Phys. Rev. X 5, 021012 (2015)

Charged colloidal particles repel or attract, depending on the signs of their charges. But the underlying forces are not necessarily governed by the standard Coulomb law. In a solvent with ions, the charge of a colloid gets shielded and is felt only weakly beyond a certain distance, known as the screening length. So, by cleverly choosing the solvent, the screened Coulomb, or Yukawa, potential of a colloid can be tuned.

Ahmet Demirörs and colleagues have now exploited charge screening to create particular colloidal clusters. The authors used binary mixtures of small and large oppositely charged, micrometre-sized particles, and observed clusters where a small particle is surrounded by up to six large satellite particles. These 'colloidal molecules' are the analogues of, for example, methane or phosphorus pentachloride.

Demirörs et al. identified the criteria for such clusters to self-assemble and remain stable: in addition to an excess of satellite particles, the right balance has to be reached between attraction and repulsion throughout the 'molecule'. A promising feature of the fabrication scheme is that a given number of satellite particles can be obtained for different core sizes, providing a handle on surface coverage and patchiness.