When we drop a coffee droplet and wait for it to evaporate, there is a coffee ring left. Because there is coffee dissolved in the solution, and when water can evaporate into the air, the coffee cannot evaporate and it will stay. Therefore, if we drop one DI water droplet onto the substrate, there will be no such ring left because nothing is dissolved in the DI water. Here comes the question: when I droplet one droplet of Toluene or Hexane onto a glass substrate, there is a ring after it evaporates! Toluene and Hexane are newly bought, and they should be good and pure. Then what does this ring come from? It means something is dissolved in the Hexane and Toluene. Any clues?