The physics of doing the laundry

Scientists have proved that cleaning dirty clothes is physically possible.

It’s something that housekeepers around the world have known for a long time, but until now, science was stumped as to exactly how washing worked, says Sangwoo Shin, lead author of a study published in the journal Physical Review Applied.

“The key mechanism for laundry detergent, which we have been using for thousands of years, is completely different to what we thought,” says Shin, a researcher at the University of Hawaii, US.

“People in the detergent industry did recognise decades ago that there must be something else going on with fabric cleaning, but it has never been identified.”

The problem concerned dirt in tiny crevices in fabric – pores that can be as narrow as one micron wide and are dead-ends through which water cannot flow.

When the fabric is dry, dirty substances get drawn into the pores by capillary wicking – the combination of surface tension and the adhesive force between the grot and the material.

Often oily and therefore repellent to water, this dirt can only be dislodged by detergents, which form a layer around the particle, negating its water-repelling properties. But rinsing does not flush out the pores because they are tiny culs-de-sac. It was thus assumed that the offending particles were removed only because they drifted out randomly.

But that was a far from satisfying explanation. Calculations of diffusion rates showed that even after being loosened by detergent, dirt particles would need hours to move out of their cosy crevices and be washed away. This clearly contradicted visible evidence achieved by both machine- and hand-washing methods.

Shin’s realisation that washer-people across the ages were not defying the laws of physics came when he began studying the diffusion of particles caused by electric fields (a process known as electrophoresis) and by chemical concentration gradients (chemophoresis). When combined, the result is known as diffusiophoresis.

The trick, it seems, is to rinse with fresh water after using detergent. If there is fresh water at the open end of a pore filled with soapy water, the detergent molecules are drawn out, bringing the dirt with them. Even though there is no fluid flow, tiny electric fields and osmotic pressure push the dirt out and keep doing so until the pore is completely rid of the unwanted particles. The process takes mere minutes.

“It is a peculiarity of the phenomenon that diffusiophoresis depends on the detergent concentration ratio, not absolute values,” Shin explains.

“Our experiments confirm diffusiophoresis continues to operate even when the solute [detergent] is nearly completely vanished.”

Shin and his colleagues demonstrated their discovery with tiny tubes, 48 microns by 10 microns in cross-section, connected to a larger vessel. They used polystyrene balls half a micron across to act as dirt particles and showed that a soapy wash followed by a fresh water rinse in the main tube drew the particles out efficiently, while using soapy water for both wash and rinse had no effect at all.

And that finding has important implications for washing machine manufacturers. The rinsing doesn’t require liquid flow, so there is no need for multiple rinse cycles.

“This suggests we can simply leave the water for over 20 minutes without exchanging water for multiple rounds of freshwater rinse, which could eventually help to save water,” Shin says.