Fast droplet impact onto slowly moving deep pools

Journal article

When a fast droplet impacts a pool, the resulting ejecta sheet dynamics determine the final impact outcome. At low Capillary numbers, the ejecta sheet remains separate from a deep static pool, while at higher values it develops into a lamella. Here, we show that the common natural scenario of a slowly moving deep pool can change the upstream impact outcome, creating highly three-dimensional dynamics no longer characterised by a single descriptor. By considering how pool movement constrains the evolution of the ejecta sheet angle, we reach a length-scale invariant parameterisation for the upstream transition that holds for a wide range of fluids and impact conditions. Direct numerical simulations show similar dynamics for an equivalent oblique impact, indicating that the air boundary layer above a moving pool does not play a decisive role for low pool-droplet speed ratios. Our results also provide insight into the physical mechanisms that underpin pool impact outcomes more generally.

Authors: Sykes, TC; Alventosa, L; Castrejón-Pita, JR; Cimpeanu, R; Harris, DM

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Cambridge University Press
• Journal: Journal of Fluid Mechanics
• Volume: 1037
• Article number: 11
• Publication date: 2026-06-15
• Acceptance date: 2026-04-21
• DOI: 10.1017/jfm.2026.11586
• EISSN: 1469-7645
• ISSN: 0022-1120
• Language: English
• Keywords: breakup/coalescence; drops; interfacial flows (free surface)