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Soft matter and liquids

Clothes washing mystery solved by physicists

03 Apr 2018
A photo of a washing machine
Rinsing is just as important as washing. (Courtesy: iStock/NoonBuSin)

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 – causes the surfactant particles, along with the dirt, to undergo diffusiophoresis, the directed motion of particles up or down a chemical gradient.

To test the theory, the team used a microfluidic channel connected to a set of 50 µm wide, dead-end pores. These pores acted like the stagnant core of a piece of fabric, with little flow into them when a fluid flowed through the channel. They filled the pores with fluorescent, 0.5 µm, polystyrene particles suspended in a surfactant solution. They then flushed the microfluidic channel with a surfactant solution of the same concentration and with a surfactant solution 100-times weaker.

Electrolyte gradient

The concentrated solution flushed the polystyrene particles out of the entrance to the pores, clearing them to a depth of roughly the width of the pore, but had no impact on the particles deep in the pores. After flushing the entrance of the pores, the weak solution, however, caused the particles in the pores to migrate out into the channel over a period of 10 min. The researchers say that a significant contribution to this diffusiophoresis is electrophoresis, with the low concentration rinse creating a surfactant gradient with an electrolyte gradient that forces particles to migrate out from the core.

Stained fabric after rinsing

The team also tested their theory on fabric. They found that a piece of cotton stained with polystyrene latex is cleaned better and faster when it is rinsed with fresh water after being soaked with detergent, than if it is rinsed with the same detergent solution (see figure).

The surfactant gradient and the resulting diffusiophoresis results in the removal of the stain from the fabric and the polystyrene particles from the pores because the surfactant binds to the particles. This also means that the processes works effectively for most types of dirt and stains.

Similar surfaces

“The surfactant (detergent) strongly adsorbs onto the dirt surface, making them possess similar surface characteristics,” explains Shin. “It is the surface potential that drives the diffusiophoresis, so any substances that have similar surface properties should behave more or less the same.”

We can design a better wash-rinse cycle for optimal cleaning

Sangwoo Shin, University of Hawaii

As well as solving the stagnant core problem, Shin says that the results could help improve washing machines. “Since the current finding suggests that the rinsing is the key player in dirt removal, we can design a better wash-rinse cycle for optimal cleaning,” he told Physics World. “For example, a chaotic, churning motion of the laundry drum might not be essential. This could reduce energy consumption and noise. Multiple rinsing cycle might not be needed, which could save fresh water.”

Shin adds: “This study could also shed light on other similar applications that require the removal of particles and droplets from deep pores through the use of chemical gradients, such as in petroleum production or skin pore cleansing.”

The research is described in Physical Review Applied.

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