Gulliver Seminar : Christian Ligoure (Université de montpellier)

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16 mai 11:30 » 12:30 — Bibliothèque PCT - F3.04

Impact of Drops and beads under large biaxial deformation : the role of viscosity,capillarity and elasticity.

We investigate freely expanding sheets formed by ultrasoft gel beads, and viscoelastic drops, produced by the impact of the bead or drop on a plate covered with a thin layer of liquid nitrogen
that suppresses shear viscous dissipation thanks to an inverse Leidenfrost effect. The time evolution of the sheet is simultaneously recorded from top and side views using high-speed cameras.

We first condider the regime of negligible visous dissipation : Our experiments show a unified behavior for the impact dynamics that holds for solids, liquids, and viscoelastic fluids and that we
rationalize by properly taking into account elastocapillary effects. In this framework, the classical impact dynamics of solids and liquids, as far as viscous dissipation is negligible, appears as the asymptotic limits of
a universal theoretical description. A novel material-dependent characteristic velocity that includes both capillary and bulk elasticity emerges from this unified description of the physics of impact.

Second , we consider the impact of viscoelastic fluids where biaxial viscous dissipation cannot be neglected. The investigated viscoelastic fluids are Maxwell
fluids, which are characterized by low elastic moduli, and relaxation times that vary over almost two orders of magnitude, thus giving access to a large spectrum of viscoelastic and elastocapillary
effects. By using a generalized damped harmonic oscillator model, we rationalize the role of capillarity, bulk elasticity and viscous dissipation in the expansion dynamics of all investigated
samples. In the model, the spring constant is a combination of the surface tension and the bulk dynamic elastic modulus. The time-varying damping coefficient is associated to biaxial extensional
viscous dissipation and is proportional to the dynamic loss modulus. For all samples, we find that the model reproduces accurately the experimental data for dmax and tmax.





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