Monday, July 16, 2012

Rheology, Theology and Deborah

If you ever studied even a little rheology, you've probably been exposed to the Deborah number, a dimensionless number that is the ratio of relaxation time of a polymer (or other non-Newtonian material) and the observation time. Or if you haven't been taught the actual number, you've no doubt been taught about the ideas it represents. If the relaxation time of a material is very long and your observation time is short, then the material will appear to be solid. But the timescales are inverted and the relaxation time is short while the observation time is long, then the material will appear to be liquid. That means that a material can appear to be either a liquid or a solid - all depending on the relaxation time and your observation time.

This idea is intimately tied to the name Deborah, far more than any other name is associated with any other dimensionless number such as the Reynolds number, the Prandtl number, or the 2nd Domköhler number, all of which are named after researchers. Instead with the Deborah number, it is named after a person that is long deceased for over 3000 years and obviously not a rheologist. The Book of Judges in the Bible has the victory song that the prophetess Deborah wrote after she gave the Canaans a good a**whooping on the battlefield. One line of that song is "The mountains flow before the Lord", which a prominent rheologist (Marcus Reiner) thought captured this concept perfectly. On the observation scale of God, mountains could appear to flow like a liquid, while for people and their short observation times, the mountains appear very solid. And so the name of this dimensionless number is "Deborah".

Reiner described this line of thought in an article (open access) published in 1964, in which he also further describes the relationship between "rheology" and "theology". Since that time, the relationship has been further blurred by spellcheckers. The first time someone tries spelling "rheology", the spellchecker doesn't recognize the word and instead suggests "theology" as a correct word. (Not that bad a choice, as the "r" and the "t" are adjacent on the QWERTY keyboard.)

Steve Carrington of Malvern Instruments however, has proposed yet the latest connection between the two subject this last week:
"How many times have you looked at some rheological data....and hoped for some divine intervention to help you understand it?!?"

More times than I ever imagined. More times...


Salomon T. said...

It's the Theo Thing!

John said...

Groan! (and praying that this is the last pun you send my way).

Chemjobber said...

My (and probably many others', too) favorite detail about this passage in Judges is the demise of the enemy commander via tent peg. Ouch.