Current Research Interests:
The rheology of concentrated suspensions; effective properties of two-phase materials; electrorheology.

Our research program seeks to study in depth a number of fundamental problems in fluid mechanics and two-phase materials, using both experimental and theoretical(analytical and numerical) methods.

For example, it has been known for some time that the rate at which a suspension settles can be drastically increased by tilting the walls of the containing vessel. This surprising phenomenon, which has very important implications in a variety of separation processes, has not been well understood until recently when our group developed the first quantitative theory on the subject. We are currently refining the theory and testing its predictions experimentally.

The transport of momentum, heat, and mass in two-phase materials plays an important role in a large variety of physical processes; the determination of effective transport parameters is a matter of considerable interest. Although in principle these parameters can be computed "exactly", the calculation often present insurmountable difficulties. We have recently developed and are currently refining and applying an effective continuum theory which, although approximate, accomplishes this task much more quickly and with acceptable accuracy. Our fundamental studies of concentrated suspensions have explained a host of seemingly unrelated phenomena such as: a) the observed resuspensions of a settled bed of particles in a viscous fluid upon being sheared; b) the existence of a shear-induced anisotropy in a concentrated suspension which manifests itself in measurable normal stresses; and c) the slow decay with time of the effective viscosity of a concentrated suspension, as measured in a Couette viscometer, together with a shear thinning behavior in such systems. Evidence now exists that many of these phenomena derive from a shear-induced diffusion mechanism that produces a flux of particles from regions of high particle concentration to low, or from regions of high shear to low.

The current research aims to: a) study the resuspension mechanism in detail experimentally and to develop a reliable explanatory theory; b) measure the shear-induced diffusion coefficient by a novel technique over a wide range of particle sizes, particle concentrations, and degrees of polydispersity, and to construct a theory for determining this coefficient; c) examine in depth the shear-induced anisotropy, as inferred from our normal stress measurements , and determine to what extent it can lead to a drift of particles in concentrated suspensions; and d) identify the mechanism responsible for the, as yet, unexplained experimentally observed shear thinning behavior in concentrated suspensions.

Our work is currently supported by the National Science Foundation, the National Aeronautical and Space Administration (NASA) and the Department of Energy.

Marco Marchioro, M.S. in Engineering, 1996, The Johns Hopkins University, Mechanical Engineering Department; Ph.D. in Mechanical Engineering, 1999, The Johns Hopkins University

Anubhav Tripathi, B. Tech., 1991, Indian Institute of Technology;M. Tech., 1993,Indian Institute of Technology, Ph, D., 1998, Chemical Engineering, the City College of the CUNY

Selected Recent Publications:
Acrivos A., "Bingham Award Lecture-1994. Shear-induced particle diffusion in concentrated suspensions on noncolloidal particles," J. Rheol., 39, 813 (1995).

Schaflinger, U., A. Acrivos and H. Stibi, "An experimental study of viscous resuspension in a pressure-driven plane channel flow," Int. J. Multiphase Flow, 21, 693 (1995).

Pelekasis, N. A. and A. Acrivos, "Forced convection and sedimentation past a flat plate," J. Fluid Mech., 294, 301(1995).

Khusid, B and A. Acrivos, "Effects of conductivity in electric field-induced aggregation in electro-rheological fluids," Physical Review E, 52, 1669 (1995).

Kapoor, B and A. Acrivos, "Flow of sediment layer on an inclined plate," J. Fluid Mech., 290, 39 (1995).

Jana, S.C., Kapoor, B. and Acrivos, A. "Apparent Wall Slip Velocity Coefficients in Concentrated Suspensions of Noncolloidal Particles" J. Rheol. 39(6), 1123 (1995).

Tripathi, A. and Acrivos, A. "A New Criterion for the Continuous Operation of Supersettlers in the Bottom Feeding Mode" Int. J. Multiphase Flow 22, 353 (1996).

Khusid, B. and Acrivos, A. "Effects of Interparticle Electric Interactions on Dielectrophoresis in Colloidal Suspensions" Phys. Rev. E 54, 5428 (1996).

Wang, Y., Mauri, R. and Acrivos, A. "The Transverse Shear-Induced Liquid and Particle Tracer Diffusivities of a Dilute Suspension of Spheres Undergoing a Simple Shear Flow" J. Fluid Mech. 327, 255 (1996).

Breedveld, V., Vanden Ende, D., Tripathi, A. and Acrivos, A. "The Measurement of Shear-Induced Particle and Fluid Tracer Diffusivities in Concentrated Suspensions by a Novel Method" J. Fluid Mech. 375, 297 (1998).

Tripathi, A. and Acrivos, A. "Viscous Resuspension in a Bidensity Suspension" Int. J. Multiphase Flow 25, 1 (1999).

Tirumkudulu, M., Tripathi, A. and Acrivos, A. "Particle Segregation in Mondisperse Sheared Suspensions" Phys. Fluids 11, 507 (1999). Erratum, ibid. 11, 1962 (1999).

Khusid, Boris and Acrivos, A. "Phase Diagrams of Electric-Field-Induced Aggregation in Conducting Colloidal Suspensions" Physical Rev. E 60, 3015-3035 (1999).