Volume 23, Issue 4 (December 2000)
The Accuracy of Hydrometer Analysis for Fine-Grained Clay Particles
This paper provides a rigorous analysis on the accuracy of Stokes' equation for calculating particle-size distributions of nonspherical, fine-grained clay particles. Analytical expressions relating the actual particle shape and size to the equivalent “Stokes' diameter” are presented for disk-shaped and rod-shaped particles. Disks and rods are chosen to represent the high aspect ratios typically exhibited by real clay particles. For particle sizes ranging from 0.1 µm to 100 µm and for aspect ratios ranging from 10 to 500, it is shown that Stokes' equation underestimates the maximum particle dimension by up to two orders of magnitude. Consequently, particle-size distributions calculated from conventional hydrometer analysis are shown to be misleading. To confirm the contention that Stokes' equation may not be appropriate for calculating particle sizes and particle-size distributions for fine-grained clays, kaolinite and pulverized mica are characterized by comparing results from hydrometer analysis, sieve analysis, laser diffraction analysis, and scanning electron microscopy (SEM). The experimental results confirm the error estimates from the theoretical evaluation.