MANY theoretical and experimental investigations have been made into particle migration in laminar tube flow. Segre and Silberberg1,2 noted that all the particles in a neutrally buoyant suspension of rigid spheres flowing down a vertical tube congregate in about 0.6 of the tube radius, and Maude and Yearn3 reported precise experimental evidence for another high concentration zone near to the tube wall, which they term the “wall peak”. Rubinow and Keller4 explained the occurrence of high concntration zones by deriving the transverse force on a spinning sphere. It was subsequently shown by Saffman5 that the lift force on a freely rotating particle caused by rotation is insignificant, and the radial velocity of the particle is given by where U0 is the maximum fluid velocity, a the particle radius, R the tube radius, ReT the Reynolds number (= U0R/v, v is the kinematic viscosity), r the radial position of the particle and ΔVz the axial relative velocity. If the particle lags behind the fluid, ΔVz>0 and the radial migration is towards the tube axis. The relative velocity ΔVz cannot directly be applied for this purpose, but Repetti and Leonard6 have introduced the effective fluid velocity caused by the presence of the particle, and they explained the Segre–Silberberg effect, using the axial particle velocity obtained experimentally by Goldsmith and Mason7.