Drop Impact of Newtonian and Elastic Fluids

We studied the dynamics of drops of a Newtonian fluid ~water! and a constant viscosity elastic ~Boger! fluid of matched shear viscosities (hN51.1 mPa•s, hE51.3 mPa•s! impacting on hydrophilic and hydrophobic surfaces and on a thin liquid film ~;1 mm! of the same fluid. The elastic solution has a Zimm relaxation time l51.1310 s. During the drop spreading, both solutions have an equivalent surface tension of sDmax572.1 mN/m. At long time, the surface tension of the elastic solution decreases to sE562.0 mN/m. In all experiments, the impact velocity and initial drop diameter are 2.48 m/s and 1.68 mm, respectively. The Reynolds and Weber numbers are Re54166 and We5142 for the Newtonian solution and Re53205, We5166 for the elastic solution. To observe the impact of liquid drops from both bottom and side views, we used a beam splitter cube setup ~Fig. 1! with a high speed video camera. On a hydrophilic surface ~Fig. 2! the spreading of the two solutions occurs over the same time interval. At the maximum diameter, both solutions display a flat disc surface with peripheral fingers with different amplitude and frequency. During recoil, the Newtonian solution exhibits capillary waves from the outer ring to the center of the disc. These waves are dampened in the elastic solution. The rate of retraction and the final shape of the drops at long time differ significantly between the two solutions. Such differences cannot be explained by the difference in surface tension of the solutions and it is believed to be due to the adsorption of the polymer to the surface during spreading FIG. 1. Experimental setup.