These Liquids Look Alive!

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 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 miscible, 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. 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, it's 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 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 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.