How Many Angels…?

Posted On March 16, 2008

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You can almost hear a snowflake drop around here now. The scientific campaign officially ended on Friday, and almost everyone has packed up and gone home.  Only five of us remain, four to make sure their atmospheric research equipment gets on the next plane to Borneo, and one to make sure her frozen frost flower samples get back to England. Obviously I picked the wrong major somewhere along the line.

Our freight is due to ship out on Monday, through Montreal back to Cambridge, and I am here to make sure it does. It includes a Cryoshipper, which is basically a giant thermos flask that is prechilled to -196 ◦C with liquid nitrogen (which is absorbed in a material in the flask so that it cannot leak or spill). The Cryoshipper contains 20 small screw-top containers of frost flowers and some brine, snow and ice core samples. I am hoping it will make it to Cambridge before it thaws, which will be in about a week.

If not, you may expect a series of excellent papers on the lifespan of an avocado in the subarctic.

Just kidding, sort of. I have been maximizing my remaining time making surface area measurements on frost flowers and snow here, so that things do not hinge on my frozen samples making it home.

You may have been wondering how one measures the surface area of a frost flower (or of snow), so here goes… We allow methane to adsorb on the surface of the frost flowers at liquid nitrogen temperature and analyze the resulting pressure difference in a known volume containing the sample using a method known as the Brunauer-Emmett-Teller, or BET, method. In less technical terms, I offer the following analogy of a similar method for calculating the area of a complex shape…

Say you want to know the volume (or area) of a heart-shaped cake pan, to compare it to the volume (or area) of a standard round cake pan, for which you can easily calculate the volume (right?). Then you can figure out whether the same amount of batter will make fatter or thinner layers in two heart-shaped pans than it will in two round pans. The geometry of a heart is not readily available, so you decide to fill the pan with M&Ms (or Smarties if you are in England) to one layer deep. You then calculate the cross-sectional area of a single candy (measure its diameter, or better yet, measure the area of many, then take the average), and multiply by the number of candies required to just cover the surface of the pan. Same thing here, except frost flowers are a lot smaller than cake pans and I already ate all the sweets! Therefore, I use methane gas (for which the molecular cross-sectional area is known) and allow it to adsorb onto the surface of the frost flower sample until the pressure difference in the sample volume tells me that it is completely covered to one monolayer.

Very simple in principle. The trick was building a manifold, for the gas, and a sample container, for the frost flowers, which are the right size relative to one another and hold a vacuum (provided by the turbopump). That I did in Cambridge. I then disassembled the system and reassembled it here in the Research Station. After some tightening of joints, replacement of valves, etc., it worked again. Since then, and especially over the past week, I have been using it to gather data on the specific surface area (surface area per gram) of frost flowers.

Here I am in my makeshift lab (with Andrew Sheppard, of Environment Canada).

rachel_in_lab_2.jpg

And here I am in the field, where I prefer to be! (Nielsen Island, the Northernmost point reached by anyone in our campaign.)

rachel_on_nielsen_island.jpg

And here are the most recent frost flowers studied, from the ice just off the islands that is regularly flooded by the tide.

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One Response to “How Many Angels…?”

  1. Mama Bear

    Will you please stop dancing on the head of icy frost flower pins, and come home?? Plenty of Smarties await you here. And also some English M & Ms.
    No wonder the British cake mix I used in your heart-shaped pan didn’t work too well; the molecular cross-sectional area of methane gas (and M & Ms) had temporarily escaped me. Plus, your one-cup Pyrex measuring cup actually measures 0.3 litres, or 1/2 pint, which is indicated at the 10 ounce mark. I have become fluent in centigrade temperatures, but cook better in my own lab.
    We miss you, Miss Snowshoe Rabbit. Hop home for Easter.

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