# Splashing

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Fig.1: The Milkdrop Coronet, arguably the world's most famous photograph of a splash, taken by Harold Eugene Edgerton.

Splashing is a phenomenon observed in fluid dynamics where an object (liquid drop or solid) is impacting on an otherwise quiescent (solid or liquid) surface, creating a disturbance which is in part characterized by small bits of the material flying away at an oblique angle to the line of impact.

Some very common examples include raindrops falling to the ground (liquid drop impacts on solid), a stone falling into water (solid object impacts on a free water surface), or water dripping from a faucet (water drop impacts on solid or free water surface).

Though the physical mechanisms which are responsible for the formation of a splash are not well understood in any system involving a fluid, recent experimental and theoretical work has provided a framework which might soon lead to an answer for this long standing question.

The broad definition used above also covers the use of the term splashing in other areas of science. This includes the impact of meteorites (solid object on solid surface), where often small pieces of ejecta are observed, flying away at oblique angles from the impact site. Another example is the impact of objects in dry quicksand (solid object on granular matter), recently reported in a paper in Nature.

At very high speeds, the impact of a solid object in a liquid does not only create a splash in the sense described above, but also a significant sound. This sound is sometimes also referred to as a splash. In fact, it has been argued that the word splash is onomatopoeic, that is it imitates the sound of the process it describes. In most case the sound arises mainly from fluid collapsing around a pocket of entrained air, in the wake of the solid impactor. The closure time is given by

$t \sim \sqrt{a/g}$

where $a$ is the size of the object and g is gravitational acceleration. Recent work has demonstrated that splashing can be affected by inertial effects, addition of polymer, vacuum and ambient air conditions, and the wetting properties of the solid impactor.