Drying clothes
In a washing machine, why do clothes not get totally dried even when the machine is put in spin-dry mode?
MOHAN J.
Chennai
When wet clothes are spread under dry shade, they get very slowly dried by slow evaporation whereas under bright sun light, they get reasonably quickly dried by brisk evaporation of the water. If the wet clothes are exposed to high dry temperatures, (slightly above the boiling point of water, i.e., 100{+o}C), like when we iron them, they get near-instantly dried by boiling away of the water. In washing machines during spin-dry mode, the wet clothes are dried by throwing away the water by a mechanical force known as centrifugal force but not by evaporation.
Remember, the centrifugal force operates on a revolving body in an outwardly direction from the axis and is akin to the type of throwing force that we experience when we ride on a merry-go-round or that when our bus takes a fast circular turn on a highway ghat road. The higher the mass and the speed of revolution (and/or farther from the axis) the higher is the amount of centrifugal force.
Wet clothes have the water in them held by more than one means; majority of water is held by mechanical retention whereas considerable by capillarity in the crisscross spaces of the network of the fabric. As water, held by these two modes, is essentially in lumps, this part of water can be easily thrown away through the sieve-like holes of the spin-dry drum by the centrifugal force that comes into action when the spin-dry mode is on.
But there is yet another portion of water in wet clothes that is held in the interiors of the fabric by a weak chemical force known as hydrogen bond. Even though a single hydrogen bond is weak, it counts in a collective action. There are a lot of terminal –OH groups on the chemical (cellulose) framework of cotton fabric and –NH- or –CO- groups in wool and polyester fabrics. These chemical moieties make multidirectional hydrogen bonds with water molecules and thus some part of water is held very intimately at molecular level in wet clothes. This force is too strong to be broken by the centrifugal force during spin-drying. Hence, wet clothes do not get completely dried in a washing machine even when the machine is put in spin-dry mode.
Prof. A. RAMACHANDRAIAH
Department of Chemistry
National Institute of Technology
Warangal, Andhra Pradesh
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