Effect of nanoparticle concentration on the convective deposition of binary suspensions.

We investigate the coupling between the suspension properties and the deposition process during convective deposition of aqueous binary suspensions of 1 microm silica microspheres and 100 nm polystyrene (PS) nanoparticles. The structures formed from this rapid and scalable process have use in a variety of optical, chemical, and biochemical sensing applications. At conditions that produce a well-ordered microsphere monolayer at a silica volume fraction of 20% in the absence of nanoparticles, we examine the effect of varying the concentration of nanoparticles from 0% to 16% on the quality of the microsphere deposition and the exposure of the microspheres within the PS layer. At low concentrations of nanoparticles, the deposition results in an instability that forms stripes parallel to the receding contact line. Optimum deposition occurs between 6% and 8% PS and forms a monolayer having the same high degree of uniformity as the monodisperse suspension is fabricated. For higher concentrations, the deposition is increasingly less uniform as a result of nanoparticle depletion destabilizing the microspheres. The degree to which each microsphere is buried by the nanoparticles in the deposited thin film increases with nanoparticle concentration. This variation in coverage also suggests interplay between deposition and nanoparticle engineered properties of the suspension that influence the deposited morphology.