Impact dynamics of drop on viscous liquid films with density contrast

This study numerically investigates drop impact dynamics on viscous liquid films, focusing on the underexplored role of the film-to-drop density ratio (λ). Using a volume-of-fluid method, we evaluate how λ and dimensionless film thickness (Ψ) dictate interfacial evolution and energy partitioning. Energy budget analysis reveals that heavy drops impacting light films (λ=0.1) retain more kinetic energy, yielding significantly higher rebounds. We identify three restitution coefficient (Γ) regimes: substrate-independent bouncing, substrate-dependent bouncing, and non-bouncing. An existing spring-damper model accurately predicts Γ for λ≤1 but deviates for heavy films (λ=10) due to unaccounted inertial resistance. Additionally, the maximum film deflection transitions from a wall-restricted linear scaling to a universal deep-pool scaling. Finally, the maximum spreading ratio exhibits a sigmoidal decline, reflecting the redirection of kinetic energy from lateral expansion to vertical penetration. These findings advance the fundamental understanding of multiphase impact phenomena for coating and printing applications.