The interaction of falling and sessile drops on a hydrophobic surface

• Concentric impacts of water drops on Teflon is characterized.
• Spreading diameter indicates increased energy dissipation at higher Weber numbers.
• Spreading diameter favorably compares with analytical predictions.

The dynamic behavior of drops impacting surfaces, liquid films or pools, and moving drops has been previously investigated; however, the behavior of falling drops impacting a sessile drop at rest has been explored to a lesser degree. In this work the coalescence, spreading, and recoil following the concentric impact between a falling water drop and a sessile water drop is studied on a Teflon substrate (hydrophobic). The influence of the Weber number, We, on drop spreading is the primary focus. For experiments at We ∼ 200, the impacts results in drops that spread a maximum diameter and then recoil to a final equilibrium diameter. At lower Weber numbers the drops spread to a maximum diameter, then recoil, and then undergo a second spreading event to the final equilibrium diameter. These observations suggest that at higher Weber numbers more energy is dissipated in the deformation of the liquid volume that occurs via crown formation, contact line movement, and spreading. Experimental observations are compared with analytical predictions based on conservation of energy with two treatments of energy dissipation. Analytical predictions for the maximum spread diameter are within 10% of measurement and indicate that approximately one half of the kinetic energy of the impacting drop is dissipated in the approach to maximum spreading diameter.