> 3) Ants carry lots of heated marbles (will have to think on how to keep 
>them heated) and they drop one at the time. Other ants follow those 
>marbles by some kind of infra red detection. I saw some cheap robots 
>that are attracted by heat, maybe this is the cheapest solution.

Can you control the environment as well as the robots?  For example,
can they be on a sheet of rubber that would hold heat and then slowly
radiate it away?  Could you use a heated roller to warm up a surface?

Or a surface of some sort of fabric, where wet spots have different
color, or become translucent revealing an underlying color?  Isopropyl
alcohol would evaporte rapidly and cleanly.

Mike H.
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