If you’ve ever used a spell checker to review an article which includes the word ‘rheology’, then you might have come across a ‘theology’ connection, as that’s what you’re usually prompted to change the word to! But rheology really does have some theological connections, aside from being considered a misprint, and one in particular highlights the importance of understanding the timescale of deformation and the effect that has on a material’s rheological properties.

One of the founding fathers* of rheology, Markus Reiner proposed the Deborah Number – named after the prophetess Deborah. In the Old Testament Book of Judges, the line ‘The mountains flowed before the Lord’ is in the song of Deborah, which Reiner attributes to Deborah’s understanding of material deformation properties. Firstly everything flows, even mountains, but secondly, there is a timescale of observation associated with this. For man – in his relatively short lifetime – the mountains are solid, but for the Lord with an infinite observation time, the mountains flow.

The dimensionless Deborah number, D, is the ratio of time of material relaxation to the time of observation, and the magnitude of D reveals either solid-like (high D) or liquid-like (low D) behavior. Depending on the timescale of deformation, a material can therefore show viscoelastic properties i.e. exhibit both solid-like and liquid-like attributes.

A discussion of Deborah Number is given in this transcript of an after dinner speech by Markus Reiner at the Fourth International Congress on Rheology, Providence, RI in 1963 – along with some other entertaining anecdotes from him about the connections between rheology and theology.

Of course, there is one other connection you might be familiar with………how many times have you looked at some rheological data………and hoped for some divine intervention to help you understand it?!?

If you are trying to get to grips with the background to rheology and what the data means, there are some multi-language presentations on the basics of rheological measurements available here – please do let us know if you have any questions though.  We can’t promise deity-like wisdom, but we are here to help you with material flow or stability problems!

* On 29 April 1929 to be exact – see here for ‘a short historical excursion’ on the genesis of rheology.