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Gouttes et billes sous grande déformation biaxiale : le rôle de la viscosité et de l'élasticité.

Abstract : Comprehending the deformation of drops upon impact constitutes a major challenge for industrial sectors such as ink-jet printing, pesticide application, spray coating, etc. For this reason, we study the biaxial deformation of drops of Newtonian, rheo-thinning and Maxwell fluids following their impact, and of soft elastic beads using another experimental configuration.First, viscous drops are impacted on a surface covered by a thin layer of liquid nitrogen, which by its evaporation creates the ideal non-wetting and slip conditions provided by the cold Leidenfrost effect. In these experimental conditions, we identify two regimes: a capillary regime where the maximal diameter reached by the sheet, dmax, does not depend on the viscosity of the fluid and a viscous regime where dmax decreases as the viscosity increases. This decrease is due to the viscous dissipation taking place during the sheet extension. The dissipation is governed by the biaxial extensional viscosity. For polymer solutions, the biaxial extensional viscosity takes into account the rheo-thinning nature of the fluid. In the same experimental conditions, we also impact viscoelastic Maxwell fluids. A non-linear damped harmonic oscillator model predicts the maximal diameter, dmax, reached by the sheet as well as the time, tmax, needed to reach dmax. Good quantitative agreement is found between experiments and theory for Newtonian fluids of various viscosities and Maxwell fluids with characteristic relaxation times smaller, larger or comparable to the characteristic time of the experiment, tmax.In a second part of this work, the drops are impacted on small solid surfaces, named targets, complicating the flow field undergone by the sheet. The part of the sheet in contact with the target is subjected to shear, and shear viscous dissipation develops. On the other hand, the part expanding freely outside of the target only suffers dissipation caused by the biaxial extensional deformation of the sheet. We predict the maximum expansion factor of the sheet as a function of the relevant viscosity by evaluating quantitatively the viscous dissipation due to shear and biaxial extensional deformations. We finally show a correlation between the maximal diameter reached by the sheet and the target size. Once again, the importance of considering the rheo-thinning behavior of the polymer solutions to rationalize the experimental results is emphasized.We then investigate the emergence and dynamics of the rim formed at the free edge of an expanding viscous sheet. We provide direct measurements of the sheet thickness field, allowing the measurement of the time evolution of the shape and volume of the rim. We develop an analytical model, which predicts the filling rate and velocity of the rim. We identify and compare the relative importance of the contributions due to surface tension, inertial forces, and viscosity to the filling velocity. We show that the contribution of the inertial forces cannot be neglected. We find a good quantitative agreement between the experimental and theoretical values for the filling rate and velocity.Finally, we use a spinning drop tensiometer to measure the biaxial deformation of soft elastic beads. The set-up allows the deformation of a soft elastic bead immersed in a denser fluid by putting in rotation along its axis a capillary containing the bead and fluid. Under the assumption of homogeneous deformation of the bead, we obtain a theoretical decoupling of the measurement of surface tension and elastic modulus, which was confirmed by a confrontation with the experimental values.
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Carole-Ann Charles. Gouttes et billes sous grande déformation biaxiale : le rôle de la viscosité et de l'élasticité.. Matière Molle [cond-mat.soft]. Université Montpellier, 2021. Français. ⟨NNT : 2021MONTS097⟩. ⟨tel-03598039⟩

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