Drop spreading and rebound at low Weber number

We investigate the spreading and rebound of liquid drops impacting a thin layer of highly viscous silicone oil in the low-Weber and low-Bond number regimes, where a thin mediating air layer prevents coalescence. Using a custom drop generator capable of producing sub-millimeter drops, we conduct experiments across a broad range of drop sizes and viscosities as well as impact velocities. High-speed imaging captures the dynamic behavior of the drops during impact. Direct numerical simulations supplement our empirical data to examine the impact dynamics. Additionally, we develop a reduced-order model from first principles that models impact by enforcing the natural geometric and kinematic constraints of the problem. The model can predict the droplet shape and trajectory throughout the process and is validated through direct comparison to experiment. Our results reveal a departure from the classic scaling laws known for drop spreading at higher Weber numbers, with these established trends recovered as the Weber number increases in our study. Our multi-pronged approach allows us to highlight numerous fundamentally distinct trends that characterize spreading and rebound in the low Weber number limit. Among other key distinctions, the spreading and rebound dynamics shift from symmetric to asymmetric in time as one transitions from the low to high Weber number regimes.