Splash (fluid mechanics)

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Water bouncing upwards after water droplet hits it

In fluid mechanics, a splash is a sudden disturbance to the otherwise quiescent free surface of a liquid (usually water). The disturbance is typically caused by a solid object suddenly hitting the surface, although splashes can occur in which moving liquid supplies the energy. This use of the word is onomatopoeic.

Splashes are characterized by transient ballistic flow, and are governed by the Reynolds number and the Weber number.

Small scale splashes in which a droplet of liquid hits a free surface can produce symmetric forms that resemble a coronet; milk is often used as it is opaque.

Sand is said to splash if hit sufficiently hard (see dry quicksand) and sometimes the impact of a meteorite is referred to as splashing, if small bits of ejecta are formed.

Physicist Lei Xu and coworkers at the University of Chicago discovered that the splash due to the impact of a small drop of ethanol onto a dry solid surface could be suppressed by reducing the pressure below a specific threshold. For drops of diameter 3.4 mm falling through air, this pressure was about 20 kilopascals, or 0.2 atmosphere.

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A splash after half a brick hits the water; the image is about half a meter across. Note the freely moving airborne water droplets, a phenomenon typical of high Reynolds number flows; the intricate non-spherical shapes of the droplets show that the Weber number is high. Also note the entrained bubbles in the body of the water, and an expanding ring of disturbance propagating away from the impact site

A drop of water splashing onto a hard surface

• Harold Eugene Edgerton, whose Milkdrop Coronet is arguably the most famous photograph of a splash