What is Rheology?

Perhaps you have not heard of the term or if you have you may be a little vague! Anything related to the flow of materials might be one explanation; whilst fluids often predominate in the activities of rheologists, it must not be forgotten that solids are also included, as are gaseous materials. The origin of the term is generally attributed to E.B.Bingham in 1929. Roger Tanner in his "Engineering Rheology" gives Bingham's classic definition as follows:

"The study of the deformation and flow of materials".

However matters go back a lot further. Newton gave his name to a class of fluids that exhibit a particular mode of flow behaviour (Linear or Newtonian Flow) and it was Heroclitus in Ancient Greece who stated: "Everything Flows". Neil Cogswell in his book "Polymer Melt Rheology" reminded us of the Biblical Song of Deborah and Barak: "The mountains melted from before the Lord" (Judges 5:5), from which we obtained the Deborah Number as the characteristic time of a process. Indeed, as we now know there is little meaning to the term "rock solid" if you choose a sufficiently long time scale. Thus the Himalayas rose up (and are still rising!) from the Indian Plains as a result of the sub-continent of India colliding with Asia in a manner analogous to the piling up of butter (the Himalayas) in front of the spreading knife (the sub-continent of India); only the viscosities and time-scales differ. Volcanoes and glaciers exhibit spectacular rheological phenomena separated again only by time and viscosity. In Europe, many medieval cathedrals have their tall windows significantly thicker at the bottom than at the top - the glass, although ostensibly a solid but actually a supercooled fluid, having flowed over the intervening centuries in response to the force of gravity. Toothpaste, lipstick, tomato ketchup, salad dressings and non-drip paint all behave as they do because of some intriguing rheological properties that are deliberately exploited. Many years ago, the journal ‘Scientific American’ featured a fascinating article on the rheology of quicksand! This being a shear-thinning material, the viscosity rapidly drops if you raise the shear-rate by thrashing about in it (consequently letting you sink quickly) whereas slow and deliberate movements reduce the rate of shear, maintain the viscosity and give you longer to call for help (whilst letting you sink more slowly!). Thanks to an understanding of the underlying rheological behaviours we can now conveniently transport coal and other minerals as slurries with water, far more efficiently than was previously the case. Flow-improvers are big business in the crude-oil industry where the pumping of large volumes via pipelines over vast distances is only possible if the viscosity can first be lowered by the appropriate use of additives.

An essential component of Rheology is the measurement of flow. The Clepsydra or Water Clock of Ancient Egypt was a moderately accurate rheometer, utilising the flow properties of water through a fixed orifice to characterise the passage of time; an early employment of capillary flow. The Sand or Hour-glass used a similar arrangement. Closer to modern times, one of the first rheometers to be designated as such was devised by Leipzig bakers in an effort to characterise; and hence be able to reproduce; a good bread dough (a modernised version is still commercially available from more than one rheometer supplier). The common meat mincer uses the "Archimedean Screw" (still used for moving water in the East) that in this century was given some heating, a thicker barrel and a substantial drive motor to become the ubiquitous screw-extruder of the plastics industry, not to mention ceramics, pharmaceuticals and foodstuffs. Flow is crucial to most industries, as the predominance of chemical engineers within them will testify. Even the medical profession now recognise the value of rheological techniques in the diagnosis of blood disorders and for characterising other biological fluids. Modern studies into Chaos Theory are giving an insight into the rational flow within hitherto unrecognised systems of order, such as weather, traffic congestion and population behaviour.

Whilst the reading of a "good" book on the subject may prove to be a somewhat intimidating experience for the novice, there are many practising rheologists who have achieved great success without necessarily needing all the rigour of the various mathematical treatments available in the literature.

Rheology is probably one of the most diverse of subjects which embraces many of the traditional (and regarded as disparate until relatively recently) disciplines within Physics, Chemistry, Engineering and the Life Sciences; an attribute which is at once both challenging and fascinating! The breadth of this diversity brings together many scientists who would not necessarily interact otherwise. It makes the defining of a Rheologist very easy; refer to Bingham’s statement above! Conversely, it can also make it exceedingly difficult to say that someone is not! Although electricity is not now generally regarded as a rheological ‘fluid’, it is interesting to reflect on the fact that we may still find ‘rheostats’ in use for adjusting current flow!