The role of normal stresses in causing particle migration and macroscopic spatial variation of the particle volume fraction in a mixture of rigid neutrally buoyant spherical particles suspended in Newtonian fluid is examined for curvilinear shear flows. The problem is studied for monodisperse noncolloidal Stokes-flow suspensions, i.e., for conditions of low-Reynolds-number flow and infinite Péclet number, Pe ϭ O(␥ a 3 /kT), where is the suspending fluid viscosity, ␥ is the shear rate, a is the particle radius, and kT is the thermal energy. Wide-gap Couette, parallel-plate torsional, and cone-and-plate torsional flows are studied. The entire dependence of the compressive shear-induced normal stresses is captured by a ''normal stress viscosity'' n (), which vanishes ͑as 2) at ϭ 0 and diverges at maximum packing in the same fashion as does the shear viscosity s (). Anisotropy of the normal stresses arising from the presence of the particles is modeled as independent of , so that ratios of any two particle contributions to the bulk normal stress components are constants, ⌺ 22 p /⌺ 11 p ϭ 2 and ⌺ 33 p /⌺ 11 p ϭ 3 ; the standard convention of ͑1,2,3͒ denoting the ͑flow, gradient, vorticity͒ directions is used so that, for example, ⌺ 11 p is the normal component of the particle stress ⌺ p in the flow direction. Predictions for the steady and unsteady flows are presented to demonstrate the influence of variation of the normal stress anisotropy parameters 2 and 3 , the rheological functions s and n , and the sedimentation hindrance function used to represent the resistance to relative motions of the phases during migration. Comparison with available experimental data shows that a single set of parameters for the rheological model is able to describe all qualitative features of the observed migrations in the flows considered.

The atomic force microscope (AFM) was used to directly measure the forces of interaction between E. coli D21 bacteria and hydrophilic glass or hydrophobic N-octadecyltrichlorosilane (OTS)-treated glass substrates coated with the block copolymers, poly(ethylene glycol) (PEG)-lysine dendron or Pluronic F127 surfactant, respectively. Short-range repulsive interactions between bacterial cells and substrates coated with the block copolymers were detected by the AFM over distances of separation comparable to the extended length of the PEG polymer chains. In contrast, glass and OTS-treated glass devoid of PEG-lysine dendron or Pluronic F127 exerted long-range attractive forces on E. coli D21 bacteria. Thus, polymeric brush layers appear to not only block the long-range attractive forces of interaction between bacteria and substrates but also introduce repulsive steric effects.