On the Convective Self-Assembly of Colloidal Particles in Nanofluid Based on in Situ Measurements of Interaction Forces.

While the currently available techniques for the self-assembly of colloidal particles show great promise owing to their simplicity and high efficiency, they are plagued by the fact that they result in colloidal crystals with defects. Here, in order to overcome this problem, we propose a strategy that uses a suspension of nanoparticles (i.e., a nanofluid) as the "solvent" for the colloidal particles. We fabricated colloidal films of microspheres using such a nanofluid suspension and performed in situ measurements of the interaction forces between the microspheres in the nanofluid. This was done in order to systematically elucidate the effects of the nanoparticle size and the thickness of the electric double layer (Debye length) on the self-assembly process. The obtained results confirm that the use of the nanofluid results in a monolayer with a higher degree of order than that in the case of films formed using pure water. Further, the optimal size of the nanoparticles is determined based on the balance between their physical size and the Debye length. We also show that the lodging of the nanoparticles between the microspheres decreases both the lubrication force and the friction force between them. Thus, in this study, we show, for the first time, that a nanofluid can be used in the self-assembly process for improving the regularity of the fabricated colloidal particle arrays, as it inhibits the aggregation of the particles and limits the lubrication and friction forces between them.