Robust fadeout profile of an evaporation stain

We propose an explanation for the commonly seen fading in the density of a stain remaining after a droplet has dried on a surface. The density decreases as a power p of the distance from the edge. For thin, dilute drops of general shape this power is determined by a flow stagnation point in the distant interior of the drop. The power p depends on the local evaporation rate J(0) at the stagnation point and the liquid depth h(0) there: p = 1- 2 (h(0)/(h) over bar)((J) over bar/ J(0)), where (h) over bar and (J) over bar are averages over the drop surface. Copyright (C) EPLA, 2009