i'm talking about the idea that you can
make something that will swim out of two
spheres alone connected by a spring this
is a mock-up of it the real object is in
a cell over here a red sphere above a
glass sphere
the idea is that the lower sphere is
denser the glass and the atmosphere is
plastic it's the less dense so these two
spheres
have different sizes but the important
thing is they have different densities
so although it's the whole thing is
neutrally buoyant the sphere at the top
is being pulled up and the sphere at the
bottom is being pulled down so that
spring is being stretched first we've
got to make one of these
on a smaller scale are they hard to make
they look fiddly yeah they're a
nightmare you've got a big sphere and a
little sphere and you've got to connect
them by a coil that's
vaguely spring-shaped and you've got to
wind that round
a rod like this and then you've got to
very carefully get the glue on one end
and the glue on the other if you get too
much glue then it gets too heavy and it
sinks if you don't get enough glue it
might be too light when i press this
switch it just causes this loudspeaker
to oscillate so this is going up up and
down at about 50 hertz so the same
frequency as your light bulbs
so it's very difficult to see under
these lighting conditions but this the
surface of the speaker is going up and
down so it's accelerating alternately
it's accelerating that way and then that
way rapidly backwards and forwards we're
going to shake this object inside a
liquid and then the lighter sphere
will move one way and the denser sphere
will move the other and it will go like
that
and similarly with this top sphere that
gets propelled forwards at a higher
acceleration than the sphere so what
happens is that when the thing is
accelerating upwards it stretches like
that so as you shake it up and down
this one wants to stay where it is that
one wants to move much more so that one
wants to go with the flow go with the
flow as you put it so what do we mean by
swimming well what we mean by swimming
is that
we've got an object in some liquid
perhaps and in order to propel itself
along this object it has to the object
has to
deform in some way it has to move its
body parts in some way it turns out that
when uh for objects that are moving very
slowly or very very small objects in
water such as bacterium if they in in
such a case this the viscosity of the
water is very important and you find
that if you do a stroke
in which you uh
so-called reciprocal stroke in which the
stroke looks the same forwards in times
backwards in time so if i'm trying a
stroke like that
and then if i'm you have to imagine i'm
doing it either very slowly or i'm very
small that will just propel me forwards
when i
do that and then backwards again when i
do that and we go nowhere so that's a
stroke in which if you run the video
forwards and back or backwards in time
you can't you can't tell
just from looking at that stroke whether
the video is being run forwards or
backwards in time an object like a
bacterium which is very small and which
is living in a world dominated by by
viscosity it's living a very low
reynolds number it has to develop
another trick in order to be able to
swim
so he can't use a stroke like this which
looks the same forwards or backwards in
time it has to develop some other type
of stroke
so one way for example a bacterium could
do it for example an e coli bacterium is
to it's got a little whip-like tail on
the back and it rotates that round just
like a corkscrew
and that motion you can tell it's not
time-symmetrical there are other tricks
as well i mean you the bacteria can use
little sillier little hairs that move
along the edge of its body and that can
propel it through the liquid so here
when we're if we if we oscillate it at a
small amplitude
it's a little bit like the bacterium
it's living at low reynolds number
and because this is a time reversible
motion it's going to go nowhere so what
we've done is that we keep increasing
the amplitude
until the inertia of the liquid becomes
important
and at that point
it starts to swim
why
essentially the inertia of the liquid
can
rectify the motion this oscillating
motion so it can turn oscillating motion
into
a jet of liquid that comes out of the
bottom and this jet of liquid is what
propels
the spheres along it was cool but
it happened so quickly in real life you
have to go back and watch the video to
see
if it was actually interesting or not so
when we went back and watched the video
we saw this huge jet being propelled out
of the bottom sphere and a couple of
vortices above the big sphere so yeah
yeah it became really interesting
retrospectively i'm too clumsy to do
this
mike did that then you put it in the
cell
you then have to get all the liquid in
here stationary you have to make sure
the density of this is exactly right to
get it to float in the middle i forgot
about the bubbles yeah bubbles right so
if we
yeah so this solution is salt water but
we have to put it under vacuum for a
while first if there's air in the cell
halfway through a little bubble will
appear on the top surface of this thing
and it will rise to the top and you'll
get a false reading
so you're constantly going against
nature's desire to screw this experiment
up the interesting question is
how does
this such an object or any object make a
transition to swimming
so a low reynolds number with low
amplitude of oscillation
it's not swimming it's just like the
bacterium that can't it can't swim with
a reciprocal motion
and we keep increasing the amplitude
suddenly it starts to swim and it goes
off we get this jet of liquid underneath
it or is it a smooth transition so does
it start if we start very very low
speeds does it gradually
start moving with faster and faster
speed
and so what we seem to have found with
this experiment is that there is indeed
a transition between
not swimming
and we reach a critical amplitude and it
starts swimming this is one of these
problems which mathematicians are
worried about they don't know how to
deal with this exactly
and they write equations down which work
for small velocities or you know if you
were doing this in treacle
but they don't correspond to the real
world this has fallen through the gaps
of mathematical analysis computing you
have to do this damned experiment i said
that before people complained i was
swearing but you have to do that in
order to make sure you're not fooling
yourself
[Music]
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 elasticity
the energy loss in these collisions
there's a
a likelihood that they will cluster
together and form little clusters