okay ferrofluid in the dish as i say it
looks fairly innocuous now very very
strong magnet
sorry
oh no that hurt
get off
these magnets are very strong
let's do the magic first
we are talking about ferrofluids this
wonderful well it looks pretty innocuous
it gets everywhere it stains everything
this evil black sort of oil-like fluid
but this time last year i went into my
daughter's school
and played around with this and i've
never seen a bunch of eight-year-olds
get so excited about a piece of science
so i really wanted to to do a 60 symbols
video and let me cut to the chase
there we go black oil evil in fact they
say that you should dispose of it just
as you dispose of oil it's pretty dirty
it stains very badly it's not toxic at
all so you don't need to stand back
birdie it's fine it's not quite level
right what we're going to do now is
capture the dynamics of how this
ferrofluid response or magnetic field
i'm going to bring the magnet in up
underneath it and and then take it away
and we're going to look with a high
speed camera i'm absolutely surrounded
by cameras here lots of lights it's um a
very big sort of hollywood production
compared to the usual brady carrying a
camera on his shoulder let's go so what
i'm gonna do
wow
ready
so as you bring the magnet up from from
below basically the influence in the
magnetic field is changing and that's
giving you different spikes as the
ferrofluid fields the greater influence
of the magnetic field and balancing
against that is the surface tension
which is trying to hold the liquid
together and so you've got these two
competing effects and from the period
distilled spikes you can work out lots
of fascinating things about the dynamics
of the fluid and also the properties of
the fluid
that's the problem it's jumped out so i
brought the magnet i stupidly brought
the magnet to the top surface and it's
jumped out and covered my hand look at
how badly it seems i now have to wash my
hands
the fun bit is when you put the magnet
on the top and watch it go
up to that that's the fun bit that's the
bit i'm looking forward to but then we
will have to stop the video because i'll
have to spend at least an hour cleaning
myself and where does magnetism come
from well magnetism comes from the
interactions of spins
and what we basically do to represent it
what physicists do is we represent we
take the electrons and we think of them
basically as either being spin up or
spin down and what happens is when you
put the magnetic field on
those spins align right and they align
right across the solid
so
with ferromagnets with the magnets
you're used to when you take the magnet
away those spins remain aligned and
you're left with what we call a
permanent magnetic moment in things
called um power magnets what happens is
that when you bring the magnetic field
in you can align them but then when you
bring it away they no longer remain
aligned that's exactly what's happening
with these ferrofluids so we're going to
move away from considering the sort of
individual atoms of the individual
electrons
and think about a nanoparticle which
comprises you know tens of thousands
from up to 100 000 atoms and each one of
those particles the electrons in them
all those spins interact and what
happens in the particle is that the
particle ends up despite the fact that
it's comprising lots and lots of atoms
ends up with one big net spin we call it
a single domain particle now what's
happening in the absence
of the
magnet in the ferrofluid so just as it
stands just as it is like here those
particles are bouncing around bumping
off other particles moving around like
you would expect you know particles in a
liquid to do and so the spin gets
scrambled
and they don't align
when you bring the magnet in what's
happening is you're getting the spins of
those particles all to align and you're
seeing the collective response of all
those particles responding to the
magnetic field lining up and interacting
with each other take it away again
and you're back to the fluid when i
bring a magnet near you why don't all
your atoms and electrons stand to attach
very good question brady certain things
are magnetic other things are not
magnetic and it depends on how the
electrons are arranged in the solid
that's the key thing lots of organic
material we say it's diamagnetic it's
not ferromagnetic or not paramagnetic
and that's because basically the
electrons are not free to align in the
same way as they are in iron for example
or other fire magnetic materials so the
particles in this liquid are magnetite
so an iron oxide and what they're coated
with is something called a surfactant
which means that um the particles are
the surfaces of the particles are quite
passive they come together but then they
don't chemically bond it's what we call
a physical interaction it's a physical
attraction um no chemical bonding and
then
when you bring the magnet away they can
release easily and then they're bumping
into each other in the solution but
they're not forming chemical bonds so
you might think that it's a fairly
esoteric you know product you really
haven't seen it before but um where it
crops up all the time is in hard disk
technology actually the spindles that
are associated with hard disks where the
hard disk is spinning around that is
lubricated actually with ferrofluid so
you have to get a first time brady
because i'm going to get covered in
stuff i'm going to bring it in
above the the ferrofluid instead of
putting the magnet from below as we've
seen before i'm gonna bring it in from
the top
let's see what happens three two one
oh man that was good from this angle
foreign
mmm
you