Sunday, April 23, 2017

Fun things about liquids illustrated by simple kitchen mixtures of oil & water: hydrophobicity, diffusion, surface tension, &c

I did my PhD partially on liquids, analyzing the hydrophobic effect. Recently, I stumbled across an account describing simple things that you can do with home kitchen ingredients to illustrate properties of water and other liquids (see reference at bottom). The basic idea, of course, is just to mix oil and water in a container. However, there are many flourishes that you can easily add - for instance, you can add syrup (a very thick liquid) at the bottom, which forms a third layer separated to some degree from the water unless you vigorously mix it.

You can then put lots of things into this three-layer stack that go to the different characteristic layers depending upon the density -- for example, corks or toothpicks, which float to the very top, or blueberries, raspberries and some other vegetables, which go to the oil-water interface. Then there are things such as light plastic beads and pasta that go to the water-syrup layer. Finally, coins or metal objects sink to the bottom. Tightly crumpled paper vs a flat piece will also go to different levels, illustrating Archimedes'.

There are other neat things you can do such as pouring little bits of dye into the mixture and then watching it go through the oil and slowly mix with the water. Or even putting small amounts of salt in and watching them clump together and eventually pass through the oil before dissolving into the water. The differential aspects of pouring and mixing illustrate the physical concepts of diffusion and viscosity.

To some degree, one can also see the differences in surface tension of water and oil when one looks at objects at the oil/water boundary and also floating on top of the oil. The surface tension of the water is illustrated by the curved shape of the oil/water boundary (where the blueberry sits) in contrast to the top of the stack on the oil (where the cork or the duck sits).

Finally, if one leaves the preparation for enough time, the syrup mixes with the water. Of course, the oil and the water will stay separate. Eventually, if you wait long enough, you will see a small layer of bacteria at the interface, which is kind of interesting to watch grow, presumably getting considerable energy from the hydrocarbons in the oil.

Altogether a lot of fun: I have taken many pictures of the mixing and so forth and I would encourage people to do the same at home.

Pictures and Movies of the oil-water separation

Playful Separations: photos of oil-water mixtures in bottles, some with a 3rd layer and suspended objects ( and
Time-lapses: at first fast; then slow to equilibrium

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Lisa Burke,

Things illustrated

Diffusion and viscosity
Viscosity and diffusion
Capillary Action and Surface Tension
Hydrophobic Effect
Archimedes' principle