Thèse : Sollicitation hydrodynamique et propriétés interfaciales des hétérogénéités dans les liquides viscoélastiques

Viscoelastic materials are those which exhibit viscous and elastic behaviour. A plethora of industrial and biomedical applications have piqued interest in their rheological properties. The interest of this project lies in the flow of Viscoelastic liquids with heterogeneities of concentration and, consequently, heterogeneities of rheological properties. The underlying scientific inquiry is in relation to the mechanical description of the transient interfaces between miscible fluids. It has been proposed previously that there might be a transient non-equilibrium surface tension between miscible phases, but the topic is still actively debated and has been the subject of recent studies (see Truzzolillo D., & Cipelletti L. 2017).

The first system under study is a semi-diluted polymer solution constituting the dispersed phase while its solvent constitutes the continuous phase. The two phases are miscible but with protracted diffusion times, therefore there is a sustained 'miscible interface' between them. This interface is probed using established biphasic phenomenon occurring in microfluidic and millifluidic geometries. These experiments are supported with numerical simulations using the Finite Volume Method.

Once such experiment surrounding the dispersion of a droplet in the continuous phase under flow through a porous medium led to counter-intuitive results. Due to the influence of higher normal forces at higher Weissenberg numbers, the droplet disperses quicker at low flow rates while it stayed intact at higher flow rates. This is also supported by simple two-dimensional numerical simulations that show the behaviour of a fluid bulk moving through a rectangular channel with a cylindrical obstruction, where the obstruction is offset from the center. The results show that the mass flow ratios between the bulk that moves through either side of the obstruction has a Weissenberg number dependence.

The counter-intuitive observations of the influence of flow velocity on droplet dispersion forms the first riddle in understanding the transient interfaces that form between miscible complex fluids. The project is outlined by the experimental inquest coupling Fast confocal microscopy into the flow behaviour of viscoelastic pseudo-biphasic mixtures, using well-demonstrated experiments of biphasic flow, including droplet formation, digitation and Saffman-Taylor instabilities, Rayleigh-Plateau instability, droplet breakage at junctions, etc.

Reference: [1] Truzzolillo, Domenico, and Luca Cipelletti. “Off-Equilibrium Surface Tension in Miscible Fluids.” Soft Matter, vol. 13, no. 1, 2017, pp. 13–21.