I would to start this post by acknowledging how impressed I am with the style of the French guy who strode confidently onto Friday’s flight from Bordeaux to London wearing a large string of cloves of garlic draped around his neck. Some people just have natural style. As someone who does not, I am a bit envious. My PhD student and I were in Bordeaux as part of an EU research network (called RAMP) on crystallising proteins. There is a research lab in the suburbs of Bordeaux that is world leading at what is called microfluidics — essentially plumbing but instead of pipes centimetres across that move litres, microfluidics has pipes that are less than 1 millimetre wide and move as little as billionth of a litre, a nanolitre. The channels above in the microfluidic device shown above are only 0.15 mm wide.
You may ask, what is very tiny plumbing used for? One use is when what you are working with is very hard/expensive to get, even in tiny quantities. For example, a typical protein being studied for research or for drug development is only available in quantities of less than a gram, and by weight is much more expensive than gold. You can forget about using litres then, and by working with nanolitre samples you can make a tenth of a gram of protein go a long way.
The other reason is that scale really matters for transport of molecules, ions, particles etc. Roughly speaking there are two ways molecules etc move from A to B. The first is diffusion. All molecules in a liquid are moving rapidly in tiny steps, and over time these steps add up and they move around. But due to the way diffusion works means if you double the distance from A to B, you quadruple the time taken.
This means diffusion is very very slow for distances over maybe a millimetre, or in many cases less. The second way molecules move about is due to flow by the liquid. For distances over a millimetre, as diffusion is then so slow, this is how essentially everything moves about. So microfluidics allows you to look at the balance between transport by diffusion and by flow.
This balance is important in many physical systems. The EU project I am on wants to make crystals out of proteins, and this typically done by mixing salt solutions and protein solutions, as mixing in salts often cause proteins to crystallise. This mixing involves transport. It is a bit like adding milk to coffee and stirring. The stirring does a lot of the mixing, it turns the black coffee with a white swirl of milk into a coffee with a uniform colour, but at the smallest lengthscales it is diffusion that is mixing up the milk and coffee.
It was an inspiring visit, the scientists in Bordeaux really know their stuff, I think together we can make progress on the apparently-simple-but-actually-kind-of-complex problem of mixing salt and protein.