Clothes washing mystery solved by physicists

Physics World ^ | 3 Apr, 2018 | Michael Allen

[MtnClimber]

A fresh water rinse is just as important as washing in detergent for getting your clothes clean, according to physicists in the US and the UK. They claim that the rinse cycle plays a key role in removing dirt from deep within textiles, by setting up chemical and electrolyte gradients that draw it out. This could lead to the development of more efficient and environmentally friendly washing machines, they add.

Washing machines wash clothes with water mixed with detergent and then rinse them with fresh water before finally spinning them. Washing detergents are surfactants, compounds that lower the surface tension between liquids and other substances, making it easier for them to mix. When washing clothes, they help the water mix with and loosen dirt on the fabric. Conventional understanding is that rinsing then flushes the fabric and washes the dirt away.

Stagnant cores

But, there is a problem with this idea. In most fabrics there are tiny pores that do not allow any significant fluid flow inside them. According to Sangwoo Shin at the University of Hawaii, Patrick Warren of Unilever in the UK and Howard Stone of Princeton University, it should take several hours for micron-sized particles to diffuse out these micrometre-sized pores. Yet significant numbers of particles do leave these pores on much faster time scales. The question as to how this is possible is known in the washing industry as the “stagnant core problem”.

Looking at this problem, the trio noted that when detergent-saturated fabric is exposed to fresh water the surfactant molecules rapidly move out of the stagnant core. They hypothesized that the surfactant gradient established when the fabric is rinsed – with a high concentration of surfactant within the fabric’s pores and a low concentration in the surrounding water –

[Mears]

Why do you even use fabric softener?

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[Don W]

I have to use the dryer through the winter, and the static build up is reduced a LOT when softener is used.

It smells nice, too.

[Mears]

Thanks-—I never used it so I was curious——I always considered it just another unnecessary product thrown at us to make money for a corporation.

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[RooRoobird20]

In the really olden days, people used to collect urine, let it set around a while, then wash their clothes in it.

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Yes, as far back as the ancient Roman times!! Mr. Roo Roo and I took a guided tour of Pompeii, and we saw the remains of a laundry service that were still in place. There were a couple of large shallow tubs where the laundry and urine would be placed and workers would stomp around on the laundry, much like stomping grapes to make wine. Being a laundry stomper was not considered a desirable job—the urine smelled really bad and was irritating to the stompers’ feet.

According to our guide, the laundry service would accept just about any kind of urine, but supposedly zebra and giraffe urine were greatly coveted. After washing laundry in urine and rinsing, the laundry would be “smoked” to help get rid of the ammonia/urine smell.

[mass55th]

Thanks for the additional info. Hadn’t known that about the zebra and giraffe urine.