Scientists Just Figured Out How Washing Machines Work?!

Scientists Just Figured Out How Washing Machines Work?!


[♪ 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
“eaten” in 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 the “stagnant
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]