The density of small diamagnetic particles can be measured by suspending them in a paramagnetic solution and applying a magnetic field. Unfortunately, this MagLev method only works in a particle density range close to that of the solution, about 0.8 to 2.3 g/cm3.
George Whitesides and colleagues, Harvard University, Cambridge, MA, USA, found a way around this limitation. They measured the densities of materials from air bubbles to osmium and iridium (23 g/cm3). A typical MagLev device consists of a vertical tube between two magnets with like poles facing each other. Particles suspended in a paramagnetic solution in the tube (aqueous MnCl2 or GdCl3) find a balance point between gravitational and magnetic forces at a height that depends on their density. Very light particles float to the top and very heavy particles sink to the bottom, rendering the technique ineffective.
By tilting the MagLev device, the researchers decreased the buoyant gravitational force along the direction of measurement, enabling them to suspend a wider range of particles without increasing the magnetic field strength. They noted that increasing the range of densities came at the expense of precision in the density measurement.
- Tilted Magnetic Levitation Enables Measurement of the Complete Range of Densities of Materials with Low Magnetic Permeability,
Alex Nemiroski, Siowling Soh, Sen Wai Kwok, Hai-Dong Yu, George M. Whitesides,
J. Am. Chem. Soc. 2016.