentation have been, taken separately, deeply studied. Yet, to date, the association of a drop stream with an immiscible liquid jet has not been considered. Preliminary experiments have demonstrated that the proposed configuration introduces new fundamental aspects. For instance, the possibility for successive drops to interact via the liquid jet essentially modifies the interactions at play. The proposed experimental and analytical study focuses on the description of the various outcomes and the conditions to obtain them. The fragmentation mechanisms at play will be analyzed and the boundaries between the observed outcomes will be modeled. In this manner, the project will further consolidate and wider the current state-of-knowledge of liquid collisions, jet and ligaments stability as well as the influence of visco-elastic liquids on the dynamics of such phenomena. The importance of this knowledge should not be underestimated since liquid ligament fragmentation and potential drop collisions can literally be found in all industrial processes involving liquids at millimetric or sub-millimetric scales. More pragmatically, such collisions provide the opportunity to generate complex liquid structures, which cannot be obtained otherwise and constitute promising candidates as intermediaries for new capsule production. Indeed, free flowing liquid jets containing regular immiscible liquid inclusions have been preliminary observed which may be turned into advanced fibers by hardening the jet liquid. Such structures preserve the essential properties of spherical core-shell capsules, including the controlled proximity with the surrounding which drives exchanges while it enables an easy manipulation and dosing of several encapsulated reservoirs. The combination of these two aspects, crucial in the booming field of tissue engineering and drug delivery, remains today a challenge. Preliminary trials were performed during which liquids were successfully jetted and subsequently hardened following well known methods: precipitation, solvent exchange and melt cooling. We will therefore combine in the second phase of this project, the mentioned collision process to obtain for example the drops in jet structure with the proven hardening methods to solidify these elements in the form of drops in fiber.

The controlled collisions between drops form a regular drop stream and a continuous liquid jet have been studied. This type of collisions, also called in-air microfluidics, enables the formation of liquid structures, which cannot be produced otherwise. The regularity of these structures makes them ideal candidates for capsules and fibres precursors. In this project, we have investigated the characteristics of the liquid structures produced by drop-jet collisions over a broad range of parameters, such relative velocity, diameters of drops and jet, spacing of the drops in the stream The observed outcomes have first been classified in four regimes. The classification criterion is the fragmentation (or not) of the drops, the jet, both drops and jet, or none of them. Two distinct fragmentation mechanisms applying to either the drops or jet, have been identified. The effects of the liquid properties and collision parameters on these fragmentation limits have been investigated. This comprises the influence of the viscosity from both drops and jet; the viscoelasticity of the jet; the miscibility and wettability of the liquid pairs; as well as all geometric and kinetic parameters. In a second step, models have been developed that predict the occurrence of each regime based only on the collisions parameters and liquid properties. This achievement made possible the rational and efficient usage of drop-jet collisions for the production of regular fibers with predefined characteristics. Such fibers were successfully produced in the last part of this project. We mostly used alginic acid, which is known to gel upon contacts with cations, into alginate, a natural and biocompatible hydrogel. Various compositions have been screened and several types of fibers have been obtained. Among others, alginate fibers gelled with calcium or strontium, containing (or not) regular glycerol inclusion, with or without the presence of a plasticizer to improve their mechanical properties. The obtained fibers have been characterized in term of regularity and mechanical properties. The results enable us to identify the best candidates for advanced biomedical applications such as tissue engineering.