Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 278

Figure 9. Comparison of the retraction dynamics of a drop of pure water and one of a dilute polymer
solution after impact and spreading on a hydrophobic surface: the dilute polymer solution exhibits a
much slower retraction rate.
dissipations caused by elongational viscosity leave nothing of the impact kinetic
energy available to propel the drop off the surface [30].
This seemed to be confirmed by the retraction velocity of the drop after max-
imum spreading, which for polymer solutions is one or two order of magnitudes
smaller than that measured for the pure solvent [29, 30], as shown in Fig. 9.
A careful experimental study aiming at establishing the independent influence
of dynamic surface tension and elongational viscosity on drop impact [37] indeed
demonstrated the existence of a correlation between the transient elongational vis-
cosity, measured with an opposed jets rheometer, and the drop rebound. Figure 10
shows in fact that both the retraction velocity after maximum spreading and the
maximum height reached by the drop decrease as the ratio between the elonga-
tional and the shear viscosity η E /η (called Trouton ratio) of the fluid grows. For
this reason, energy dissipation associated to elongational viscosity has been trusted
to be the cause of the anti-rebound effect of polymer additives on drops impacting
onto hydrophobic surfaces until recently [38]. However, an empirical correlation
between two phenomena does not necessarily imply a cause-effect relationship: in
fact, this scenario was not supported by the results of other independent investiga-
tions, which demonstrated that in the absence of direct contact between the liquid
and the substrate the anti-rebound effect is no longer observed.
A study of drops impacting on small targets [39] suggests that polymer additives
do not change the retraction velocity at all, while independent experiments on Lei-
denfrost drops (where the liquid is separated from the target surface by a thin vapor
layer, as discussed more in detail in Section D.2) show that they cause only a slight
reduction of this quantity [40]. Because in these experiments wetting effects are
absent or negligible, one must conclude that the retraction velocity reduction ob-