Why are coffee stains darker at the edges?

Answer There are many things around us that are so taken for granted that it never occurs to us to give them a second thought until we look a little closer. Coffee stains are one such thing. When coffee dries it leaves stains that are dark at the edges and lighter inside. The explanation may not be obvious but it is in fact rather simple. When a drop lands on a table, it spreads and tries to dampen as large an area as possible. Though the table surface may seem smooth there are always some imperfections where the edges of the drop will stop. This can be compared to deep ruts in a track from which it is difficult to steer a vehicle's wheels. In the same way the movement of the edges of the drop is hindered and it needs energy to diminish its circumference on the table. Near the edge of the drop the upper surface slopes at an angle to the table surface. So the area of the drop's upper surface is greater than the area of its lower surface. Evaporation is in proportion to the drop's upp ... Read full article.