Relative particle velocities in two-species settling

Abstract Settling velocities have been measured for two particle fractions settling together through a liquid. The two fractions differed only in particle density but not in particle size. It is shown that the settling velocity of each fraction may be determined from the Richardson-Zaki equation, if the total particle concentration is used to correct the terminal fall velocity of the particles in each fraction. The concept of a critical concentration in binary sedimentation has been examined. At this concentration the two fractions appear to settle at the same velocity. Previous authors have attributed the effect to mechanical interlocking of the particles, but the present work shows that an alternative hydrodynamic argument can account for the phenomenon.