Frontiers Physics World  May 2018

Physicists dish the dirt on clothes washing mystery

Rinse cycle The top row shows fabric that is rinsed in fresh water, while in the bottom row it was rinsed in a detergent solution. (S Shin et al., Phys. Rev. Applied (2018))

A fresh water rinse is just as important as washing in detergent for getting your clothes clean, according to physicists in the US and 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 insight could lead to the development of more efficient and environmentally friendly washing machines.

Washing machines clean clothes using water mixed with detergent, followed by a fresh-water rinse and then a spin-cycle. The detergents contain surfactants – compounds that lower the surface tension between liquids and other substances. They are usually ionic and can attach to particles (such as dirt) in the solution, therefore causing a neutral particle to become charged.

When washing clothes, the lowered surface tension makes it easier for the water to mix with and loosen dirt on the fabric. Conventional understanding is that the detergent draws out the dirt and the water rinse then washes the dirt away. But there is a problem with this idea – most fabrics contain 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 in the US, it should take several hours for micron-sized particles to diffuse out of these micron-sized pores. Yet lots 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”. The researchers hypothesized that it is related to diffusiophoresis – the directed motion of particles up or down a chemical gradient.

To investigate, the team mimicked stagnant cores in detergent-saturated fabrics by using microfluidic channels with dead-end pores filled with polystyrene particles (the dirt) in a surfactant solution. They found that if they flushed the pores with detergent-filled water, the flow did not remove the particles from within – there was no concentration gradient to encourage diffusiophoresis.

In contrast, a fresh water rinse washed away most of the detergent except for deep within the pores, where the surfactant concentration, now attached to particles, remained high. As the surfactants are ionic, the resulting gradient created an electric field that encouraged diffusiophoresis to such an extent that the pores nearly emptied over 10 minutes of rinsing.

The team repeated the tests using stained cotton and found that rinsing a detergent-saturated fabric with fresh water cleaned it much faster than a detergent solution rinse. (Phys. Rev. Appl. 9 034012).

Michael Allen