Kind: captions
Language: en
[Applause]
watch what happens when I place some
small drops of food coloring onto this
slide some are attracted to each other
and merge While others repel and Chase
each
other it looks just like the tiny world
of
microorganisms but why well if you want
to try this out for yourself you need to
get some ordinary food coloring and
dilute it with distilled water using a
pip hat then pass the microscope slide
through a flame for about 30 seconds let
it cool and then put some drops of
different concentration onto the
slide you'll find that drops of similar
concentration attract each other and
merge while drops of different
concentrations Chase each other
[Music]
you can draw guides on the slide using
permanent marker which is hydrophobic
and set up long-distance
Pursuits or you can find that drops of
similar concentration attract each other
despite the hydrophobic
barriers but how is this possible just
using ordinary food coloring and water
well the key is evaporation each drop is
constantly evaporating so around it is
an envelope of vapor the rate of
evaporation depends on the humidity
around the drop so the drier the air is
the faster the rate of evaporation so
when any two drops are close enough the
humidity between them is greater than
the humidity around them and therefore
there's more evaporation around the
droplets than in between them and this
pushes them together so differential
evaporation makes the droplets attract
but that's not the whole story food
coloring is a mixture of mainly two
molecules water and propylene glycol
these two liquids mix well so we say
they are missable but they have
different properties for example water
evaporates more readily because it's
lighter and it has a stronger surface
tension due to hydrogen bonding and this
is important because interesting things
happen when there are gradients in
surface tension for example if I add
some pepper to this bowl of water to
allow me to see the motion and then I
add A little dab of soap right in the
middle you see that all of the water
rushes outwards and this is because soap
has a lower surface tension than water
you can think of it a bit like a tug-of
warar before I added the soap all of
those water molecules were pulling on
each other equally but once I add the
soap molecules in the middle their
weaker surface tension means the water
molecules around them are pulling harder
on each other than the soap is and so
they Rush outwards away from that
spreading soap drop and this motion is
called Maring goni flow and a similar
thing is actually happening with our
food coloring droplets if you have drops
of similar concentration they attract
each other due to the higher humidity in
between them and they merge and although
drops of different concentration also
attract each other they don't merge when
they come into contact this is because
the water in the lower concentration
drop pulls together and away from the
drop with the higher concentration of
propylene glycol just like with the soap
and water and differential evaporation
drives the two drops forwards it is
fascinating to watch these droplets and
think about how closely their motion
mimics life living organisms seek out
molecules like food and this is a
process known as chemotaxis but what
these droplets are doing is really not
all that dis similar in fact it's been
called artificial
chemotaxis and as amazingly lifelike as
their motion appears in a way I think it
shouldn't be all that surprising after
all Evolution began with a natural
tendencies of molecules to form and
break apart to attract and repel and
then over billions of years of
refinement through natural selection
Evolution has produced bodies whose
utilization of these natural tendencies
of molecules appears
miraculous and it's that that we call
life I tried using slides that hadn't
been passed through a flame and then I
couldn't get the drops to move so on
researching this further I found out
that passing the microscope slide
through the flame creates a high energy
surface and what that means is the flame
is essentially breaking open some of the
bonds the glass Bonds in the surface of
this slide and when you put the droplet
on
top it's those open bonds it's that high
energy surface that draws some of the
water molecules away from the droplet
and that actually makes the water
molecule more likely to evaporate but as
just happened right there some of the
microscope
some of the microscope slides have
shattered and so you should be
particularly careful when doing this
procedure obviously uh heating them up
to a very high temperature causes them
to expand a lot and when they contract
again as they're cooling um well then
they sometimes break apart because of
those stresses