Splash and crown formation of a falling drop

Partly settled. Some of it is understood, some is still argued about.

A. M. Worthington's pioneering 1876 high-speed splash photographs first documented the crown-shaped splash but offered no mechanism. Over a century later, in 2005 Lei Xu, Wendy Zhang and Sidney Nagel at the University of Chicago showed that ambient air pressure is a key, previously overlooked control parameter: reducing chamber pressure below a threshold suppresses splashing entirely for an otherwise identical drop, implicating gas dynamics rather than the liquid or solid surface alone. This resolved the immediate mystery of why splashing thresholds seemed inconsistent across labs, but subsequent research through the 2010s (Latka et al. 2018 and others) found that surface wetting, roughness and other factors also modulate splashing in ways that are not yet fully unified with the air-pressure picture, so the complete quantitative theory of when a drop splashes is still being assembled.

A phenomenon photographed since the 1870s turned out to hinge on something as seemingly irrelevant as the pressure of the surrounding air.

Explanations that circulated

• That splashing is governed only by impact speed and liquid viscosity, independent of the surrounding gas; Xu, Zhang & Nagel's 2005 finding that lowering ambient air pressure suppresses splashing overturned this, though later work (e.g. de Goede, Laan, de Bruin & Bonn 2018) found substrate wetting properties also matter in some regimes, in apparent tension with Latka et al.'s claim that wetting is irrelevant at high capillary numbers.

References

• Drop Splashing on a Dry Smooth Surface, L. Xu, W. W. Zhang, S. R. Nagel, Physical Review Letters, 2005
• Drop splashing is independent of substrate wetting, A. Latka, A. M. P. Boelens, S. R. Nagel, J. J. de Pablo, Physics of Fluids, 2018