WE鈥橵E been washing our clothes for millennia. Homer even mentioned the chore in the Odyssey. But it鈥檚 always been a puzzle how washing gets the dirt out of clothes so quickly. Now Dutch researchers think they鈥檝e worked it out鈥攁nd cleaner clothes could be on the way as a result.
Until now, most people thought that the tiny pores that trap dirt particles between the fibres in your clothing were too narrow for soapy water to flow through. The rationale went something like this: when the water wets the walls of a pore the water molecules form a 鈥渂oundary layer鈥� of stationary water, that halts the flow in its tracks. True, detergent lowers the surface tension of the water, but not by enough to clear the blockage and let the soapy water pour through.
But if this was what actually happened, the only mechanism for getting the dirt out would be by diffusion of the water through the boundary layer inside the pores. This would work鈥攂ut would take an age. So how come washing machines and even hand washing gets the job done in only 10 to 20 minutes? 鈥淚t was a mystery,鈥� agrees Jacques van der Donck, a detergent expert at the Netherlands Organisation for Applied Scientific Research in Delft.
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Annemoon Timmerman and her colleagues at the Delft University of Technology now think they know what鈥檚 happening. And it鈥檚 counterintuitive. They have worked out that when the pores in fabric are below a certain size, soapy water can get through. She thinks that a similar process gets the grime out from between fibres in our clothes.
Timmerman discovered the phenomenon after making a computer model of a single pore. At first, when she tried to push a soapy water mixture through the pore, the water remained steadfastly stuck. But when she tried reducing the pore鈥檚 size, she hit a critical diameter where the boundary layers breaks down and the water suddenly gushes past.
Timmerman thinks that the switch occurs because at small pore sizes, flow without boundary layers becomes the most disordered state for the system. It takes less energy to lose the boundary layers than to resist the force pushing water through. She confirmed her idea by forcing a soap solution through a polycarbonate sheet peppered with pores of the critical size. The liquid went through much faster than you would expect if only diffusion was at work.
Once the liquid flows into a pore, the detergent gets to work. Detergent molecules build up behind a piece of dirt and the lack of detergent in front of it creates a difference in surface tension between the two sides. 鈥淲hen you have a force difference over a distance you have a torque,鈥� says van der Donck. This torque rips the dirt off the fabric, he adds.
If the idea is confirmed, it will help us design better detergents, says Vincent Nierstrasz, a detergent expert at Twente University, also in the Netherlands. 鈥淚t is an interesting possibility. She has some interesting clues,鈥� he says of Timmerman鈥檚 work.
Knowing the importance of pore size might mean some researchers try to dream up super-washable clothes. But the very tight weave they鈥檇 need means they are unlikely to happen. 鈥淢aking the pores too small affects water absorption and ventilation,鈥� says Nierstrasz. Few people would want super-clean clothes that were too sticky to wear.