Lift on a sphere near a plane wall in a second-order fluid

In this paper, we examine the lift on a sphere moving very close to an infi nite plane wall in a shear flow of a second-order fluid. The sphere is allo wed to both translate and rotate along the plane. We focus on the limit whe n the sphere touches the wall. We found that due to the normal stress effec t, the flow gives rise to a positive elastic lift force on the sphere when the gap between the sphere and the wall is small. For a moving particle dri ven by shear flow, the ratio of the elastic lift to the buoyant weight of t he particle is inversely proportional to the particle radius, such that sma ller particles will be easier to suspend, in contrast to the result that th e ratio of the inertial lift to the buoyant weight of the particle is propo rtional to the particle radius, such that the inertial lift does not suspen d small particles. Furthermore, the elastic Lift force is singular when the minimum gap between the sphere and the wall approaches zero. Consequently, a moving particle in a viscoelastic fluid will always be suspended from a smooth surface. (C) 1999 Elsevier Science B.V. All rights reserved.