The thread title basically says it all. How come the edges of stains seem to be the most tenacious?
Because you can see the obvious contrast between the stain and the unstained cloth? In the center of the stain it’s fairly uniform and if you don’t get it all out you can’t really see the difference between the stained area and the unstained. But by the edge the two are obviously right next to each other and you’re seeing any slight difference.
There also may be something with surface tension and/or capillary action that brings more of the staining substance to the edges.
That is where the drying out began and lasted longest, the wetness flowed out till it reached the far ends and was slowed down to a stop (by evaporation and/or absorption). This process continues till that fluid could not reach that point anymore (achieved equilibrium - the flow outwards stopped). That would seem to indicate that that area had more of that stuff dry over time then the rest of the cloth which remained wet. The rest of the fabric then was uniformly wet and there was no longer any pressure for the fluid to ‘flow’ so the rest of the stain dried evenly and all at one time.
It also does bring to mind the elementary school experiment, which may be a secondary factor, when you placed a drop of colored stuff on a paper like product and it spread out. At the fringe usually the colors would separate (the color X spread just a bit faster then the color Y, out of the multi color dye).
From the above it would seem like the fringe is a combination of multiple ‘colors’ separated out just ‘shy’ of each other.
Just to add to the above. The wetted area also served as a channel that allowed the flow much easier then the fringe area (going into non-wetted area), which allowed the flow to uniformly distribute itself in the wetted area. So the fringe area had resistance to growth, but still being fed from behind. So once the stain achieved it’s max size it does not grow, but more material will be still be transported to that edge, but no further.
Telemark: I don’t think that’s quite it. I just washed a reddish stain out of white fabric, and the middle part of the stain really did wash out much faster. It was almost white again, whereas the edges were still red. It wasn’t just the contrast.
kanicbird: I think that makes sense, but I don’t understand why stains often have defined borders. If there is still liquid being pushed out toward the edges, why would accumulate and not just spread out more?
Thank you both for the replies!
Surface tension.
The stain material is made up of a mixture of different molecules. The more polar molecules tend to move more easily away from the centre of the stain as the liquid wicks out into the fabric. As with liquid chromatography, they associate more with the mobile phase. This is due to intermolecular forces, rather than surface tension.
Since the more polar molecules move with the solvent front, and based on your observation, these molecules most be more resistant to stain removal. Detergents work by removing organic molecules, whereas the polar water soluble remnants possible fix into the substrate to cause the stain. Of course, polar materials such as simple salts are removed with water, however the polar dyes would fix on drying.
This observation suggests that polar dyes are more wash-fast.
Can you please carry out further experimentation. We need more data.
I was going to go a different direction. My thought is that the stain substance remains thickest at the center of the stain. The fringes are thinner and dry faster - set up sooner and become color fast. The center maintains some liquidity or retains some solvent and can be “attacked” by the cleaning agent easier.
– most definitely not a chemist –
all of the above and some of the below, except for that one answer which is clearly wrong.
when a dissolved substance migrates along a solid, some of the substance can be retarded on the solid. you may see some liquid slightly ahead of the stain.