General Monthly Meeting
Soft Matter, Rheology and Leonardo
Professor Roger Ian Tanner
P.N. Russell Professor of Mechanical Engineering
University of Sydney
Date: Wednesday, 7th November, 2001
Time: 6:00 for 6:30 pm
Venue: Search & Discover Room, Australian Museum,
Collins St., Sydney (William St. entrance)
Soft matter is a general term describing polymers, foods, colloid systems, surfactants and granular materials. Rheology is the science of deformation and flow of these substances.
In this talk we will see how Leonardo da Vinci's philosophy can be applied to complex engineering problems arising with soft materials and their flows. Two detailed examples of the application of this philosophy will be given, one from the life of AGM Michell, the great Australian inventor of the ship's thrust bearing, and one from the author's own experiences with the plastics industry.
Professor Roger Ian Tanner is the P.N. Russell Professor of Mechanical Engineering at the University of Sydney. He has done work in experimental, theoretical and computational rheology and is the author of the book Engineering Rheology, a co-author of a book on the history of rheology, plus over 200 research papers. His main contributions lie (a) in the development of relations between stress and deformation for nonlinear viscoelastic materials, (b) in the development of computational methods suitable for industrial polymer processing, and (c) in the development of theories and experiments explaining rheological phenomena. He has successfully applied this knowledge to pioneering studies of extrusion and lubrication, and has thereby contributed to the development of polymer processing technology.
Professor Tanner was awarded the Edgeworth David Medal of the Royal Society of New South Wales in 1967 and amongst numerous awards has been elected as a Fellow of the Australian Academy of Science and as a Fellow of the Australian Academy of Technological Sciences & Engineering. In 1993 he was awarded the inaugural Australian Society of Rheology Medallion.
Report on the General Monthly Meeting
by Edmund Potter
Question: What have rubber, liquid crystals, powders, emulsions, toiletries, foods, foams and most body parts got in common? If you don't know the answer (which is they are all examples of soft materials), then you should have been present at the 1101st meeting of our Society at the Australian Museum on 7th November last when Prof. R.I. Tanner, FRS, spoke to the title heading this report.
Roger Ian Tanner, who is P.N. Russell Professor of Mechanical Engineering at the University of Sydney, covered a range of property changes that soft materials undergo when subjected to quite minor forces and compositional changes. As an example of the former he cited the familiar rubber band, easily stretched to grip loose objects and readily relaxing for re-use. As for the latter, he recalled the Goodyear story (1839) in which rubber latex oozing from some types of tree was found after coagulation to firm up to tough flexibility by chemical action with elemental sulfur. Later on, chemists learned that the latex empirical formula, C5H8, hides the fact that the latex is a tangle of polymeric fragments and that sulfur cross-links the chains producing a sturdy but yielding "solid" suitable for motor tyres. The study of the deformation and flow of matter was termed rheology by Eugene C. Bingham in 1929, and the subject now encompasses knowledge from the wholly practical to the highly mathematical. It was Clerk Maxwell (1831-1879) who reconciled the earlier approaches of Isaac Newton (friction, lubrication) and Robert Hooke (metal springs) by advancing that everything has springiness and a resistance to it. All substances have a relaxation time, when half of an applied force that induces flow has dissipated. Pure liquid water has a relaxation time of around 10-12 seconds, but for window glass at room temperature it is over 108 seconds.
For rheological demonstrations Prof. Tanner recommended bread dough, particularly that containing wheat starch. A length of dough subjected to a constant strain rate undergoes thinning at first, but the molecular structure then aligns and shows behaviour dependent on gene structure of the wheat, enabling a useful prediction of how the dough will bake.
Prof. Tanner related applied rheological phenomena to the artistic and inventive genius of Leonardo who, early in the sixteenth century, had summarized his philosophy in the words "Theory is the Captain, Practice is the Soldier". The speaker drew a parallel to the invention of the ship's thrust bearing (Michel 1905) where the principle of closely fitting metal moving parts (the theory) is brought to practical fruition by the lubrication provided by an oil film.
In a second example of this blending of theory and practice, Prof. Tanner dealt with the large-scale production of transparent polymer film. The hot polymer is pumped continuously into free space where it is stretched like a balloon in two directions at once, producing a film of great toughness. Two intermeshing wheels of the pump have up to 200 atmospheres across them, but on changing to a different polymer it was found that they were prone to seize. Study of the temperature/force relationship for the two polymers showed what adjustment of temperature would bring theory and practice into alignment, thus solving a costly problem.
After a number of questions to the speaker, President David Craddock asked your reporter, Edmund Potter, to propose the vote of thanks, which was carried with acclamation.