This tiny plastic boat is floating, 
upside down, in a layer of levitating liquid.
It looks like a gravity-defying magic trick,
but in this case the magic is in the science
and the mathematics behind this 
counter-intuitive new phenomenon.
To start with, we need to explain 
how a layer of dense liquid
can be suspended on a layer of less dense air.
The answer is vigorous vertical shaking.
This part isn’t a newly discovered phenomenon,
though it can be tricky to demonstrate.
Take this dense blue liquid in a glass container.
When you flip it upside down 
it starts to fall due to gravity.
It doesn’t just move down all in one mass though.
Like paint falling down a wall, it starts to drip.
The bottom surface bulges 
into droplets which fall,
displacing the air underneath,
until it ends up all in a layer 
at the bottom - as you’d expect.
But if you place the container on a shaker,
set to a suitable frequency,
something unusual happens.
Once the blue liquid is fully upside
down it stops dripping.
The vibrations prevent the 
droplets from forming
and keep whole bottom
layer of the liquid nearly flat.
And if the droplets can’t form, 
the liquid can’t fall.
It’s floating on a trapped cushion of air
and contained by the glass on either side.
It’s compressing the air below it 
but it can’t displace it,
so it hovers in place.
While this was already known,
researchers in France 
have been exploring another
even more counter-intuitive effect of the shaking.
When they place a boat, or a light plastic ball, 
at the bottom surface of the liquid,
it floats in place 
just like it would at the top surface.
This is partly because the air pressure 
underneath the ball is quite high,
since the air’s being compressed by the
 weight of the liquid pressing down on it.
But as you go up the liquid layer, 
the pressure decreases,
as there’s less water above it pushing down.
This results in the ball, or boat, being pushed
up by the pressure underneath,
that is until it reaches an equilibrium point
where that upwards force is 
balanced out by the downwards force of gravity,
and it floats.
This balance of forces 
is what causes buoyancy
and that is exactly what's happening 
here too, albeit upside down!
But the most puzzling thing about this 
upside down floating effect is that it’s stable.
If you gently push or pull the boat 
it will bob back to its equilibrium position,
just like its right-side-up counterpart 
does on the surface.
The researchers mathematically 
modelled the forces involved,
and sure enough the figures predict 
this bizarre, stable, reversed buoyancy.
It may seem contradictory to all common sense,
but the maths checks out.
The effect does depend on the system being shaken hard enough though, and at the right frequency.
If the vibrations get too small and gentle, 
the ball and the liquid will eventually fall.
But get the shaking right, and that is how
science can lead to a boat
floating upside down on a levitating layer of liquid -
not with magic but with maths.
