  We investigate freely expanding liquid sheets made of either simple Newtonian
fluids or solutions of high molecular water-soluble polymer chains. A sheet is
produced by the impact of a drop on a quartz plate covered with a thin layer of
liquid nitrogen that suppresses shear viscous dissipation thanks to an inverse
Leidenfrost effect. The sheet expands radially until reaching a maximum
diameter and subsequently recedes. Experiments indicate the presence of two
expansion regimes: the capillary regime, where the maximum expansion is
controlled by surface tension forces and does not depend on the viscosity, and
the viscous regime, where the expansion is reduced with increasing viscosity.
In the viscous regime, the sheet expansion for polymeric samples is strongly
enhanced as compared to that of Newtonian samples with comparable zero-shear
viscosity. We show that data for Newtonian and non-Newtonian fluids collapse on
a unique master curve where the maximum expansion factor is plotted against the
relevant effective biaxial extensional Ohnesorge number that depends on fluid
density, surface tension and the biaxial extensional viscosity. For Newtonian
fluids, this biaxial extensional viscosity is six times the shear viscosity. By
contrast, for the non- Newtonian fluids, a characteristic Weissenberg
number-dependent biaxial extensional viscosity is identified, which is in
quantitative agreement with experimental and theoretical results reported in
the literature for biaxial extensional flows of polymeric liquids.
