Single Drop Impacts of Complex Fluids: A Review - Drops and Bubbles in Contact with Solid Surfaces - page 281

Figure 11. Dynamic contact angles observed during the retraction of drops of pure water and of a
dilute polymer solution after impact on a hydrophobic surface.

1,000 s − 1 ),
and the fluid elements are in compression rather than extension, making the stretch-
ing of molecules in the drop interior unlikely.
A comparison of the fluid velocity in the bulk of the droplet during retraction with
the velocity extracted from macroscopic observations of the contact line shows a
dramatic difference between water and PEO drops. Figure 12 shows that the motion
of the contact line for droplets of pure water is similar to that of the fluid inside the
drop. By contrast, the motion of the contact line for PEO drops is one order of
magnitude slower than that of the corresponding bulk velocity measurements, and
further confirms that the difference between the behavior of the two fluids lies solely
at the droplet edge.
The nature of the dissipative force arising on the contact line was revealed by
direct visualization of fluorescent λ -DNA molecules [44, 45]: immediately after the
transit of the receding contact line, DNA molecules can be observed on the impact
surface in a stretched conformation, oriented in the direction perpendicular to the
contact line. Thus, it was suggested that during drop retraction polymer molecules
are stretched by a combination of hydrodynamic and surface forces arising when a
liquid meniscus moves on a solid surface, which is a phenomenon known with the
name of 'molecular combing' and can be exploited, for example, in order to stretch
DNA molecules using a flow field [47]. This mechanism is illustrated qualitatively
in Fig. 13. The ensemble of polymer molecules stretching as the drop edge sweeps
the surface provide the dissipative force necessary to retard the displacement of the
contact line. This also explains the reduction of the dynamic contact angle observed
in experiments: to overcome the action of polymer molecules on the contact line,

During retraction, radial velocity gradients in both cases are small (