Get a sink full of water with a grease slick on top. Then, apply a tiny dab of detergent on your finger and then just lightly touch the surface of the water with your finger. Almost instantly, the grease in the water dissolves in a very rapidly expanding circle.
However, if you had a drop of red dye on your finger and did the same thing, it would take many minutes for it to diffuse as far.
Is there some mechanism by which detergent can travel very quickly in water? Does the grease slick pull the detergent along as it travels or something?