I spilled something on my shirt the other day. My girlfriend told me I ought to hold it under cold water. I asked her why it should be cold water, and she claimed that it’s because hot water sets the stain.
She’s usually right about this sort of thing, but from a physics standpoint this made no sense to me. First of all, hot water has a lower surface tension, so I would think it would penetrate between the fibers of the fabric more easily (since it’s not forming large droplets). And furthermore, I would have thought that the heat would make the fibers of the material expand (although I’m not sure if it would be by a significant amount), making it easier to remove the particles of the stain from between them.
So, who’s right? And if my girlfriend is right and cold water really does work better, what’s the physics behind this? Obviously I’m looking for something a little more in depth than “hot water sets the stain.”
Overall I think you’re right, but I can think of two situations where heat would cause a problem.
If the stain would be soluble in the fabric, for instance if it’s a smallish organic molecule, heat could increase the ability of stain molecules to move into the polymeric crystal of the fabric material. Heat is a sort of plasticizer in this sense.
Heat can make some things crosslink and lose their solubility. The only example I can think of is egg whites, but I know there are others. In fact, whatever proteins they add enzymes to detergent for, would be candidates for this problem.
I would imagine this would be more of a chemistry question than a physics question. IANAC, but it seems reasonable to suppose that the setting of a stain represents some sort of chemical bonding between the fabric and the staining substance. Most chemical reactions proceed quicker at higher temperature, so hot water could help speed the stain-setting reaction.
Well, I can only figure it out in terms of fabric dyeing (which is essentially what you want to prevent your spill from doing, right?) To dye a fabric, you need pigment (your spill), and something to open up the fibers of the fabric so that the pigment can get in them. Otherwise, the pigment sits on top of the fibers and will rinse out in water. Some pigments (blood, grape juice, wine) are made of up tiny molecules, and can slide into the fibers without assistance. Most pigments, however, have larger molecules. When I’m dyeing fabric, I use alkaline soda ash, which breaks down a bit of the cellulose structure of the fabric, making the spaces in the fibers larger, and letting the pigment molecules in. Heat will also, as you say, open the fibers. If I’m dying in the winter, the wet, dye-soaked fabric gets set on the radiator overnight - the heat yields a brighter and more permanent color. Therefore, if your stain hasn’t “set” (that is, if the pigment hasn’t yet penetrated the fiber) the heat of the hot water could open the fibers a bit and increase the dyeing - “set the stain,” if you will.
That makes sense. So perhaps cold water is best when you just spilled something on a fabric, and you can rinse it off before it’s penetrated fibers, but after the stain has already set in you might be better off using hot water.