From jets to sprays

Controlling liquids, whether for depositing a component on a substrate, filling an object with said liquid, transporting particles in a controlled amount, or delivering drugs or chemicals to very specific parts of a human or a plant, often appears as a simple task but is crucial in our society and can be very complex to understand. Multiple factors influence the motion of a liquid: the liquid’s physical properties (such as its viscosity or surface tension), or interaction with its surroundings (including air friction, turbulence, gravity...). In this Thesis, we look at different liquids being forced through nozzles and try to shed light on the effects of both the internal properties of the liquid as well as the interaction with its surroundings on the shape and motion of the resulting jets and droplets. We study coalescence along a single jet or within a spray cloud, and show that droplet size increases with distance from the nozzle in both cases, which leads to the droplets being less affected by air drag. Moreover, we show that the well-known conical spray shape is caused by the Kelvin-Helmholtz instability, and that droplets in the middle of the cone grow larger than the ones on the side of the cone due to increased droplet density (so more coalescence) in the center. Finally, we look at viscoelastic elliptic jets that produce a chain pattern, and show that the elasticity modifies the jet shape compared to Newtonian fluids.