Controlled, rapid deposition of structured coatings from micro- and nanoparticle suspensions

The objective of the study was to develop the operational basis for rapid and controlled deposition of crystal coatings from particles of a wide size range. We deposited such structured coatings by dragging with constant velocity a small volume of liquid confined in a meniscus between two plates. Two types of structured coatings were characterized: latex colloidal crystals and thin layers from metallic nanoparticles. The crystal deposition was sped up by use of preconcentrated suspensions. Crystal coatings larger than a few square centimeters were deposited in minutes from aqueous suspension volumes of approximately 10 microL. The governing mechanism of crystal deposition is convective assembly at high volume fractions. The two major process parameters that allow control over the coating thickness and structure were the deposition speed and particle volume fraction. The evaporation rate was not found to affect the process to a large extent. A volumetric flux balance was used to relate the deposition parameters to coating structure and properties. Operational "phase" diagrams were constructed, relating the crystal layer thickness and packing symmetry to the process parameters. These diagrams could be instrumental in transforming the convective colloidal deposition into a robust scaleable technology.