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It has been taken for granted for the better part of the last century that the flow behavior of a suspension of non-colloidal particles in a Newtonian liquid is identical to that of the pure solvent when account is taken of the difference in their respective viscosities. It has been shown recently, however, that this is not so because the presence of the particles introduces a new stress into the system which induces normal stresses as well as particle migration from regions of high shear to low. In addition, the random motion of the particles owing to their hydrodynamic interactions with their neighbors renders their trajectories chaotic, the result being that such suspensions lose the well known reversibility property associated with flows at vanishing Reynolds numbers. A number of examples will be presented which illustrate these new rheological features of suspension flows that are absent in particle-free fluids.

Born in Athens, Greece, Andreas Acrivos received his BS from Syracuse University and his Ph.D. from the University of Minnesota in Chemical Engineering. He joined the faculty of the University of California Berkeley, and then moved to Stanford University. He was appointed New York State's Albert Einstein Professor of Science and Engineering at the City College of CUNY in 1988 and became emeritus in 2001. Professor Acrivos is known internationally for his work in fluid mechanics. His research interests focus on the rheology of concentrated suspensions, effective properties of two-phase materials and electrorheology. He is a member of the National Academy of Engineering, the American Academy of Arts and Sciences, and recipient of the 2001 National Medal of Science. He has authored or co-authored about 200 publications. He served as Editor of the Physics of Fluids from 1982 through 1997.