In an effort to better characterize nonnewtonian fluids, Dr. Aditya Khair is pushing the limits of rheometry at Carnegie Mellon. He came all the way up the mountain last Wednesday to discuss his ideas before an audience of professors and students convened at Iaccoca. Professor Khair’s visit was sponsored by the Department of Chemical and Biomolecular Engineering as part of their Fall Seminar Series.
Rheology, the science of the deformation and flow of matter, has enabled the prediction of a fluid’s behavior via measurement of its intrinsic properties. Of the various methods of measurement implemented in pursuit of this goal, oscillatory rheometry has been of particular interest in respect to Dr. Khair’s recent ideas. In this method, a fluid is pressed between two horizontal plates. The top plate is held stationary as the bottom plate is rotated sinusoidally, imposing a time dependent strain on the fluid, by which the fluid then enacts a quantifiable torque upon the top plate. A comparison of the frequency of oscillation and the measured stress response can then be used to infer properties of the fluid in question. This allows for calculations of viscosity and elasticity to be extrapolated and applied to a wide range of conditions – at least, that’s what we tend to assume.
As Dr. Khair demonstrated to us, in the case of conditions under which either strain or oscillation rates are very high, the stress is no longer directly proportional to the strain, creating a gap between our assumed extrapolations and the reality of a nonnewtonian fluid’s behavior.
In order to better understand this reality, Dr. Khair applied mathematical models to two types of fluids: those characterized at the molecular level as rigid rods and those seen as flexible coils, enabling the interpretation of rheometric data from the region beyond ideal conditions. In this way, the use of the rheometer itself as a platform for fluid mechanics experimentation has enabled Khair to better define and more accurately explain the behavior of these fluids under nonlinear conditions.
Extensive discussion followed Dr. Khair’s presentation. From the questions raised by Lehigh’s ChemE faculty, a sentiment of cautious skepticism was made clear – after all, as Dr. Anthony McHugh remarked, “I’ve always used a rheometer as a rheometer,” reflecting some doubt in being able to learn something new about a fluid from applications of a rheometer outside its typical range of use. He also mentioned, however, that there’s always been a “problem of extrapolating [rheometric] data to nonlinear conditions,” and that deeper understanding of a fluid’s behavior at high strain levels/oscillation rates might lead to industrial applications concerning fluids under highly turbulent conditions.
Despite his unconventional approach, the potential of Dr. Khair’s ideas to yield new information concerning the properties of nonnewtonian fluids is certainly something to be optimistic about.