Laying Microscale Tiles
Microcrystalline monolayers: Laying by hand is superior to self-organization process

Craftsmen tile walls or floors by hand; but how can you get an ordered monolayer onto a substrate when the ¡°tiles¡± are microscopically small instead of big and easy to handle? Previously, self-organization processes have been the method of choice for this scale. Korean researchers have now come to the realization that even such tiny components can be arranged in a ¡°do-it-yourself¡± process. As they describe in the journal Angewandte Chemie, their manually produced monolayers of microcrystals are qualitatively superior to the self-organized variety.
How small can components be such that they can still be glued to a surface by hand? And conversely, how big can microscale components be such that they can still be arranged by self-organization? Which method is best in the size range where both techniques work? These questions have been investigated by a team led by Kyung Byung Yoon at Sogang University in Seoul. To find answers, they carried out experiments with zeolite crystals in a variety of sizes. Zeolites are silicate minerals with applications in many technical fields.
The powdered zeolite was applied by simply rubbing it on with a finger (with wearing a latex glove). Alternatively, they were applied in solution, and ultrasound was used to kick-start the self-organization process. The ¡°glue¡± between the ¡°mini-tiles¡± and the substrate was the attraction between oppositely charged groups of atoms, hydrogen bridge bonds, and chemical bonds between reactive groups of atoms.
The experiments demonstrated that self-organization only works for particles smaller than about 3 µm. Hand-application works for crystals as small as 0.5 µm in diameter. In the overlapping range (0.5 to 3 µm), hand application is preferable to self-organization for quality: the packing is denser and the microcrystals are oriented more regularly. Whereas self-organization produces individual crystals grown at a 90 ° angle onto the monolayer, such ¡°parasites¡± are simply rubbed off by hand. There are other ¡°handy¡± advantages of the manual process as well: it is simpler, doesn¡¯t require a solvent or special equipment, runs more smoothly, and allows for treatment of larger surfaces.


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