Abstract
The micro-scale morphology of the receding contact line of dilute polyethylene oxide solution drops (c ∼ 100 ppm) after impact and inertial spreading on a fluorinated hydrophobic surface is investigated. One can observe the formation of transient liquid filaments and dendritic structures that evolve into a bead-on-a-string structure similar to the well-known capillary breakup mechanism of dilute polymer solutions, which confirm the interaction between stetched polymer coils and the receding three-phase contact line. The estimation of the average polymer force per unit contact line lenght provides a quantitative explanation for the reduction of the contact line retraction velocity reduction observed experimentally.
Highlights
Formation and Contact Line ForcesThe wetting dynamics of complex fluids, such as polymer or surfactant solutions, can be significantly different with respect to simple liquids
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; for about 20 years it has been known that the addition of very small quantities (c∼100 ppm) of a high-molecular weight 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 [1,2]
Experiments reveal the formation of transient microscopic dendritic structures generated by the receding contact line, which evolve in a similar fashion to the well–known beads–on–a–string mechanism [11]
Summary
Formation and Contact Line ForcesThe wetting dynamics of complex fluids, such as polymer or surfactant solutions, can be significantly different with respect to simple liquids. 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; for about 20 years it has been known that the addition of very small quantities (c∼100 ppm) of a high-molecular weight 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 [1,2] This is surprising since the shear viscosity and surface tension of such drops are almost identical to those of pure water. This phenomenon was initially understood as a direct consequence of the nonlinear bulk rheology of the fluid, namely of the elongational viscosity, and normal stresses [1,2,3]
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