Science & Technology

Why Is Tumble-Dried Laundry Softer Than Air-Dried Laundry?

Whether it's because the dryer broke, you were trying to save energy, or you just left your wet towel in the sun after a dip in the pool, pretty much everyone has felt the uncomfortable crunch of air-dried fabric. What exactly is it about letting laundry dry naturally that makes it so stiff and scratchy? And what sorcery goes on inside a dryer to make clothes so supple and soft? And most importantly, is there a way to make air-dried fabric softer? To understand, we need to zoom way in on the fabric fibers.

We Gotta Stick Together

Cotton and rayon fibers are made up of a natural polymer called cellulose. It's the same substance people extract from wood to make paper — in fact, while cotton is from, well, cotton plants, rayon is actually produced from wood. But by the time the wood or cotton is processed enough to create fabric, the fibers are almost completely composed of cellulose.

A triple strand of cellulose showing the hydrogen bonds (cyan lines) between glucose strands.

Cellulose is a polymer, or basically a long chain of similar molecules — in this case, glucose molecules. Those chains are held in place with hydrogen bonding, one of the strongest bonds there is. But not all cellulose is created equal. Cotton cellulose is stronger than cellulose from wood pulp for two reasons. One, it has a higher degree of polymerization, which means each cotton cellulose macromolecule contains a lot of individual units. Two, it has a higher degree of crystallinity, which means that the fiber molecules are closely packed in a parallel arrangement. Put these two qualities together, and you've got one incredibly strong fiber.

Cellulose molecules also contain what are known as hydroxyl groups, or oxygen atoms paired up with hydrogen atoms. As you might guess from their molecular makeup, hydroxyl groups absolutely love water molecules. In the presence of water, they grab every H2O molecule they can. That causes the fibers to swell and those hydrogen bonds to break.

Never fear! As the fabric dries, new hydrogen bonds form. But how they form depends on how the fabric dries. When you hang a cotton towel outside on a sunny, windless day, stiff, sturdy hydrogen bonds form because there's nothing to disturb them. When you dry a cotton towel in an electric dryer, however, the fabric is constantly moving, which keeps the molecules from being able to re-bond as firmly. That makes the fabric softer and fluffier.

I Like to Move It, Move It

But sometimes, air drying is the best option. It's definitely the most eco-friendly option since the electricity it takes to dry six loads of clothes is about the same as it would take to run a blender nonstop for two full days. If you're doing laundry every week, that power adds up.

So how do you line-dry your clothes without suffering through the stiff, scratchy results? Well, for one thing, try using less detergent. Not only does too much detergent actually leave your clothes less clean, but it can also exacerbate the stiffness problem.

But your best trick is to do what you can to keep the fabric moving while it dries. You can periodically shake out the clothes while they're drying, or take advantage of windy days to schedule laundry day. You can also try a hybrid approach: Run your clothes in the dryer on the "fluff" setting for 10 minutes, either before you hang them or when they're almost dry. That can help loosen up those hydrogen bonds to make for fluffier fabrics.

Our final tip? Don't worry about it; the psychological concept of habituation has you covered. That's the decrease in response that your senses experience after they're repeatedly stimulated by the same thing. Just like you eventually stop smelling your own house, you'll eventually stop feeling the texture of the clothes you're wearing. Of course, that won't help with your bath towel. For that, maybe just remember that you're saving the planet one towel at a time — and give yourself a pat on your well-exfoliated back.

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For more everyday explainers, check out "Ask a Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works" by Larry Scheckel. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer May 24, 2019

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