Sangwoo Shin, Patrick B. Warren, and Howard A. Stone from the University of Hawaii at Manoa call this nagging problem the "stagnant core mystery." Here's the issue: detergents contain surfactants, which surround otherwise water-repelling particles of oil and grease and make it so water can wash them away. They're what give detergent its stain-penetrating power. The problem is that detergent can only penetrate so far: soapy water can get between the threads in a T-shirt, but it's less effective at burrowing between the tiny fibers that make up each thread. If there are miniscule particles of dirt between those fibers, there's no way that detergent and water should be able to draw them out. And yet, they come out anyway.

But Shin isn't just a laundry hobbyist. He's a mechanical engineer who has published studies on diffusiophoresis: basically, the way a particle will migrate toward increasing or decreasing concentration of a chemical. If that chemical is made up of charged molecules, the differences in concentration will actually create an electric field that moves the particle along. Shin's previous research looked into how to use this phenomenon to insert particles into "dead-end pores," which could be a handy way to deliver drugs.

Wait a minute. Cotton threads are full of dead-end pores. Instead of putting particles into those pores, couldn't that same phenomenon help take them out? With that question, Shin and his team got to work.

For a study published in the journal Physical Review Applied, the research team started with a tiny fluid channel connected to a series of dead-end pores that were each just 50 micrometers wide, or half the width of a human hair, and filled the pores with fake dirt particles. Next, they mixed detergent with water and sent it through the fluid channel. As they expected, the detergent got into the pores, but did nothing to sweep away the dirt lodged within. But when they performed a "rinse cycle" with fresh water, that all changed: the dirt particles began to wash away. After about 10 minutes of rinsing, the pores were almost completely empty.

They followed up this success with an experiment on stained cotton fabric. Both swatches of fabric in the video below were previously cleaned with detergent. On the left, the fabric is rinsed with fresh water; on the right, it's rinsed with detergent-filled water. You can see a clear difference.

That's because when detergent — and its charged surfactant molecules — goes from a high concentration in the wash cycle to a gradually lowered concentration in the rinse cycle, it creates an electric field that draws those dirt particles out. The charged surfactant molecules that surround each particle carry them away through the electric field like a microscopic Maglev train. You may have thought the detergent was what gets your clothes clean, but it'd be nothing without the magical physics of the rinse cycle.

This discovery does more than just help the researchers sleep a little more soundly. Shin and his team discovered that this phenomenon doesn't actually require the water to be moving, which means that laundry machine manufacturers don't need to program in 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 told Cosmos Magazine. Imagine that: what seemed like a whimsical area of study ended up helping conserve natural resources. Science cleans up good.

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