What Makes Air-dried Towels So Stiff?

Main Points

Hanging your laundry outside in the sunshine feels refreshing, doesn’t it? It’s an eco-friendly way to save energy (clothes dryers consume a lot of electricity) and gives fabrics a fresh, natural scent. However, air drying isn’t ideal for every item, and one of the most affected is the bath towel.

Why does a towel dried outdoors become as stiff and rough as beef jerky? This question has puzzled scientists for years, but a team from Hokkaido University and the Kao Corporation in Japan believes they’ve uncovered the secret behind air drying—and in doing so, they’ve gained new insights into the behavior of water.

At their core, most fabrics (excluding silk and wool) that aren’t synthetic are derived from plants. Cotton comes from the fluffy seed casings of a shrub, while fabrics like rayon, modal, viscose, acetate, and bamboo are made from wood pulp. Plants are rich in cellulose, a compound that strengthens plant cell walls. Cellulose excels at absorbing water, which is why cotton is preferred for towels over polyester. Water molecules bond to cellulose and move along it through capillary action—sometimes even overcoming gravity to do so.

Water, being a polar molecule with positive and negative charges, is highly responsive to electrical forces. In a study published in the February 2020 issue of The Journal of Physical Chemistry C, researchers discovered that individual, cross-linked fibers in air-dried fabrics like cotton towels contain "bound water." This water behaves uniquely as it adheres to surfaces due to their charge, becoming trapped between fibers and causing them to stick together.

The research team’s experiments revealed that bound water on cotton fibers creates "capillary adhesion," causing the tiny fibers to cling to each other. This bonding results in the fabric’s stiffness, as the fibers essentially glue themselves together.

"Bound water exhibits a distinct hydrogen bonding state, unlike regular water," explained Ken-ichiro Murata of Hokkaido University in a press release.

Takako Igarashi from Kao Corporation noted, "While fabric softeners were believed to reduce friction between cotton fibers, our findings reveal that bound water plays a key role in cotton hardening. This offers fresh perspectives on how fabric softeners function and paves the way for improved agents, formulations, and systems."