Sublime


Created with Admarket’s flickrSLiDR.

My flatmate needed some dry ice for a vortex cannon demonstration which, even though we’re mathematicians, turns out to be surprisingly easy to get hold of through the university. In fact, she ended up with so much that I was able to use some of the excess for experiments of a photographic, rather than scientific, nature.

I quickly discovered that for seeing what’s going on, it’s better to add just a couple of pellets to plenty of water. That way you get to see individual bubbles of smoke before they burst – if you’re lucky, producing a smoke ring in the process. Carbon Dioxide is heavier than air, so it’s not a natural choice for smoke photography. But I did stumble on an approach that yielded an interesting set of abstract images. Placing a pellet in sufficiently shallow water prevents the gas forming large bubbles on the surface- instead, you get tiny streams of gas within the water, with the combined effect reminding me of magnetic field lines or iris patterns.

So what is actually going on here? I asked Dr Helen Maynard-Casely, the domestic scientist, for some insight:

These fantastic pictures are coming from the dry ice (solid carbon-dioxide) sublimating straight from its solid form to a vapour. This process of ‘cutting out the middle man’ (the liquid phase) happens when the atmospheric pressure surrounding it is insufficient to keep the solid together. When you expose molecules in dry ice to temperatures well above their melting point they vibrate: in the case of dry ice this is so vigorous that it is not contained by the air pushing in back. So instead of losing its crystal (solid) form and becoming a liquid the molecules fly off as a gas. The bigger the temperature difference the more vigorous the smoke will be (try putting dry ice in your tea….). Dry ice is only stable at -78°C so the large difference means you get more molecules flying off and a thicker ‘smoke effect’. Water ice often sublimates too, when sun shines directly on snow for instance. Also, this is how people who live in cold environments get their washing dry!

Sublimation may have also killed Napoleon Bonaparte. Arsenic also sublimates at the correct temperatures, and was used frequently as dye for wallpaper by British manufacturers in the 18th century. That was fine in Britain, where it was usually quite cold, but in a British-decorated house in St.Helena……

As well as photos, I captured a fair amount of slow-motion video too, which you can find in this playlist (the first few seconds of the first clip have unfortunately been mangled by youtube). For both stills and video, I was using a Casio Exilim FH20 camera, which is a bridge camera designed for high speed work. I’ve been playing around with this for several months, but was particularly pleased with this project: at high frame rates it suffers badly in low light conditions, rendering it all but useless indoors, so this time I borrowed a 500W floodlight which did wonders. So, not an expensive or sophisticated setup, nor all that complicated: camera stabilised on a tripod, either in super macro mode or manually focused, but otherwise making its own decisions about settings. Then it was just a case of sieving through 1300 photos and several GB of video for the best bits! Choosing between shots 1/30th of a second apart is near-impossible, but a set I’m happy with is up on flickr (and cycling through on the slideshow above).

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