Drop Impact Oscillations
Authors:Darren Banks, Hamza Surti
Mentor:Dr. Guillermo Aguilar, Professor and Graduate Advisor, Mechanical Engineering, University of California Riverside
Sprays of atomized liquid are used in many applications, such as fuel injection, thermal management, and 3-D printing. The behavior of a spray is one of the most complex challenges in fluid dynamics today. Single droplet behavior is an important part of understanding complete spray behavior. One of the less-studied aspects of single droplet impacts is the behavior of a droplet for an extended time after impact Generally, impact will cause the droplet to splash and rebound almost immediately. However, in some cases, a droplet striking a surface may only spread and recoil, without splashing. The droplet oscillates between a disk and a hemisphere until the kinetic energy of the impact is dissipated. Hydrophilic and hydrophobic surfaces are especially interesting when studying droplet oscillations, as the interaction of the droplet fluid and the surface can significantly alter the oscillatory frequency and magnitude. In this study, oscillations of droplets with kinematic viscosities between 1.0E-6 and 1.1E-4 m2/s are explored, focused on the viscously-driven damping behavior. The droplets were made using mixtures of water and glycerol; in conjunction, three impact substrates were used to vary the hydrophilicity of the surface. Droplets freefall onto the target with velocities of 0.5-1.5 m/s. The Weber number of the drops ranged from 10-100 and the Reynolds number from 15-4000. Impact velocity, kinematic viscosity, and surface tension are the three key factors which contribute to oscillations. A highly viscous drop tends to restrict oscillations much more quickly than a less viscous one. Similarly, hydrophobic surfaces facilitate oscillations while hydrophilic surfaces do the opposite. Increasing the velocity of impact restricts oscillations, by spreading the drop fluid across a larger area which releases the energy needed for oscillation. We explore the interplay between viscous and surface tension effects in droplet oscillations.