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  • [♪ INTRO]

  • There are all kinds of inventions that make modern living possible, but some fly under the radar.

  • Some so much so that we've only just figured out how they truly work.

  • Case in point: the washing machine.

  • Because apparently, no published paper was able to totally explain

  • how these things got your clothes clean; not until 2018.

  • Now, to be clear, it's not like we had no idea how these machines worked.

  • The oldest washing machines that resemble our modern appliances date back to the 19th century,

  • so these things have been around for quite a while.

  • And for years, we've had a really good understanding of

  • how they use soap and detergent to get stuff off the surface of your clothes.

  • Soap is a surfactant, which is short for surface active agent.

  • That means its molecules can attach to two substances that don't normally interact,

  • for example, oil and water.

  • Usually, one end of the molecule is hydrophobic, meaning it repels water,

  • and the other end is hydrophilic and regularly bonds with water.

  • So, when soap is dumped into a big bath of water,

  • the surfactant molecules group together to form spherical structures called micelles.

  • The hydrophilic ends stick out toward the water, and the hydrophobic ends hide inside.

  • When a micelle lands on a dirty section of fabric,

  • the hydrophobic ends pop out and attach to the dirt, or whatever is soiling your clothes.

  • Then, the hydrophilic ends pull the rest of the micelle off the fabric's surface,

  • and the micelle reforms with the soil it's “eatenin its center.

  • Detergents like the ones we use for washing clothes use anionic surfactants,

  • meaning the hydrophilic end has a negative electrical charge.

  • But, while there are different types of surfactants, they generally all work the same way.

  • The reason washing machines spin everything around is to help the soap solution

  • flow through your fabric and pick up all the dirt hiding in the crevices.

  • But here's where the mystery came in: Water can't flow through every spot in your clothes.

  • That's because fabric is usually made of yarn, which is itself made of multiple fibers.

  • Soap generally has no problem getting into inter-yarn pores,

  • or the spaces between separate strands of yarn.

  • But it does have a problem getting into intra-yarn pores,

  • or the spaces between the fibers in a single strand.

  • These pores are at least an order of magnitude smaller than the inter-yarn ones,

  • and only about 0.1 percent of the soap solution can actually get inside them,

  • and even then, it doesn't get in all the way.

  • Micelles actually get stuck in there and are only able to move

  • when they're struck by nearby water molecules.

  • According to the math, all of the micelles would eventually get knocked out,

  • and that soap and dirt would go flowing down the drain.

  • But this process would take several hours.

  • And that's just not how washing machines work.

  • They get the soap and crud off your clothes in usually under one hour.

  • This phenomenon was called thestagnant core problem”,

  • and its what scientists hadn't been able to explain until that 2018 paper.

  • In their research, the authors learned that the solution to

  • getting dirt out of those intra-yarn pores wasn't the soap, which is kind of surprising.

  • Instead, it was the rinse cycle, along with a process called diffusiophoresis.

  • Diffusiophoresis is the movement of colloidal particles caused by a gradient.

  • In other words, it's the movement of tiny particles suspended but not dissolved in a fluid.

  • When you swap out the soapy water with the clean stuff,

  • the surfactant micelles are way more concentrated in the fiber pores than they are elsewhere.

  • And when you're using anionic surfactants, this creates an electric field

  • that makes the micelles migrate out of those intra-yarn pores.

  • In their paper, scientists figured this out by doing a series of experiments.

  • Instead of dirt, they used micrometer-sized, fluorescent balls,

  • and they picked a standard detergent called sodium dodecyl sulfate.

  • It also goes by sodium lauryl sulfate,

  • and you can probably find it in most of the cleaning products you own.

  • The team did one trial with no detergent at all,

  • just to see how much cleaning the water and agitation could do all by itself.

  • Then, they did a trial that rinsed sudsy fabric with detergent-filled water,

  • and no diffusiophoresis was observed.

  • Finally, their third run mimicked a regular wash/rinse cycle,

  • where the soapy fabric was rinsed with clean water.

  • This time, the team observed that the fabric pores

  • nearly emptied out after just 10 minutes of rinsing,

  • cleaning out the intra-yarn pores over 100 times faster than in the soapy run.

  • So yeah, surfactants get the dirt off your clothes,

  • but you need clean water to actually get dirt out of them.

  • This research doesn't answer all the questions about cleaning your clothes,

  • for example, why some stains are far more persistent than others,

  • but it does have some worthwhile applications.

  • With knowledge like this, we might be able to maximize detergent efficiency

  • and minimize the amount of water and energy we use to do our laundry.

  • That would make washing machines, both in your home and in industry, much better for the planet.

  • Which is kind of a big deal, considering how much water this process takes.

  • On a different note, other research suggests that faster rinsing speeds

  • could also create a better diffusiophoresis effect.

  • So there may be turbo-charged washers in our future that get clothes clean even faster.

  • But, hey. As long as my laundry comes out smelling fresh, I'm all for it.

  • Thanks for watching this episode of SciShow!

  • If you want to learn more about laundry science,

  • you can check out our episode about fabric brighteners.

  • Apparently, they don't just make the colors in your fabric more vibrant,

  • they make your clothes glow.

  • [♪ OUTRO]

[♪ INTRO]

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