Your probably know of usual fluids, like water,
or ethanol or olive oil. A ferrofluid is
different, unlike those fluids. The first main
difference is that these fluids are all composed
on one or similar kinds of a molecule: water of
water molecules, ethanol of ethanol molecules and
olive oil of oleic acid, palmitic acid and other
fatty acids. So they are all molecular fluids.
Ferrofluids are not! They have a
molecular fluid as the main component, which can
be water or some organic solvent, but what makes
them respond to magnetic fields are small magnetic
nanoparticles that are dispersed in there. The
nanoparticles are so small that their random
thermal motion is larger than their motion due to
gravity, so they do not settle down. If they were
big, like the pulp in your orange juice, they
would fall at the bottom exactly like the pulp.
But they are small, exactly like the fat droplets
in your homogenized milk you have in your fridge
and keep randomly moving around, making the
ferrofluid a continuous, homogeneous fluid, even
though it has solid particles in a liquid.
But these particles are magnetic, so why
don't they come together by attracting each other
? First of all, because they constantly move
and rotate, their magnetic dipole (their
orientation) is changing all the time, so they are
not like permanent magnets. But we also help them
stay separated by adding a bit of soap. The soap
sits on the surface of the nanoparticles,
preventing them from touching. This way, they
cannot form big clusters which would fall at the
bottom and behave like one big magnet.
So, when you don't have a magnet near a
ferrofluid, it behaves like a fluid and its
surface is pretty flat. But if you put a magnet
near it, first the entire fluid becomes attracted
to the magnet (so you can raise it inside a bottle
with a magnet outside the bottle, which is pretty
much what they did in NASA to move ferrofluid
rocket fuel to the engine when there's no
gravity). Secondly, you can make very pretty
shapes like the beautiful art of the artist
Sachiko Kodama sachiko .
But how does the surface make these spiky
shapes? It is all an interplay between the
competition of (1) the energy required to deform a
surface called surface tension which likes
to keep surfaces flat and droplets spherical, and
(2) the energy of the magnetic field and how it
wants to follow the field lines.
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