How do damp clothes dry at room temperature, and can you unshrink wool?

How do damp clothes dry at room temperature?

Water molecules on the clothes are attracted to other water molecules and also to the molecules that make up the clothing. Each water molecule finds itself in a “sticky” environment, which makes it hard for it to escape into the air, but it does have enough energy to move, swapping positions with other water molecules.

Heat can be thought of as energy that molecules possess. The more heat, the greater the amount of energy they have, and the easier they find it to overcome the stickiness of their surroundings.

Temperature is a measure of the average energy that the molecules possess, but a few will have less, and a few more than the average. At a room temperature of 20C, some water molecules will have enough energy to overcome the forces from the other molecules, and will be able to escape from the clothes into the air. As this process continues there will be less and less water on the clothes, until eventually they are dry.

If the temperature is higher than 20C, more molecules will have enough energy to evaporate, and the clothes will dry more quickly. Some of the water molecules that have escaped into the air may fall back on to the clothes and get stuck again. Putting clothes out on a windy day speeds up the drying process, because the wind blows away the water molecules that have escaped, so they are less likely to get stuck again.

Why does cold water sputter and fizz when sprayed on a very hot surface?

“Normally, water will boil at 100C. If the temperature of the surface is much greater than that, the water will “evaporate”, in other words boil, very rapidly. This evaporating water forms a cushion of air that the blobs of water can float around on.

As they float around, more and more of their water evaporates and replaces the cushion of air that is gradually lost. This in turn creates a large amount of turbulence underneath the blob of water and it is this turbulence that sends it speeding all over the tin plate.

In an aeroplane at 30,000ft, you can often see water droplets, even though the temperature is below freezing point. How can this be?

Water can exist as a liquid below 0C, and often does. There are two main reasons. Impurities in water reduce its freezing temperature, which is why salt is put on roads in winter. Adding salt can lower the freezing point as far as -13C. Pure liquid water can also exist below 0C. For water to freeze, molecules must form the correct structure. A container, such as a cup or bowl, offers plenty of chances for this structure to form. With a small amount of water, the structure is less likely. Hence drops of liquid water can exist in the atmosphere at temperatures as low as -40C.

Is it possible to ‘unshrink’ a wool garment that has been shrunk in the tumble dryer?

Sorry, no. But you might want to know why shrinking happens in the first place...

Two conditions are required for wool to shrink: water and heat. The outside of the wool fibre is hydrophobic (it repels water). The inside of the wool fibre is hollow, and hydrophilic (it absorbs water). Even though wool wants to repel water around the fibre, at a certain point of getting wet it reverses its process of repelling water to one of absorbing water. When this happens, it brings water into its core and holds it.

At the microscopic level, the outside of the wool fibres look as if they are made from overlapping filaments – a bit like tiles on the roof of your house. When wool gets wet and heat and agitation are present, these filaments have a tendency to overlap and lock together. Result: the wool shrinks.

The only way to avoid shrinking is to let wool dry in a cool area, and block or stretch out the fibres when the fabric is still wet. Dry cleaning is the surest means of cleaning without shrinking. You can hand wash, but the clothing must be cared for properly. The only way you can safely wash wool is if it has been chemically treated to “descale” the fibre.