Oil drop impact on inclined thin oil films

Abstract

Viscous drop impacts occur in various modalities across numerous natural and commercial processes. In most practical applications, such as spray deposition, oblique impact is commonplace as well as the formation of a thin deposited film. In this study, impact dynamics of silicone oil drops on inclined ($\varphi$ = 30°) glass slides pre-wetted with the same liquid, both spanning a viscosity range of 4–10,000 $\mathrm{mPa\; s}$ were investigated. Using high-speed imaging techniques from both the side and bottom views, three distinct air entrainment dynamics were identified: single, double, and peripheral — governed by the viscous, capillary and inertial dynamics of the drop and the thin oil film. Additionally, the introduction of carbon black (0.005–0.1 wt.$\text{\%}$) particles significantly altered the wetting behavior by accelerating the air film rupture. Our results highlight the importance of drop and film viscosities and impact inertia in wetting dynamics and contact line propagation, and also underscores the need for multi-angle imaging to fully capture the transient wetting phenomena.