STUDY ON THE DYNAMIC BEHAVIOR OF THE ENTRY OF A SPHERE INTO THE OIL-FILM COVERED LIQUID

The dynamic behavior of objects entering water involves complex processes of multiphase flow coupling and interfacial energy dissipation, holding significant application value in military equipment, marine engineering, and biomimetic robotics. In real marine environments, the presence of heterogeneous medium layers (e.g., oil films) necessitates consideration of air-oil-water three-phase coupling effects for water entry problems, with dynamic characteristics differing markedly from traditional air-water two-phase systems. To investigate the regulatory mechanisms of the viscous oil film on the motion resistance, cavity evolution, and closure characteristics of hydrophobic spheres during water entry, thereby providing theoretical support for water entry problems in complex media environments, this study employed a combination of high-speed photographic experiments and theoretical modeling. It comparatively analyzed the dynamic differences between hydrophobic spheres entering water directly and those traversing silicone oil films of varying viscosities (100-1000 cSt), as well as the unique influence of oil films on closure characteristics and cavity morphology. A theoretical model was established to quantitatively derive characteristic quantities of sphere motion and cavity evolution. The results demonstrate that oil films significantly increase the motion resistance of spheres through viscous shear forces, causing pronounced deceleration post-entry. By quantitatively estimating viscous resistance and motion resistance, a theoretical model was developed to predict the time-dependent variation of water entry depth. Shear effects at the oil-water interface induce Kelvin-Helmholtz instability, generating ripples with wavelengths and amplitudes governed by oil viscosity and impact velocity. High-viscosity oil films (1000 cSt) promote high-amplitude regular ripples dominated by long wavelengths (approximately 3-10 mm), while low-viscosity films (100 cSt) primarily exhibit small-amplitude random ripples with short wavelengths (0.5-1 mm). The presence of oil films reduces cavity closure depth and, through the formation of corrugated cavity walls, enables a "dual closure" phenomenon where cavities nearly simultaneously pinch off at two positions under specific conditions. Building upon existing cavity closure models, a modified model incorporating oil film properties was established, quantitatively characterizing the reduction in closure depth caused by oil film-induced deceleration. For cavity wall corrugations, the theoretical model accurately predicts the wavelength of ripples formed by spheres impacting high-viscosity oil films and its quantitative relationship with sphere velocity. The suppression of short-wavelength perturbations by viscous dissipation qualitatively explains the viscosity-dependent ripple morphology. This work provides critical insights into water entry dynamics in multiphase media and advances predictive modeling capabilities for such complex interfacial phenomena.