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The Marangoni effect

Liquids form approximately spherical droplets because of surface tension. Surface tension is a force acting at the surfaces of liquids that tends to contract the area of these surfaces, and a sphere has the minimum surface area for a given volume. This is cute. But even cuter is that any time the surface tension of a liquid is not the same everywhere ,then the areas of the surface at lower surface tension tend to expand while those areas at higher surface tension contract – this is called the Marangoni effect and is illustrated in the YouTube clip above.

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Evolution in action

Seeing is believing, so it is good to see evolution in action. The above movie shows a huge petri dish with bacteria starting at both edges where there is no antibiotic. The dish has a gradient of increasing concentration of this antibiotic towards the centre of the plate. Initially they can’t grow in the regions where the antibiotic concentration is high. But they evolve resistance and just march up the gradient of the antibiotic. This takes about two weeks. Impressive.

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Interwoven networks of crystals

This is an animation of a rotating cubic box containing a few thousand molecules. It is from computer simulations by James Mithen, who postdoced with me. Most of the molecules are in one of two different crystals, the yellow ones are in a type of crystal called a face-centred cubic (fcc) crystal, while the green ones are in a different type of crystal called body-centred cubic (bcc). Both are crystals in the sense that the molecules are arranged in a regular lattice but the two lattices are different, for example, in the fcc crystal each molecule has 12 neighbours while in bcc each molecule has 8.

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A personal study of nucleation

I study nucleation, but mainly via modelling on a computer. The guy below, Harley Morenstein, took a more personal approach. Incidentally, if you are bored of the Vine looping just click on it.

Drinks like Diet Coke, lemonade etc, are carbonated, i.e., have carbon dioxide pumped into them under pressure to make them fizzy. If you carefully take the top off and are gentle with then (as opposed to giving them a shake) most of the carbon dioxide remains in the drink. This means that the amount of carbon dioxide dissolved in the water is actually above the solubility of carbon dioxide in water (at atmospheric pressure), and so this carbon dioxide wants out (technically speaking it is thermodynamically ‘downhill’ for the carbon dioxide to leave the water and go into the atmosphere).

The carbon dioxide comes out as bubbles and if the drink is not shaken these bubbles can find it hard to start to form. This initial step when a tiny bubble starts to form is called nucleation. If nucleation is not possible then this traps the carbon dioxide in the Diet Coke or whatever the drink is.

Until something comes along to make nucleation easier, like Mentos. Mentos are an American sweet, and for reasons nobody really understands, carbon dioxide bubbles nucleate like crazy on the surface of Mentos. So when the guy in the Mentos suit drops into the Diet Coke tub, bubbles of carbon dioxide nucleate like crazy, and you saw the result above.

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A murmuration of starlings

Murmuration is my new a favourite word, it means a flock of starlings. It is one of the old English collective nouns for a group of animals, like a murder of crows, a skulk of foxes or a gaggle of geese. And as the YouTube clip above shows murmurations are simply astonishing.  The Guardian also has a gallery with some pretty amazing pictures. Thousands of starlings flying through the air as if they were a single organism. Flicking back and fore like cat’s tail, not like the thousands of bird spread across maybe 100 m that they are.

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Using analogy to understand how chameleons change colour

The video shows artificial models of the key structures in a type of cell called a melanophore. This is from a nice paper by Aoyama et alMelanophores and similar cells are how animals like chameleons change colour. The blobs that show up as bright here in the fluoresence microscopy images are actually dark brown under natural conditions. They contain eumelanin, the brown pigment that makes brown hair brown.

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