The fate of an object in a simple homogeneous fluid is easy to guess. If it weighs more than the fluid it displaces, it sinks; otherwise, it floats, just as Archimedes predicted 23 centuries ago. In both natural and industrial settings, though, the suspending fluid is usually complex, filled with several other dispersed species in a variety of sizes and densities.
Some of that complexity may be deliberate. Food scientists or cell biologists, for instance, often add heavy salts or colloidal nanoparticles to an already crowded fluid to create a density gradient in the solvent that will separate the different components in suspension. Proteins, nucleic acids, cell organelles, and other components sink or float to levels where their densities match that of the local solvent, in a process typically aided by centrifugation. The technique is a common way to efficiently resolve or extract the different species.
As early as the...
