Osaka, Japan – Researchers have uncovered a counterintuitive phenomenon in collision dynamics: high-speed particles bounce back from wet walls much more strongly than expected. Integrating experimental observations with advanced numerical simulations revealed that increasing the impact speed induces a morphological transition in the post-collision liquid film, shifting it from a bridge to a dome shape. Further, it clarified the relevance of cavitation to such a dramatic change and to the stronger bounce. The outcomes, published in the International Journal of Multiphase Flow, provide critical guidelines for predicting high-speed particle collisions on wet surfaces and pave the way for safer and optimized designs in applications such as next-generation aerospace and automotive rotors operating at higher speeds.
The coefficient of restitution (COR) is a fundamental metric taught in introductory physics that represents the amount of kinetic energy a particle retains after a collision. Upon bouncing off walls, a particle slows down because a portion of its kinetic energy is converted into sound, heat, and material deformation. The COR condenses these complex energy conversion processes into a single value, facilitating the understanding of collision phenomena in both science and engineering.
Understanding the COR is essential across a wide range of industries, such as in coating technologies for pharmaceuticals and food, as well as in heavy machinery exposed to destructive debris. Recently, the drive toward carbon neutrality has accelerated electrification in the aviation and automotive sectors through the adoption of ultra-fast motors. Consequently, the risk of internal component damage from high-speed debris has sharply increased. A common engineering countermeasure to prevent machine failures is to coat internal walls with a liquid film to cushion impacts. However, until now, the mechanical behavior and effects of these liquid films under high-speed impact conditions, typically reaching tens of meters per second, have remained unresolved.
The key outcomes of this research on high-speed particle collisions on wet surfaces are as follows:
- Morphological transition of the liquid film: As impact speed increases, the shape of the post-collision liquid film changes from a stringy "bridge" into a "dome" that encapsulates the particle-wall gap.
- Enhanced rebound at higher speeds: The COR ratio (relative to a collision against a perfectly dry wall) increases significantly upon the formation of the dome-shaped film.
- Cavitation as the driving mechanism: Immediately after impact, the pressure within the particle-wall gap drops intensely. Once it falls below the saturated vapor pressure, a vapor cavity forms, giving rise to the dome shape.
- Suppression of the attractive force: The formation of the vapor cavity drastically weakens the liquid attractive force, which pulls the rebounding particle back toward the wall. With less energy absorbed by the liquid, the braking effect is released, resulting in a stronger bounce.
"Although particle collisions on wet walls have been extensively studied, this research focused on high-speed collisions and identified phenomena that differ from those reported in previous literature." says Hironori Hashimoto, the lead author of the study. He affirmed, "Despite the conceptual simplicity of collisions, the dynamics of the liquid film and the subsequent modification of the particle motion are highly nontrivial. Integrating experimental observations with numerical simulations, we elucidated the mechanisms underlying these complex phenomena. We shall continue our research based on these findings and further improve the performance and safety of industrial equipment."
