  The present article highlights the role of non-Newtonian (elastic) effects on
the droplet impact phenomenology at temperatures considerably higher than the
boiling point, especially at or above the Leidenfrost regime. The Leidenfrost
point (LFP) was found to decrease with increase in the impact Weber number
(based on velocity just before the impact) for fixed polymer (Polyacrylamide,
PAAM) concentrations. Water droplets fragmented at very low Weber numbers
(~22), whereas the polymer droplets resisted fragmentation at much higher Weber
numbers (~155). We also varied the polymer concentration and observed that till
1000 ppm, the LFP was higher compared to water. This signifies that the effect
can be delayed by the use of elastic fluids. We have showed the possible role
of elastic effects (manifested by the formation of long lasting filaments)
during retraction in the improvement of the LFP. However for 1500 ppm, LFP was
lower than water, but with similar residence time during initial impact. In
addition, we studied the role of Weber number and viscoelastic effects on the
rebound behaviour at 405o C. We observed that the critical Weber number till
which the droplet resisted fragmentation at 405o C increased with the polymer
concentration. In addition, for a fixed Weber number, the droplet rebound
height and the hovering time period increased up to 500 ppm, and then
decreased. Similarly, for fixed polymer concentrations like 1000 and 1500 ppm,
the rebound height showed an increasing trend up to certain a certain Weber
number and then decreased. This non-monotonic behaviour of rebound heights was
attributed to the observed diversion of rebound kinetic energy to rotational
energy during the hovering phase.
