Why is it, when you try to wipe away drops of blood off of a surface, it leaves a behind a thin ring around it which is harder to clean?
In: Physics
Because the blood at the edge of the drop dries more quickly. When a drop of blood hits a surface it tries to stay together, but it will obviously spread out some into a circle. The inner most part of the drop will be more dense than the outer part. Since the outer part has less blood, it will dry more quickly, leaving dried residue behind when it’s wiped away
Edit: as many have pointed out, i should not have used the term dense. The the amount of atoms packed in any given area will pretty much remain the same throughout. I meant the middle will contain more blood than the outside due to inter-molecular forces.
Edit 2: Also i understand blood ‘clots’ and doesn’t just ‘dry’ like i said. I just gave a basic explanation for why most liquids dry at the outside before the inside. Blood just does it much much faster due to its ability to clot.
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