The last time I mentioned gels was over their use as the matrix for a cockroach repellent. I recently ran across another example of gels being used to repel undesired fauna - as a bird repellent. Apparently if you apply the gel to bird roosting areas, the birds dislike the sticky feeling on their feet and take flight. Speaking strictly from a rheological viewpoint, "tackiness" is largely independent of the thickness of an material, but only after reaching a certain critical thickness. Excessively thin layers will have lower levels of tack - something that the British city of Market Rasen recently discovered. They were testing a gel such is this on bridges, but applied it too thin. The material for 1 bridge was spread out over 3, so the gel was too thin and the tack was no where near where it should have been.
Another option for repelling birds, although in this case, it appears to be limited to bronze statues, is to have some arsenic incorporated in the alloy. The mechanism by which this repels birds is unexplained at present. The only problem with this approach is that a new report ($) which looked at 6 elements of the "geologic copper family" (Cu, As, Se, Ag, Te, and Au) [*], concludes that the supply risk for arsenic is critical at a global level. As such, making it available for use in semiconductors is in my mind more important than for keeping statues clean of bird droppings.
[*] Although these elements are scattered across the periodic table, they are a "geologic family" as they commonly occur together in copper ore deposits. Hopefully Th' Gaussling (a chemist with an intent interest in geology) can explain sometime how this occurs.
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