DROPLET IMPACT ON SHEAR-DRIVEN LIQUID FILMS

This paper investigates the interaction between the impact of a liquid droplet and a moving liquid film in which a shearing air flow is present above the liquid film. Visualization of droplet trajectory, deformation, and impact outcome are provided. Droplet diameters used in this study range from 2.4 to 4.8 mm, water film thicknesses are between 0.2 and 1.7 mm, and the shearing air velocity within the rectangular, duct is between 4.5 and 15 m/s. Two high-speed cameras are used to track the droplet and visualize the film surface. A laser focus displacement meter is used to obtain the instantaneous and average film height. By analyzing the acquired images, along with the film height, different regimes for both droplet deformations in shearing air and impact outcomes are observed. A regime map is produced relating the air, film, and droplet properties with the outcomes of the interaction between the droplet and the moving water film. The shearing air has an important effect on the interaction process before the impact by deforming the droplet and confining the film thickness; it also influences the impact process during and after the impact. Where the droplets retain their spherical shape before impact (low shearing air velocity), the transition between splash and deposition agree with published results obtained for droplet impact onto static films. The waviness of the film has no clear effect on the impact outcomes but does have an important effect on the way splash occurs with a big difference in the size of secondary droplets.