Fundamental study and applications of convective deposition of microsphere monolayers.

摘要

We examine the fundamental nature of the rapid convective deposition of micro- and nanoscale particles. This process uses a blade to draw a suspension droplet across a substrate thus advancing thin film where particles assemble due to capillary force and create monolayer close-packed structures. The optimal operating ranges to form 2D close-packed microsphere arrays are obtained by varying deposition rate and blade angle. Previous deposition models do not fully describe various aspects of this deposition process. For instance, blade angle and hydrophobicity affect the rapid 2D crystal formation by varying shape and flow within the thin film. High speed confocal laser scanning microscopy reveals the dynamic self-assembly of colloidal particles under convective evaporation. Image analysis of deposited layer quantifies crystal quality through radial distribution function, the fraction of the number of nearest neighbors, and local bond order. Lubrication theory describes flow inside the extended meniscus and explains accelerated and reversal flow phenomena during monolayer deposition.;Building on this foundation, we investigate the coupling between suspension properties and the deposition process during the convective deposition of aqueous binary suspensions of 1 mum silica microspheres and 100 nm polystyrene nanoparticles. At optimal conditions, this binary colloidal suspension creates higher-quality and longer-range monolayer of microsphere with nanoparticle-filled interstitial region. A model is developed to predict the optimum ratio of micro to nanoparticle fluxes during the deposition for creating long-range 2D crystal of microspheres. With unmatched flux, instabilities arise that result in the formation of stripes perpendicular to the direction of deposition.;We successfully utilize 2D crystal formation in various applications. Microlens arrays convectively assembled on the GaN surface improve the efficiency of LEDs by increasing photon extraction efficiency. Our process increased LEDs efficiency by as much as 262%. Furthermore, potential application of this periodic structure on cell capture devices, biological membranes, and photoelectrode of dye sensitized solar cells are discussed.