Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 280

Unfortunately, this approach is deeply flawed for several reasons. First of all,
whilst the anti-rebound effect is observed for drops of dilute polymer solutions (i.e.,
when the polymer concentration is less than the overlap concentration), Eq. (5) has
been validated with experimental data based on solutions in the semi-dilute regime
whose properties, and in particular viscosity, are different from the dilute regime
[35]. Thus, the slow retraction observed in these experiments can be simply due to
a higher shear viscosity of the solution. Second, Eq. (5) is derived for large contact
angles, but this is in open contradiction with the fact that it is used only in the
case of small contact angles, which correspond to the experimental situation as also
confirmed by more recent and accurate measurements [43].
It is not clear at all how the velocity gradients responsible for normal stresses
have been measured or estimated: according to rheometric data, for the most di-
lute solution normal stresses are significant only for shear rates much greater than
10 3 s − 1 , corresponding to a timescale < 1 ms, whereas drop retraction takes times
of the order of one second. The total confusion about basic kinematics is further
confirmed when the microscopic lengths extracted from the fit of experimental data
to Eq. (5) are compared with the lengths of the fully extended polymer chains: this
suggests that polymer chains are thought to stretch in the vertical direction, whereas
the main velocity component is parallel to the impact surface. Finally, if it was true
that large normal stresses arise in the fluid near the contact line, they should work in
the direction of increasing the contact angle with respect to the case with no normal
stresses (Newtonian fluid), which is exactly the opposite of what can be observed
during experiments [43].
Figure 11 shows the difference between the dynamic contact angles measured
during retraction on a hydrophobic substrate for a drop of pure water and one of
a 200 ppm PEO solution. The smaller dynamic contact angle observed during re-
traction of polymer solution drops indicates that the horizontal component of the
driving force required in order to move the contact line is much larger than in the
case of pure water, which is in contrast with the fact that the contact line velocity is
one order of magnitude smaller. However, this cannot be explained by the modest
reduction of surface tension ( ∼ 0.2 mN/m) and the modest increase of shear viscos-

ity ( 0.23 mPa s). Thus, one must conclude that if polymer solution drops exhibit

a smaller dynamic contact angle, there must be an additional dissipative force op-
posed to the force at the surface/air interface.
Direct experimental evidence to rule out the dissipation mechanism provided
by bulk elongational viscosity is provided by recent particle velocimetry measure-
ments inside impacting drops [44-46], showing that the local velocities measured at
different times during expansion and retraction are similar for the drops of polymer
solution and for those of pure water.
Velocity gradients in the fluid measured with respect to time during expansion
(a parameter which determines whether a polymer undergoes a coil-stretch tran-
sition) are almost identical for the dilute PEO solutions and for water, suggesting
similar amounts of dissipation throughout the spreading of the droplet.

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