Figure 1.

Schematics of the three phases of sedimentation–diffusion equilibrium for nanoparticles in solution. (a) Once injected in solution nanoparticle dynamics is mainly regulated by diffusion forces. Nanoparticles are dispersed over the entire available volume. (b) Nanoparticles are randomly but homogeneously dispersed in solution. Gravity tends to direct particles to the bottom of the solution. (c) Sedimentation–diffusion equilibrium is attained. The rate of transport of nanoparticles in any one direction due to sedimentation equals the rate of transport in the opposite direction due to diffusion.