Matching Constituent Fluxes for Convective Deposition of Binary Suspensions

Rapid convective deposition is an effective method for depositing well-ordered monolayers from monodisperse suspensions; however, much less is known about polydisperse suspension deposition. The addition of a much smaller species can enhance deposition by extending the range of ordered deposition and can induce instability, producing stripes and other complex morphologies. By considering relative species flux, we predict the volume fraction ratio of smaller to larger constituents necessary for steady well-ordered deposition. Experiments varying the 1 microm microsphere and 100 nm nanoparticle concentrations exhibit an optimum nanoparticle to microsphere volume fraction ratio at moderate volume fractions that agrees well with theory. Average local bond order and surface density characterize crystallinity and coverage, respectively. At lower microsphere volume fraction, monolayer crystallinity is optimized at a constant nanoparticle volume fraction of 0.04. At lower-than-optimum nanoparticle concentrations for each microsphere concentration, instability occurs and alternating stripes of monolayer and submonolayer morphologies form. At higher-than-optimum nanoparticle concentration, the microspheres become disordered and/or form multilayer regions. Additionally, the degree of microsphere burial in deposited nanoparticles depends solely on nanoparticle concentration.