Rebounding Drop

12 October 2008


Milk Milk Milky Tea A cup of tea
A spoon A spoon


Gently drip milk off the spoon into the tea.

What colour is the drop?

You can use water instead of tea, ans you will get a slightly better view.


You should find that a spike forms with a droplet falling off the top that is made almost entirely of milk.

The whole process is very quick so we have slowed down the process for you.

A drop hitting slowed down by a factor of 10 A drop hitting slowed down by a factor of 20

It appears that the drop spreads out forming a hole in the surface of the fluid, and then a combination of gravity and surface tension creates a wave returning to the centre which creates the spike out of about 1/3 of the original droplet.

At 1200fps viewed from the side At 1200fps viewed from the top



Drop hitting the water A cavity forms
The drop hits the water moving quite quickly, it starts to spread out and it makes a hole in the water. Eventually the milk stops moving and it is spread out over a cavity in the water. The surface tension of the water tries to shrink this cavity and a circle of waves move into the centre.
The circular waves collide in the centre The jet forms a drop
When the waves meet the smash into one another and create a jet of milk going upwards and downwards. The end of the jet falls off forming a drop that is made up almost entirely of milk, as the jet was formed out of the flattened drop.

Why does the jet sometimes form rings moving down though the liquid?

ring vortex 1 Ring wortex 2 The vortex moves
The jet going downwards is moving through stationary water. this tends to slow down the sides making the sides of the jet rotate outwards slightly. This continues creating a ring of spinning liquid called a ring vortex. One side of the ring creates a downdraught which causes the other side to move and vice versa, so the ring moves down the glass.

Drops hitting surfaces is a very complex subject which may scientists are studying in order to make ink jet printers that don't splatter, understand how to make pesticides stick onto plants and not just fall on the soil, and how to make surfaces which clean themselves.

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