High-Sensitivity Measurement of Density by Magnetic Levitation

Abstract

This paper presents methods that use Magnetic Levitation (MagLev) to measure very small differences in density of solid diamagnetic objects suspended in a paramagnetic medium. Previous work in this field has shown that, while it is a convenient method, standard MagLev cannot resolve differences in density < 0.0001 g/mm3 for macroscopic objects (> mm) because i) objects close in density prevent each other from reaching equilibrium height due to hard contact and excluded volume and ii) using weaker magnets or reducing the magnetic susceptibility of the medium destabilizes the magnetic trap. The present work investigates ways to increase the sensitivity of MagLev without destabilization by i) rotating the standard configuration relative to the gravitational field, and therefore, exploiting the weak magnetic gradients along alternative axes of measurement, and ii) tuning the sensitivity by manipulating the geometries of the magnets. These modifications enable an improvement in the resolution up to 1300 over the standard configuration, and measurements with resolution down to 10-6 g/cm3. Three examples of characterizing the small differences density among “identical” samples of materials—Nylon spheres, PMMA spheres, and drug spheres—demonstrate the applicability of high-sensitivity, rotated Maglev to measure the density of small (0.1 – 1 mm) objects with high sensitivity, for use in materials science, separations, and quality control of manufactured products.