Tristan XABADA – From filament to spit production during speech

Tristan XABADA

The breakup of liquid filaments plays a crucial role in industrial processes (such as inkjet printing and spray technologies) and biomedical contexts, particularly in the airborne transmission of pathogens via saliva droplets generated during coughing, sneezing, or speaking. These droplets carry viruses and thus represent a major vector for disease propagation. To assess and mitigate this risk, it is essential to understand how these droplets form, originating from the deformation and rupture of thin saliva filaments stretched between the lips and broken by the airflow during phonation. This thesis investigates the dynamics of liquid filaments subjected to uniaxial stretching, which thin and eventually break under the combined influence of viscous, inertial, and capillary forces, along with a controlled perpendicular airflow. An original experimental setup was developed to reproduce rapid and violent extensions in the lab. It enabled the analysis of thinning in Newtonian filaments (water–glycerol mixtures) and how this process is altered by airflow. The filament shape under perpendicular airflow was studied using a quasi-static approximation (neglecting inertia), testing two models: viscous friction (“slender body”, low Reynolds numbers) and aerodynamic drag (Taylor model, high Reynolds numbers). These models allowed fitting the experimental filament shapes and directly estimating internal tension, which was then compared to theoretical predictions. Numerical simulations validated the quasi-static framework, helping identify the most relevant model and providing a quantitative tool for filament behavior analysis. This approach was extended to viscoelastic fluids, particularly human saliva, showing that their geometries could still be quantitatively characterized. These measurements enabled the definition of an effective Ohnesorge number for saliva, allowing direct comparison with reference Newtonian fluids. The second part of the work focuses on the rupture modes of filaments and how they fragment into droplets depending on viscosity, initial volume, and airflow velocity. It was shown that airflow can accelerate or delay rupture and even alter the fragmentation mechanism. Model viscoelastic fluids (giant micelles) and saliva were studied to highlight the role of elasticity. These experiments revealed novel behaviors, such as significantly prolonged lifetimes and the emergence of specific instabilities along the filaments. Key findings include: (i) the quasi-static model accurately describes the pure deformation phase, between the end of initial extension and the onset of non-uniformities, but becomes invalid when drainage and inertia dominate; (ii) viscosity strongly stabilizes filaments at low Ohnesorge numbers, while air inertia promotes deformation and alters rupture regimes; (iii) saliva’s elasticity allows extreme stretching and significantly delays rupture. Some limitations remain: the study mainly uses glycerol as a model fluid, with relatively few experiments on saliva and micelles, and lacks systematic quantification of the number and size of droplets produced after rupture. Nevertheless, these results pave the way for a broader understanding of the atomization of complex biological fluids, linking laboratory observations to the deformation and fragmentation of saliva filaments at the lips.