Nanoparticles have a very cool way of self-assembling (science talk for arranging themselves) in a drying solvent. At the right concentrations, the solvent surface-tension draws particles together into a monolayer. These TEM images show the awesome self-assembly of 10-nm magnetite nanoparticles suspended in water. Water has a high-surface tension and relatively low boiling point, which helps drive the self-assembly.
Read “Clustering behaviour of magnetic nanoparticles” to find out why solvent drying effects might be an issue when studying magnetic nanoparticles.