Interfacial dynamics induced by impacts across rigid and soft substrates
We investigate impact-induced gas-liquid interfacial dynamics through experiments in which a liquid-filled container impacts substrates with elastic moduli from $O(10^{-1})$ MPa to $O(10^{5})$ MPa. Upon impact, the concave gas-liquid interface inside the container deforms and emits a focused jet. When the jet velocity is normalized by the container impact velocity, all data collapse onto a single curve when plotted against the Cauchy number, $Ca = ρ_{\rm e} V_{\rm i}^2 / E$, which represents the ratio of the inertial force of the container-liquid system to the elastic restoring force of the substrate. The dimensionless jet velocity remains nearly constant for $Ca< 10^{-4}$, but decreases significantly for $Ca > 10^{-4}$. Based on this observation, we define the boundary between the rigid-impact and soft-impact regimes using the Cauchy number, providing a quantitative criterion for what constitutes ``softness'' in impact-driven interfacial flows. To explain the reduction in jet velocity observed in the soft-impact regime, we introduce a framework in which only the impulse transferred within the effective time window for jet formation contributes to interface acceleration. This concept, referred to as the partial impulse, captures the situation where the impact interval (the duration of contact between the container and the substrate) exceeds the focusing interval (the time required for jet formation). By modelling the contact force using an elastic foundation model and solving the resulting momentum equation over the finite impulse window, we quantitatively reproduce the experimental results. This partial impulse framework unifies the dynamics of impact-driven jetting across both rigid and soft substrate regimes, extending the applicability of classical impulse-based models.