Laboratoire Matière et Systèmes Complexes

Thèse de Antoine Gaillard effectuée sous la direction de Laurent Limat

Soutenance le mercredi 19 décembre 2018 à 14h00.

Lieu : bâtiment Condorcet, Amphithéâtre Pierre-Gilles de Gennes (niveau -1). La soutenance sera suivie d’un pot au 6e étage.

Écoulement et stabilité d’un rideau liquide viscoélastique

Abstract:

Liquid curtains are used in industrial processes such as curtain coating which aims at depositing a liquid film (of paint for example) on a solid substrate. The liquid falls freely at constant flow rate from a slot under gravity (see figure) before impacting a substrate moving horizontally underneath. The liquids used in industry are generally non-Newtonian and the curtain properties thus depend on the liquid rheology.

In this thesis, we experimentally investigate the flow and the stability of curtains made of a viscoelastic liquid.

We use solutions of flexible and semi-rigid polymer chains. The flow U(z) of these liquids is measured by velocimetry (PIV), U being the liquid velocity and z being the distance from the slot exit. It is characterised by an initial balance between gravity and the elastic stresses arising from the stretching of polymer molecules, until inertia finally dominates and the classical free-fall behaviour is recovered. We show that the flow is mostly influenced by the value of the extensional relaxation time of the solution measured by a filament thinning technique. New insights on the theoretical description of Newtonian curtains allow us to find the master curve of the viscoelastic curtain flow by analogy.

Concerning the curtain stability, we show that the critical flow rate for curtain formation is not affected by the presence of polymers whereas the minimum flow rate below which the curtain rapidly breaks is reduced by polymer addition, thus revealing a greater resistance of the sheet to hole initiations. Furthermore, we observe the onset of a flow instability for the most shear-thinning solutions where thick bands (where the liquid velocity is larger than average) are formed within the curtain. Visualisations of the flow inside the die reveal that this phenomenon is linked to a flow instability at the contraction plane upstream of the slot, where the flow is unsteady and three-dimensional in nature.