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The wetting dynamics of dilute polymer solutions can be remarkably different from the case of simple Newtonian liquids, revealing significant differences in the behaviour of the moving contact line during the spreading and/or receding phase, in the amplitude of the dynamic contact angle, as well as in the intrinsic time of the phenomenon. A well-known example is the so-called anti-rebound effect of polymer additives. When a droplet of water falls on to a hydrophobic surface, such as the waxy leaf of a plant, the drop is often observed to bounce off. However, for about 15 years it has been known that the addition of very small quantities (~100 ppm) of a flexible polymer such as poly-(ethylene oxide) (PEO) can completely prevent rebound, by reducing the recoil velocity of the drop after the inertial spreading of two orders of magnitude. This is surprising since the shear viscosity and surface tension of such drops are almost identical to those of pure water.
High-speed, high-magnification images reveal that, as opposed to pure water, the contact line of dilute polymer solutions is pinned at several points on the impacting surface, and the polymer solution forms dendritic structures and filaments on the substrate, slowing down the recoiling phase and leading to the formation of secondary droplets having diameter size comparable to the filaments width (15 μm). The ensemble of polymer molecules stretching as the drop edge sweeps the surface provide the dissipative force necessary to slow down the displacement of the contact line. This can be interpreted, from a macroscopic point of view, as an additional, dissipative force acting on the contact line and opposed to its movement, or an effective contact line friction. This also explains the reduction of the dynamic contact angle observed in experiments: to overcome the action of polymer molecules on the contact line, the horizontal component of the surface force driving the droplet retraction must be larger than in a Newtonian fluid, therefore the apparent dynamic contact angle must be smaller.
In the present work, the Young-Laplace force balance is modified by introducing an additional term representing the polymer stretching force, obtained explicitly using the classical finite extensibility approach. Results are in good agreement with dynamic contact angle measurements reported in the literature.
