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My guess is that it's something akin to salt creep, except the evaporation rate at the meniscus is what is keeping the droplets of concentrated tincture above the bulk fluid. Once the fan is turned off and the evaporation slows below a critical rate, gravity takes over and pulls the accumulated droplets of concentrate back down into the bulk fluid.
But this is a guess, so
-------------------- “The Universe is under no obligation to make sense to you” -NDT