The science behind a good wash

“I put my favourite woollen sweater in the washing machine, expecting it to be cleaned. I did not expect it to shrink. Now, it barely fits anymore,” says Lipi Akter, a homemaker from Mugda, Dhaka.

A washing machine is supposed to clean clothes. However, the process may have some unwanted side effects too. A cleaned shirt may become faded, or a sweater may no longer fit. How can something designed to remove dirt also damage the clothes? To understand why these things happen, we must first follow a load through the machine.

How clothes are cleaned

Professor Muhammad Abdur Rahman Bhuiyan of the Department of Textile Engineering at Dhaka University of Engineering & Technology says effective washing depends on two things working together: detergent and movement. Neither can be used excessively without negative effects on the fabric.

Imagine putting a sweaty cotton T-shirt, dusty pair of jeans and a polyester sports jersey into a machine. Once a cleaning routine starts, water enters through an inlet and detergent mixes with it. The clothes begin moving through the water. In a front-loader machine, the rotating drum lifts them and lets them fall. Some top-loaders instead move them with an agitator or impeller.

According to Bhuiyan, the washing process starts with loading the clothes and adding detergent, followed by water entering through an inlet valve. The machine then moves the clothes at a controlled speed to loosen dirt. Once the wash is complete, the dirty water is drained, the clothes are rinsed, and a final high-speed spin removes excess moisture.

Each stage has a different purpose. While washing loosens dirt, rinsing carries it away. The final spin mainly removes excess water at the end of cleaning cycle.

Detergents are essential

Water alone can can carry away some sweat and loose dust. But body oils and greasy stains cling to the fabric and no amount of mechanical force or water can remove them adequately. Detergent contains cleaning ingredients called surfactants, which help water spread through clothing and loosen those oily marks.

Bhuiyan explains that surfactants have two parts, one attracted to water and the other to oil and grease. When detergent comes into contact with dirty fabric, the oil-attracting part attaches to greasy particles, while the water-attracting part helps lift them away from the fibres and disperse them in the wash water.

Once loosened, oily dirt can remain dispersed in the wash water instead of immediately settling back on the cloth. Moving water also carries away particles of dust. That is why draining and rinsing matter as much as the first wash: dirt has to leave the machine, not merely leave the shirt.

The force conundrum

The movement that makes washing effective also has a downside. As clothes move inside the drum, they rub against one another and the machine’s surface. While this friction helps dislodge dirt, Bhuiyan points out that excessive rubbing can loosen fibres, causing them to gather into tiny balls known as pilling.

Over time, this can weaken the fabric and affect its original appearance.

This does not affect every kind of clothing the same way. Cotton is absorbent and usually tolerates everyday washing, but a dyed T-shirt can fade and certain cotton garments can shrink. Denim is sturdy, although repeated rubbing may wear down its surface colour. A polyester sports jersey may hold on to body oils and odours, yet aggressive heat is not necessarily the answer.

Wool is particularly sensitive to the wrong kind of movement. Each fibre has tiny scales on its surface. In water, rubbing can make those scales catch on neighbouring fibres. The fibres become more tightly packed, and the garment shrinks. This process, known as felting, is one possible explanation for what happened to Akter’s sweater, although her wash settings are unknown.

Different modes for different fabrics

A 2008 study in Ultrasonics Sonochemistry compared different ways of agitating wool during laundering. Researchers Christopher Hurren, Peter Cookson and Xungai Wang found that experimental ultrasonic washing caused less shifting of fibres and less felting than hand washing, while removing more of the tested stains. The experiment did not test ordinary home washing machines, but it showed why the kind of movement matters.

Some woollen garments are specially treated to resist shrinkage and can be machine washed if their care labels permit it. Silk shirts and delicate blouses may also need gentler handling, depending on their weave, dyes and finishing. The label matters as much as the fibre name.

Modern machines offer programmes to manage these differences. Cotton, synthetics, wool and delicate cycles may use different temperatures, drum movements, washing times and spin speeds. A quick wash is generally meant for a smaller, lightly soiled load. Load sensing can help some machines adjust the water level, but it does not mean they can automatically recognise every fabric.

Proper practices matter

Even simple choices make a difference. “If we put too many clothes into a machine, they become crowded, and water and detergent cannot move properly between them,” Bhuiyan says. “If we use too much detergent, more rinsing may be needed. Otherwise, detergent can remain in the clothes. The aim is to get them clean without damaging their properties.”

A clean garment is not necessarily a well-washed one, especially if it comes out faded, stretched or smaller than before. Akter’s sweater is a reminder that what happens inside the drum matters just as much as what comes out of it. After all, a good wash should take the dirt out of clothes, not the life out of them.