Surface charge and interactions of 20-nm nanocolloids in a nematic liquid crystal.

We studied real-time motion of individual 20-nm silica nanoparticles in a thin layer of a nematic liquid crystal using a dark-field optical videomicroscopy. By tracking the positions of individual nanoparticles we observed that particle pair interactions are not only mediated by strong thermal fluctuations of the nematic liquid crystal, but also with a repulsive force of electric origin. We determined the total electric charge of silanated silica particles in the nematic liquid crystal 5CB by observing the electric-force-driven drift. Surprisingly, the surface electric charge density depends on colloidal size and is ∼4.5×10(-3)C/m(2) for 20-nm nanocolloids, and two orders of magnitude lower, i.e., ∼2.3×10(-5)C/m(2), for 1-μm colloids. We conclude that electrostatic repulsion between like-charged particles prevents the formation of permanent colloidal assemblies of nanometer size. We also observed strong attraction of 20-nm silica nanoparticles to confining polyimide surfaces and larger clusters, which gradually results in complete expulsion of nanoparticles from the nematic liquid crystal to the surfaces of the confining cell.