Increasing attention is being given to collecting and analyzing extraterrestrial particles. For the larger particles, one of the physical parameters desired is density. For years, sedimentologists have used Stokes' law for determining the size of a particle falling in a liquid, and recently, the same method has been used to determine density. A thorough investigation of Stokes', Rubey's, Oseen's, and Wadell's settling velocity equations was performed in the laboratory to test the validity of the various equations and to establish the range of applicability for each in terms of Reynolds number.
This study was conducted with spheres of known density. Sizes ranged from 25 to 100 microns and densities from 2.61 to 18.02 g/cm3. The settling velocity of each sphere was determined after measuring repetitive fall times in liquids of known density and viscosity. The range of experimental conditions was from a Reynolds number of 0.02 to 30.1. Results obtained in terms of Reynolds number are: (1) upper limit of Stokes' equation is 0.02; (2) upper limit of Oseen's equation is 2.0; (3) limit of Rubey's equation is between 2 and 12; (4) Wadell's equation is not found to be applicable under the conditions tested; (5) from a Reynolds number of 12.0 to 30.1 no equation appears to be applicable.
Graphs are presented which can be used to determine the density of a spherule whose diameter is between 25 and 100 μ when its fall velocity is known.