Interplay between viscosity and elasticity in freely expanding liquid sheets

Abstract

We investigate the dynamics of freely expanding liquid sheets prepared with fluids with different rheological properties: (i) Viscous fluids with a zero-shear viscosity η0 in the range 1-1000mPas and (ii) viscoelastic fluids whose linear viscoelastic behavior in the frequency range 0.1-100 rad/s can be accounted for by a Maxwell fluid model, whose characteristic elastic modulus G0, relaxation time τ, and zero-shear viscosity η0=G0τ, can be tuned over several orders of magnitude. The sheets are produced by a drop of fluid impacting a small cylindrical solid target. For viscoelastic fluids, we show that, when τ is shorter than the typical lifetime of the sheet (∼10 ms), the dynamics of the sheet is similar to that of Newtonian viscous liquids with equal zero-shear viscosity. In that case, for little viscous samples (η0 30 mPa s), the maximal expansion of the sheet dmax is independent of η0, whereas for more viscous samples, dmax decreases as η0 increases. We provide a simple model for the dependence of the maximal expansion of the sheet with the viscosity that accounts well for our experimental data. By contrast, when τ is longer than the typical lifetime of the sheet, the behavior drastically differs. The sheet expansion is strongly enhanced as compared to that of viscous samples with comparable zero-shear viscosity, but is heterogeneous with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid.