The Physics of Droplets | ScienceTake | The New York Times

Looks a bit
like the Dance of the

Sugar Plum Fairies,

if they were made
of water

and propylene glycol.

That's the key.

Each droplet is composed
of two fluids.

The coloring is just
to help you

see what's going on.

The different fluids
evaporate

at different rates,
and they

have different surface
tension.

And the result,
in each droplet,

is kind of a constant
internal tornado.

Scientists at Stanford
found that if

that internal tornado
is disrupted,

then the droplets
chase each other around,

or form a line,

or even climb vertically.

And the reason
is that just a hint of

evaporation from one drop

changes the relative
humidity

near the surface
of the other drop.

The change in humidity
affects how the

water evaporates
from the other drop.

That changes
its internal tornado,

and the drop
starts to move.

The result is totally
fun to look at,

and it could be a
brilliant teaching tool.

Simple materials,

complicated physics.