Lather, rinse and rinse again. That鈥檚 the way to remove deep stains, according to a study of the fluid dynamics within fabric-like pores.
The exact mechanism of how dirt particles are removed from fabrics has long eluded us, says study leader at the University of Hawaii at Manoa. While it may seem obvious to rinse your clothes to remove soap, a fresh water rinse does more than that. It creates a gradient of detergent within a material鈥檚 pores that draws dirt particles up and out of the fabric.
Detergents contain a surfactant that can bind to dirt particles and loosen them from fabric. But this doesn鈥檛 explain how particles deep down in the pores between fibre yarns are removed, as only about 0.1 per cent聽of flowing water reaches there. The dirt lodged there should take a lot of rinsing to break free, but it doesn鈥檛.
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To find out why that is, Shin and his colleagues carved microchannels about 50 micrometres wide in a polymer and filled them with tiny fluorescent polystyrene beads in a soap solution to mimic dirt particles.
Dirt on the move
When they ran soapy water across them, it only removed particles near the mouth of the channel, whereas flowing fresh, non-soapy water removed most of the particles, even deep down in these pores, after about 10 minutes.
They found that fresh water creates a gradient in the concentration of surfactant from the bottom to the top of the channel. The unequal distribution of ions聽produces an electric field that drives the motion of dirt particles bound up in soap. So, particles from a region of high concentration, deep in the channel, are drawn to the low concentration region at the top.
These results provide a physical framework for explaining how dirt is removed quickly even when embedded deep into clothes, says who works on paper microfluidics at the Indian Institute of Science in Bangalore. He says these results could help develop designs for washing away unwanted molecules efficiently in paper-based or textile-based medical diagnostic tools.
Physical Review Applied
Read more: Dishing the dirt: How clean does your home really need to be?
