Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 286

Figure 16. 'Dynamic' Leidenfrost temperature observed for water drops and dilute polymer solution
drops ( D 0 = 3 . 2 mm) impacting on a polished aluminum surface.

This plot shows that the dynamic Leidenfrost temperature of polymer solution
drops is significantly lower than that of water drops, and almost independent of
the impact Weber number; moreover, its value is close to that of the conventional
Leidenfrost temperature for water on polished aluminum [60].
However, in the case of viscoelastic fluids the definition of a dynamic Leiden-
frost temperature is arguable. In fact, for drops of pure water, secondary atomization
actually disappears when a continuous and stable vapor cushion prevents the drop
from making contact with the hot surface, which is indeed analogous to the Leiden-
frost phenomenon in sessile drops. This is no longer true when polymer additives
are dissolved into the impacting drop: in fact, even if the film is unstable and the
liquid locally touches the hot wall, there are other physical mechanisms that pre-
vent scattering of satellite droplets from the free surface of the liquid, as discussed
above. In this case, using the expression 'dynamic Leidenfrost temperature' may
be misleading, because it would suggest that the impacting drop never wets the
surface, whereas wetting might occur without secondary atomization.
Above the dynamic Leidenfrost point, the vapor film between the drop and the
hot surface is stable, and the liquid is never in contact with the wall: thus, one can
study drop bouncing in the absence of wetting effects and wall friction, for both
Newtonian and viscoelastic drops [40, 61].
The maximum diameter of drops after the inertial spreading is plotted in Fig. 17
with respect to the impact Weber number. Theories based on the conservation of
energy [6, 24] suggest that this quantity should scale as We 1 / 2 . However, it was
proposed that for We 1 a momentum conservation approach is more accurate,

because it is difficult if not impossible to quantify energy dissipation during im-
pact [62, 63]. Whilst the latter approach, which leads to a scaling of the maximum
spreading diameter as We 1 / 4 , seems to be confirmed by the trends obtained for