The beautiful behaviour of this flock of starlings is an example of a class of phenomena variously known as emergent, collective or more-is-different behaviour. The point is that a single starling, or two starlings cannot show this striking phenomenon, you need hundreds or thousands of starlings, to see it. A liquid is a less obviously exciting example of an emergent phenomenon. One or two water molecules aren’t a liquid, you need at least about a hundred to make even a tiny water droplet.
Chemical engineers, chemists, physicists and food scientists have been studying coexisting liquids like the (black) balsamic vinegar and (yellow) olive oil above, for over a hundred years. Cell biologists have been busy with other things over this time. But over the last few years at least some cell biologists seem to not only be studying coexisting liquids but arguing over them.
The people at Brilliant have done a lovely short (few minutes, last minute is an ad for Brilliant) video explainer on the physics of how masks work. It does a good job of saying why droplets a fraction of a micrometre across are the tough ones to catch, and why you can’t think of a face mask as a simple sieve. It compares a mask to a spider’s web, a comparison I like very much. But one thing it skips over is the physics of why when a droplet hits one of the fibres inside a face mask, we expect the droplet to stick.
The picture above shows three trajectories — red, green and orange curves — of particles through a model of a face mask. Face masks are meshes of long thin fibres and the brown discs are cross-sections through these fibres — in a simple model. The blue lines are what are called streamlines, they show the the paths taken by air flowing through the mask, due to the wearer breathing. The trajectories show (at least part of) why masks filter out the bigger droplets from a person’s breath, and it is not because the droplets are too big to fit through holes in the mask.
Way back in the innocent days of 2018, I had a very mild tussle with that part of the BBC that runs BBC Bitesize, which is the part of the BBC’s website dedicated to providing educational resources for school children (original blog post here). The BBC’s webpage originally claimed that the smell of coffee could travel across a (then open) coffee shop, via diffusion. This is not correct, it is carried by air currents, and the BBC did update the webpage.
When you have as few genuinely original ideas as I do, one way to make progress is to borrow (with appropriate attribution) other people’s ideas. I have been wondering about how a virus such as the corona virus (shown on the left as the knobbly object*) gets through the mucus (pale blue) that lines the inside of nose and throat, to attack the cells (pink) underneath this mucus. Viruses need to get inside our cells to take them over and allow the virus to reproduce.
One of the functions of mucus may be to trap or somehow to provide a barrier to the viruses. After some Googling it occurs to me that this problem of blocking movement of a virus is more-or-less the same as the problem water companies have, although the scales are very different.
One of the giants of twentieth century physics, Philip Anderson, died at the weekend, aged 96. He retired from Bell labs in the 1980s and from Princeton University maybe 20 years later, but a couple of years ago we were briefly at the same workshop at Princeton. Bell labs was where the solid state transistor — at the heart of all modern computers — was invented.
A lot of my research looks at how to move particles around, particles about 100 nanometres across. For example when we made stratified coatings we did this with a mixture of particles of sizes 60 and 400 nanometres. The corona virus (see above) is about 100 nanometres across. Viruses are barely alive. They have no metabolism themselves, no more than the inert colloidal particles my colleagues and I study. The virus above has the genetic material in the centre, surrounded by a protein coat. The red things are its spike proteins, that help it enter and infect a cell. The colours above are false, I think it is reconstructed from electron (not light) microscopy.
Convection is a fact of life, it is occurring right now in the air around your body. Your body temperature of 37 C is higher than the room temperature, so your body heat is warming air, and this warm air is rising — which is convection. Warm air is lighter than colder air and so due to gravity the lighter warmer air rises, and the heavier colder air falls. So convection occurs in the air in the rooms of your house and place of work. Convection is also key to how both the Earth’s atmosphere and oceans behave. Hot air is constantly rising in the atmosphere, and dense water is constantly falling in the oceans (and seas, lakes, …).
With excellent timing, just as I prepared to tell students in my biological physics lectures that we are roughly 20% protein (by mass), and that gelatin is a very abundant protein, which holds our tissues together, The Guardian ran an article on how cool gelatin desserts, aka jellies are. The article included the, slightly odd in my opinion, YouTube video above.