Central Michigan University

Rheology

What is rheology anyway?

From the Greek rheos, rheology is the study of the flow and deformation of a fluid under an applied force or stress.  Air, water, paint, honey, mayonnaise, tooth paste, Silly Putty, molten plastic, and asphalt are all fluids, that is, they all can be made to flow, but they exhibit a huge range of properties.  When you re-open a jar of mayonnaise, you can see the knife marks left in the top of the mayo by the last person who put some on their sandwich, yet it spreads very easily on the bread.  Honey spreads much less easily, but the surface of the honey in the jar does not retain the knife marks of the last user.  Silly Putty flows into a ‘puddle’ if you let it sit on the table, but, if you form it into a ball and through it against the wall, it bounces.  Rheologists study all these properties and more!

What do we measure?  For starters, we measure viscosity, a fluid’s resistance to deformation and flow in response to an applied force or stress.  For a particular fluid, its viscosity can vary with temperature and stress level, among other variables.  But fluids have other physical properties.  For example, fluids can also have elastic properties–they can store energy as well as dissipate it. 

Knowledge of the flow properties of materials is important for many industrial and engineering applications.  You want tooth paste to flow easily from its container, but you want it to hold its shape on the tooth brush.  The mayonnaise and catsup “plumbing” at McDonald’s needs to deliver controlled amounts of these condiments to your sandwich.  When you paint a wall, you want the paint to flow easily onto the wall, but resist sagging during the drying process.  When you injection mold a thermoplastic like polycarbonate to make automobile door panels, you want the plastic to flow and fill the entire mold quickly at the lowest possible temperature and leave no voids.

On the other hand, the physical properties–sometimes exotic–of a material must fundamentally depend on its structure at the atomic and molecular scale.  This makes it possible to learn something about the structure of a material by examining its response to stresses.  It is possible to Further, this structure-property relation can be exploited using computer simulation techniques to predict the flow properties of new materials before they are actually made, thus shortening substantially the development time needed to bring modern materials to market.

History

The Rheology Laboratory in the Department of Physics was started in 1989 by Dr. Leela Rakesh (Mathematics) and Dr. Stan Hirschi (Physics)  with the help of a Research Excellence Fund (REF) award.  Dr. Rakesh had formal training in rheology, but the Math Department had no lab space and no equipment budget.  Having been trained in medium energy nuclear physics, I was anxious to find a viable new research area for CMU in which students could readily become involved and where collaborations both with other departments and with local industries were possible.  Our lab was installed in an underutilized corner of our electronics lab in Brooks Hall.  Our first rheometer–the instrument used to measure the deformation and flow properties of fluids–a CSL500 from Carri-Med, is still operational, although it is used today only for back-up.

Present equipment

The workhorse rheometer in today’s lab, located in the basement of Dow Science in space shared with the department’s Beowulf super-computering cluster, is an AR2000 from TA Instruments, a modern and more sensitive version of the CSL500.  It can handle fluids over a wide range of viscosities, stresses and temperatures (0°C - 400°C.).  Some materials change their flow properties in the presence of electric or magnetic fields.  We have designed and built supplementary units for the AR2000 that let us investigate either electro-rheological or magneto-rheological materials.  (Chocolate, one of our ‘basic’ foods, has electric field-dependent flow properties!)  Our newest rheometer, a Vilastic-3, allows us to accurately analyze complex biofluids of low viscosity.

Simulation efforts

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People and Support

Our rheology group has included faculty from several departments, undergraduates, M.S. students, and post-doctoral associates.  Over time we have had joint research projects with and/or funding from almost a dozen partners, including research (Midland Molecular Institute and Dendritic NanoTechnologies), industrial (Dow Chemical and Dow Corning) and military (Army Research Laboratory.)

Current Research

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