Cotton towels that are air-dried often become as stiff as wood after drying, even with fabric softener, yet the same towel, when washed and tumble-dried, feels quite soft.
Many of us typically wash clothes and hang them in the sunlight to dry naturally, while others prefer to wash and dry clothes using a machine. However, a phenomenon that leaves many puzzled is why cotton fabrics, especially towels, feel soft when machine-dried but become hard as wood when air-dried on a line?
Natural cotton towels air-dried become as stiff as wood. (Photo: Getty Images)
This question puzzled scientists for some time until a research team from Hokkaido University and Kao Corporation in Japan unraveled the mystery, providing new insights into the unique behavior of water on the surface of materials, thereby aiding in the development of better fabric softening technologies.
According to the research, a residual amount of water on the surface of cotton fabric has caused individual cotton fibers to bond together, resulting in stiffness after natural drying.
Even in the driest climates, cotton retains natural moisture because its main component, cellulose, attracts water molecules. At a temperature of 25 degrees Celsius and a relative humidity of 60%, about 8% of the weight of cotton comprises water.
Cotton fibers on a towel. (Source: iStock).
Explaining this, scientists noted that most fabrics (except silk and wool) that are not made from plastics are derived from plants. Cotton is made from the fibrous shell of a small shrub, while rayon, modal, viscose, acetate, bamboo, and similar fabrics are made from wood pulp.
Plants are rich in cellulose, an organic compound that plays a crucial role in providing structure to plant cells, helping them maintain their shape and rigidity. Cellulose absorbs water very effectively, which is why towel manufacturers often prefer cotton over polyester.
Water molecules cling to cellulose and move along it through a process known as capillarity. Cellulose can even defy gravity to pull water along its surface.
Meanwhile, water is a polar molecule, meaning one side carries a positive charge while the other carries a negative charge, making water easily attracted to electrical charges.
The research team discovered that individual cross-linked fibers in air-dried fabrics, such as cotton towels, actually have “bound water” between the fibers, causing them to stick together.
Experiments conducted by the research team found that the bound water on the surface of cotton fibers creates “capillary adhesion”—a phenomenon where liquid trapped between solid surfaces leads to cohesion, resulting in stiffness as the fibers adhere to one another.
Illustration of individual cotton fibers bound together by water. (Source: AIP).
Researcher Takako Igarashi from Kao Corporation stated that bound water in this state is stickier than regular water, similar to hair spray.
In hair spray, a polymer binds hair strands together, while in air-dried towels, bound water holds cotton fibers tightly together. This occurs through a network of strong hydrogen bonds, between water molecules and between the bound water and hydroxyl groups (composed of one oxygen atom and one hydrogen atom) in cellulose.
The research team also found that mechanical forces—occurring when clothes are tossed around in a dryer—make fabrics softer, potentially due to the disruption of hydrogen bonds in bound water. They also discovered that when all water is removed from air-dried towels through vacuum heating, the towels become fluffy again.
Takako Igarashi mentioned that understanding the impact of bound water on cotton fabric stiffness will provide deeper insights into how fabric softeners work, thus helping them design improved softening products.
