we're excited we're very very happy we
had a paper accepted about 10 days ago a
paper that we've worked on for a couple
of years and the reason we're happy is
because we've done a fantastic piece of
work and it's been accepted by physical
review letters which is one of the top
journals in the field
now you have to appreciate that this
took me a while this morning brady so i
need you to be impressed what i've done
here is i've just got a bowl of water
and floating there on the surface is a
paperclip this is a
preprint version of it emergent surface
tension in vibrated non-cohesive
granular matter well it's impressive
because you know
that
a steel paper clip should be denser than
water and so it should sink
but it doesn't sink and the reason it
doesn't sink is because it's sitting on
the surface tension of the water
normally it's associated with molecules
such as water and two water molecules
suppose these are water molecules are
attracted to each other by long-range
forces the water molecules which have
this sort of shape
they bond together in little triangles
electrostatically so they're so small
that the electrostatic bonding between
the molecules is quite strong so what
we've done is something completely
different we've taken a box full of
grains of bronze so this not to scale is
a grain of bronze
our grains of bronze 150 microns in
diameter and we put them in a box 20
centimeters
square let's get to the point let me
show you the experiment that we actually
did i'm going to have to turn on the
amps that means it's going to get a
little bit noisy
and then we're going to shake some sand
then we put it in the cell and shake it
up and down this way
at a frequency of 60 hertz and so these
all start jiggling about all over the
place and you would imagine if you're
shaking these about by jig giggling them
up and down they would become randomized
and they do to some but under some
circumstances
all of them want to go to one side and
you get a dense phase over here and
there's just a few particles over there
this is our experiment okay
so we flip it on and we're expecting
that it's going to face separate into a
dense and a dilute phase
and there it goes
these two particles
when they're moving together
bash together and come out with a lesser
speed so if they hit each other smack on
at 10 meters a second bam they come out
with a fraction of that speed
why was i always told energy is
conserved well energy is conserved but
the kinetic energy of the coming in and
going out is reduced because some of
this goes into heating up the particles
and some of it goes into sound so there
is energy loss in the collision and
about 30 percent of the velocity is lost
in these collisions if they're smack on
if they're gentle once from the side
it's not so damaging
so what happens is if these come
together you get a little cluster
forming and they bash together so many
times that they almost come to a
standstill and when they made a little
cluster another one comes in and bashes
it and it makes a little cluster because
of the inelasticity
the energy loss in these collisions
there's a
a likelihood that they will cluster
together and form little clusters
what we're seeing here
is that it's not separating
instantaneously
but it's undergoing some separation
through a pattern which we call spinodal
decomposition which just the scientific
term for the way that these things
separate
first of all we did the experiment this
is a picture after 0.3
seconds that's a third of a second after
the thing has been turned on it starts
off uniform but after 0.3 seconds in
this frame you can see little patches of
bronze color that's the bronze particles
and the darker bits are where few bronze
particles are and it's starting to want
to cluster together into these little
heaps and evolved into something which
you now look at and say oh there's a
bridge of a continuous path i can wander
around here
but what's interesting we've got a
really nice one here actually what's
interesting is that we've got this
little circular droplet here and the
very fact that that's circular tells me
that there's a surface tension at play
here but remember what this is this is
just sand shaking up and down in a box
there's no intermolecular forces holding
things together this is just me shaking
marbles in a box but it's acting like
it's got a surface tension how bizarre
nobody knew that there would be surface
tension in this if you put water in
there's going to be surface tension of
the grains because the water will make
little bridges and pull them together
but before when you start off with dry
grains of bronze which are nice and
shiny with no water there it wasn't
obvious at all that they were going to
stick together if i put a big bale of
hay in the middle of a field and 100
cows all came because they wanted to
have it and they all kind of gathered
around it i haven't created surface
tension within cows have i surely just
because there's a big round blob doesn't
mean that has to be surface tension
it's ironic that you choose that example
because you have that's exactly what
you've done when you have a system where
an animal for example comes in
i'm going to say normally it comes in at
right angles to all of the other animals
then it might slow down it'll slow down
more than if it comes in tangentially to
all of the other animals because then it
can keep going it's a glancing blow if
you think about billiard balls moving
around on a table if i've got a pack of
them and one comes in and hits the pack
square it's going to stop dead it loses
all of its energy but if one takes a
glancing blow it just bounces off and it
keeps most of its energy and it's that
loss of energy at the interface which is
what creates the surface tension by the
very definition we're stretching the
interface just a little bit
we've gone ahead and we've measured this
surface tension i've got a beautiful
graph where we can actually see the
difference between the forces pulled
this way and the force is pulled this
way just like the paper clip we've got a
weight and then we've got some tension
like a trampoline skin supporting that
weight or we can see that in our graph
this beautiful circular form and the
dark blue region there is showing you
where the surface tension actually is
and we're the first people to have
actually measured that in a granular
system well the idea that you take
something and shake it is pervasive i
mean i get my bag of
muesli every morning and i open the new
packet and there's always brazil nuts at
the top or raisins and you shake it up
and you think it's going to do it and
people try and shake it up or they try
and shake it up powders in medicines to
try and make it uniform
and it doesn't work because for some
reason things like to cluster together
and in this case they seem to cluster
together and separate out into different
bits so this is something which is not
understood at all
in terms of physics this is one of the
big difficult problems of the age as it
were what happens when you have a system
which is not in equilibrium it's driven
from equilibrium energy is dissipated in
some way and yet it seems to reach a
steady state which you can't describe
using the normal laws of physics
so in here
this is our
economical supercomputer shall we call
it and what we've got here as well as a
thumping great dual core cpu