Over the last six months I have been thinking a lot about the movement of small solid particles in liquids, but a couple of weeks ago I came across examples, that were new to me, of the reverse. The motion of liquid droplets or gas bubbles in solids. I think they are fascinating. One example of liquid droplets moving inside a solid, are pockets of brine (i.e., salty water) moving inside ice.
It is now winter in the northern hemisphere, so it is now very cold in the arctic. This –causes some of water near the surface of the arctic ocean to freeze — the temperature in the air above the ocean can be −20 C or lower. This water is salty of course, this an ocean after all. Now, salt is almost completely insoluble ice so as salty water freezes it rejects the salt and forms crystals of almost pure water. As an ice crystal grows, brine can accumulate in front of it, and sometimes ice crystals even grow round a pocket of brine, completely enclosing it.
Ice is of course a solid. You might (and I did until two weeks ago) think that the brine pocket is trapped and immobile, inside this solid. Not so.
The ice on the surface of the ocean is between the arctic air, maybe − 20 C or lower at this time of year, and the arctic ocean underneath, about − 2 C. So there is a temperature gradient across the layer of ice floating on top of parts of the arctic ocean. This means that the top of a brine pocket is a little colder than at it is at bottom. This causes the warmer bottom of the pocket of brine to slowly melt, while the cold top of the pocket slowly freezes. This makes the ice recede slowly at the bottom of the pocket, while it grows at the top, which has the net effect of moving the pocket down. We are talking maybe a millimetre a month, so very slow, but these pockets in the ice are, even as you read this, inching slowly downwards towards the (relatively) warm ocean beneath.
I think this is quite cool. And there are other situations where bubbles in solids can move. For example, the fuel rods in nuclear reactors are solids but may have small bubbles trapped in them. The centres of fuel rods in an active reactor are extremely hot, while at their edges they are just very hot. So, a similar effect occurs here, with the bubbles migrating to the centre of the rods.
I read about these brine pockets in Peter Wadhams’ A Farewell to Ice, an interesting and scary book on the arctic ocean and the ice there that forms and melts. Due to climate change, it is mostly melting, as you can see above. The extent of the sea ice varies during the year, and it is typically lowest in September. In 1984 there was a lot more ice than in 2012, only 28 years later. Sobering. Wadhams’ book is worth a read, but it won’t make you sleep any easier.