SEDIMENTATION OF MONODISPERSE AND BIDISPERSE HARD-SPHERE COLLOIDAL SUSPENSIONS

The effect of concentration on the sedimentation rate of uncharged rigid spheres was investigated. Three submicron sizes of silica spheres were prepared according to the method of Stober (1968). The particles were sterically stabilized by chemisorption of stearyl alcohol at their surface by the method developed by van Helden (1981). The sterically stabilized silica particles dispersed in cyclohexane are known to behave as hard spheres. Monodisperse gravity sedimentation experiments were carried out for the various particle species over a wide concentration range of phi = 0.003 to phi = 0.37, where phi is the particle volume fraction in cyclohexane. The data for dilute suspensions (phi < 0.03) were found to be well described by Batchelor's equation: U/U0 = 1-6.55phi, where U is the sedimentation velocity and U0 is the Stokes velocity of a sphere in isolation. The data over the entire concentration range (0 < phi < 0.37) were found to be well described by the equation: U/U0 = (1-phi)6.55. Bidisperse sedimentation experiments were also carried out, and the dilute data were found to be well represented by Batchelor's 1982 theory for polydisperse suspensions. The high concentration data were analyzed in terms of a model that does not distinguish between interactions of like and unlike particles.