On the role of liquid viscosity during droplet-pair impacts on solid surfaces

This study elucidates the influence of liquid viscosity on the hydrodynamics of simultaneous and non-simultaneous droplet-pair impacts on solid substrates. Using synchronised high-speed imaging and quantitative analysis, the spreading dynamics of droplet lamellae and their interaction-driven central sheet evolution are examined across a range of viscosities from 1.01 to 91.46 mPa s, representing Ohnesorge numbers of 0.002–0.177, under controlled impact Weber numbers in the range of 81–131 and dimensionless inter-droplet spacings in the range of 1.43–1.85. The findings reveal that increasing viscosity results in thicker lamella fronts, reduced spreading and a lower maximum central sheet height. In addition, the central sheet morphology transitions from ‘semilunar’ sheets to ephemeral liquid bumps, accompanied by suppressed capillary waves and reduced rim instabilities. A novel scaling law is derived for the maximum sheet extension, demonstrating its robust applicability to both simultaneous and non-simultaneous impacts of droplet pairs across varying viscosities and impact conditions. Furthermore, distinct morphological differences emerge between simultaneous and non-simultaneous impacts, primarily governed by lamella–lamella interactions and the momentum transfer dynamics. These findings enhance our understanding of the interplay between viscous and inertial forces in droplet-pair impacts, offering valuable insights for optimising spray-based technologies and multiphase fluid systems.