Question

When a drop of water falls into a reservoir of water from a high enough altitude, water droplets will splash

Does the water in those droplets come from the original drop or from the reservoir?

Answer 1

When a drop of water impacts onto a liquid surface 'pool' of water, we can observer one or more phenomena:

• The drop 'bounces' then 'floats' the surface of the pool.
• The drop 'coalesces' into the pool.
• The drop 'splashes' on the pool, creating a 'crown' around a 'crater'.

Which will occur depends upon the size and velocity of the droplet. Specifically, the collision can be characterised by the ratio of the inertial forces to the surface force, given by the (dimensionless) Weber number, We defines as:

where ? is the density of the droplet, v is the velocity and d is droplet diameter and ? is the surface tension of the droplet fluid.

When the We is above a threshold of ?84, the impact is characterised as a 'splashing', whereby a 'crown' is formed around the crater and a column of water rises from the middle.1

In order to determine whether the original drop is present in the 'recoiled' water, a series of photos were taken at successive time intervals 0.0003s apart, using a 4.83mm diameter drop (with coloured dye) dropped from a height of 175mm into a transparent pool of water. The number in the corner of each photo represents the sequence number of the photo (at 0.0003s intervals).