Watching paint dry is, obviously, deadly dull. But it turns out that watching blood dry is surprisingly interesting — and particularly useful.
Fuck Yeah Fluid Dynamics explains that there's a lot to be seeing going on in this accelerated footage of blood evaporating:
As water from the blood evaporates, it causes relative changes in surface tension. These surface tension gradients cause convection inside the drop and carry red blood cells toward the outer portion of the drop. As the blood evaporates further, it leaves behind different patterns that depend on which parts of the whole blood mixture were deposited in each region.
Perhaps most interesting, though, is that what's ultimately left behind can reveal patient health, too. In the image below, for instance, you can see the stark contrast between a healthy human's dry blood and a similar sample from an anaemic patient.