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?

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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

17 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

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Anonymous 0 Comments

Non-ELI5 version: (I) When blood is deposited upon a surface, the RBC’s are evenly dis- tributed throughout the pool, which then will sediment. At this point the blood has a dark red colour and starts to coagulate [15]. It is possible that due to wetting and capillary action, the area of the blood pool increases as the blood spreads slowly over the surface during the initial 15 to 30 minutes. During this first stage there is a change in colour from dark red to lighter red, mainly due to coagulation. **(II) As fluid evaporates from the blood pool red blood cells that are not constricted in the fibrin web, are transported to the rim of the stain and deposited, due to flow caused by evaporation [14]. The transition from the fluidal to a gel state is referred to as the gelation front. The second stage starts when the gelation rim is created around the pool. The gelation front propagates inwards, towards the centre of the stain, as the pool continues to dry.** (Nick Laan, Fiona Smith, Celine Nicloux, David Brutin. Morphology of drying blood pools. Forensic
Science International, Elsevier, 2016, 267, pp.104-109.)

ELI5 version: As blood dries, it separates into different parts with more ~~viscous~~ gooey materials forming a gel rim around the blood. Gel is harder to wipe up than a standard liquid, which the rest of the pool is.

Anonymous 0 Comments

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Anonymous 0 Comments

Blood when exposed to air tries to clot, the different components in the blood meant to close wounds bind together.

This happens faster at the edges.

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Anonymous 0 Comments

Jesus, how much blood do you have to clean up before you start to notice these things? With that being said, the outside of the blood puddle is coagulating from the oxygen, you leave it there long enough and the whole mess will harden.