Happy Thanksgiving from Malvern

by Alan Rawle, Wednesday, 25th November 2015

Ever thought about measuring the size of a potato?  We’ll probably be consuming vast quantities of these in various forms (fries, roasted, mashed, even potato salad which signals summer in the US). Well, the humble potato is a good example of the philosophy of the most basic particle size question “What size is my particle?”. Do we buy our potatoes on the basis of number (I can understand this for large baking potatoes or the tiny new potatoes we used to see every year in my youth in England – now of course available year round…) or do we buy a certain weight (in a bag) – 2 pounds, 5 pounds, or even in kilos (how can the rest of the world other than the US be so wrong?!)?

In Finland, I remember potatoes being sold on the basis of the amount the customer could pack into a cubic wooden box (a volume descriptor) in the market – an interesting packing issue that we started to explore in a recent webinar “Perfect packing“.

Returning to my youth when scientific inventions were better received by a less skeptical public, then Cadbury’s aired a bunch of amusing adverts for their Smash Instant Mashed Potatoes where alien lifeforms rolled about the floor at the concept of peeling potatoes, boiling them for 20 minutes, and then smashing them into little pieces…

So we all have a picture in our mind of the humble potato. As sort of asteroid shaped I guess…. So we start to realize that size and shape are important in particle sizing…

In the Basic Principles of Particle Size Analysis, the most popular download over the years at Malvern, there’s a sort of model potato shape (bottom right – “well rounded” “high sphericity” – hey that’s a description of me…). To digress these shapes were stolen and adapted from a 1953 paper written by Powers (M C Powers “A new roundness scale for sedimentary particles” and didn’t show real pictures. Rather they were clay models created by “Mrs Josephine B. Thoms”. And that sends you on a Google chase (“surfing” is the term that I believe the young people of the world use) to find out a little about her – you may be able to discover that she was a “part-time instructor in art”….

OK, back to the potato (as a representation of the ubiquitous particle problem), how might we size this? We could screen the potatoes – indeed this is done to grade them and to remove extraneous material such as stones (so instantly we may see advantages in screening or sieving as a particle size technique :

The scattering angle of a potato – say 4 inches (1 cm) in diameter? …. Well from a chapter written by J R Hodkinson “The optical measurement of aerosols” (who tragically died in a sailing accident in Sweden) in Davies’ Aerosol Science book (Academic Press, London & New York, page 343, 1966) we learn that the scattering angle is roughly 35/d where d is the size in microns… “Consider the potato to be a sphere” ….. So, 35/10000 or 0.000350…… We’d need the detector a long way away to capture that scattering. Stick to the sieve I say…

Now the classic treatise on the potato is called “The Potato” (Doubleday Page & Co. ‘The Farm Library’, 1912) as you would expect. As you would also expect I have a copy of this tome (indeed 2 copies of different editions). The lead author is called Grubb (Eugene H.) who is also the inventor of the corrugating field roller (whatever that is)…. Another one to Google…

So when you think of Thanksgiving, then give thanks for the humble potato and a thought for the sizing (and shape) of irregular shaped particles…but also give a thought to those less fortunate than ourselves at this time of year, as Kristen Wilkinson wrote so well about in her Thanksgiving’s blog last year.

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Happy Thanksgiving to you all!