Drop-jet collisions for advanced fiber production

In-flight collisions between a regular stream of droplets and a continuous liquid jet, and between two regular streams of droplets are experimentally investigated. Basic research on the physics of these drop collision processes as well as the technical application of drop-jet collisions for advanced fiber production are presented in this thesis. First investigations focus on the influence of wettability and miscibility on the collision outcomes of drop-jet collisions. Using Newtonian liquids for the droplets and the jet with matching viscosities and whereby the jet totally wets the droplets, four main collision outcomes (regimes) can be observed, named as drops-in-jet , fragmented drops-in-jet , encapsulated drops , and mixed fragmentation . They are categorized according to the fragmentation of the drop, the jet, none or both. The four regimes can totally be recovered using a jet, which partially wets the droplets and, moreover, when using miscible liquids beside the absence of a true interface. This however only applies as long as the surface tension of the droplet exceeds the one of the jet. In the opposite situation, it is impossible to force the droplets inside the jet within the studied parameter range. Instead, the droplets spread around the jet, forming a coaxial structure. The transitions between stable and fragmented jets are modelled for different viscosities and visco- elasticities, keeping the wetting conditions unchanged. The jet fragmentation mechanism is not of purely capillary origin, as reported in previous drop-jet collision studies. The first transition, between drops-in-jet and encapsulated drops , takes place when the jet portions located between two consecutive drop impacts are sufficiently stretched by the droplets. Motivated by the similarities with the stretching separation of off-centre drop-drop collisions, a model can be developed, which perfectly predicts the transitions for both types of collisions, namely between coalescence and stretching separation for drop-drop collisions and between stable and broken jet for drop-jet collisions. A second jet fragmentation limit is identified, which corresponds to the emergence of satellite drops beside the stream of capsules. Before the capsules and satellite drops effectively separate, they are connected by liquid filaments. Using Newtonian liquids, we show that the capillary-viscous thinning of these filaments is faster than the overall recoil of the compound jet and therefore leads to the emergence of satellites. Yet, employing a viscoelastic jet liquid, long-lived filaments appear and these two jet fragmentations can partially or even totally be suppressed. This is attributed to the much slower drainage of viscoelastic filaments. The transition between stable and fragmented droplets are studied varying both the wetting be- haviours and the liquid viscosities. The drops are found to fragment following an end-pinching mech- anism, which develops after full encapsulation and, more precisely, during their recoiling phase. The prediction of the fragmentation threshold is based on two ingredients: the description of the drop ge- ometry during recoil, and the definition of a geometric threshold. The former can, at first order, be well approached by the drop maximal extension and require second order corrections to account for the dynamic of the jet recoil when the wetting conditions are changed. The geometric threshold corresponds to a critical aspect ratio of the deformed drop and is found to be only a function of the liquid viscosity ratio. Combining these two elements, the drop fragmentation limit is satisfactorily predicted over a very wide range of parameters. (author)