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  • Watch what happens when I place some small drops of food coloring on to 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 micro-organisms, 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 pipette.

  • Then pass the microscope slide through a flame for about thirty seconds.

  • Let it cool, and then put some drops of different concentration on to the slide.

  • You'll find that drops of similar concentration attract each other and merge

  • while drops of different concentrations chase each other.

  • 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 vapour.

  • 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 than the humidity around them,

  • and therefore there's more evaporation around the droplets

  • then 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 miscible,

  • but they have different properties.

  • For example, water evaporates more readily because its 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 war.

  • 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 "Marangoni 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 dissimilar,

  • in fact its been called artificial chemotaxis.

  • And as amazingly life like as their motion appears,

  • in a way, I think it shouldn't be all that surprising.

  • After all, evolution began with the 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 utilisation of

  • these natural tendencies of molecules appears miraculous.

  • And its 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 sufrace

  • 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,

  • Its those open bonds, its 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 microscopes,

  • some of the microscope slides have shattered.

  • And so you should be particularly careful when doing this procedure,

  • obviously heating them up to a very high temperature causes them to expand a lot

  • and when they contract again as they're cooling, well then,

  • they sometimes break apart because of those stresses.

Watch what happens when I place some small drops of food coloring on to this slide.

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B1 中級

これらの液体は生きているように見える! (These Liquids Look Alive!)

  • 13 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語