iMerino
Wicking
Wicking
Wicking describes the movement of moisture within a fabric by capillary action. In some situations, a weakly wicking fabric is appropriate, while in others a strongly wicking fabric will be favoured. Merino is weakly wicking in its natural state, which makes it especially suitable for socks, but this wicking behaviour can be increased or decreased by the application of different finishing treatments.
Surface phenomenon
Wicking involves only the external surface of a fibre, and this surface can be either hydrophilic if it attracts liquid water or hydrophobic if it is water-repellent.
Capillary action
The parallel alignment of fibres in yarns and the small physical dimensions of the spaces between them mean that fabric wicking behaviour is governed by similar principles to the wicking of liquids in capillaries. The key drivers in this process are the contact angle of the liquid on the fibre surface, the surface tension of the liquid and the physical size of the capillaries themselves.
The influence of surface energy
Most apparel fibres have similar surface energies, with the notable exception of cellulosic fibres such as cotton, linen and flax (Fig 1).
The high surface energy of cellulosic fibres is the reason they are used widely for water-absorbing products such as bath and tea towels.
Modifying wicking behaviour
It is relatively easy to modify wicking behaviour by increasing or decreasing fibre surface energy. Active sportswear fabrics are commonly treated with hydrophilic agents to increase their surface energy and hence increase the ability of the fabric to absorb sweat off the skin. At the other end of the spectrum, hydrophobic polymers such as silicons and fluorocarbons are used to reduce the surface energy of fabrics to stop wicking altogether for end-uses such as rainwear.
Reducing wicking in merino
In its natural state, the outer surface of the merino fibre is covered with a layer of lipids or waxy materials that are relatively hydrophobic. Merino fabrics manufactured from fibres that have not been chemically treated do not tend to wick water or only very poorly. Some traditional merino products used before modern surface treatments were available involved coating the fabric with a natural grease to enhance the water repellence.
Scandinavian fisherman used this trick on their merino gloves.
Increasing wicking in merino
If the lipids are removed by processes such as chlorination or oxidised by plasma treatment the fibre surface becomes quite hydrophilic. Merino fabrics treated in this way wick water quite well. Depending on the nature of the treatment, machine-wash treated merino fabrics often wick noticeably better than their untreated equivalents. The merino used on the inner face of Sportwool™ for example is machine-wash treated. Sportwool™ fabrics are treated after manufacture with a hydrophilic agent that substantially increases the surface energy of the outer polyester face. This difference in surface energy between the treated polyester and wool is what drives the one-way wicking behaviour of Sportwool™ (Fig 2).
Wicking and drying
Wicking is also a key factor in the drying of fabrics. It is a popular misconception that synthetic fabrics dry more quickly than their natural counterparts. In fact the rate of evaporation from fabrics depends on the prevailing climate conditions and structure of the fabric and is quite independent of the fibres involved. Drying time is determined by the amount of water in the fabric that must be evaporated. After a wash and spin-dry cycle, fabrics that wick strongly retain more water and take longer to dry than poorly wicking fabrics.
Wicking in socks
Laboratory simulation
Tests were done to simulate the transfer of sweat from the foot through the sock and shoe to the surroundings. They showed that if the sock was less strongly wicking than the shoe upper, sweat could easily transfer to the shoe upper and then evaporateto the surroundings. Conversely, if the sock was more strongly wicking than the upper, then the escape of moisture was restricted to the slow process of diffusion as vapour. Merino is therefore the ideal fibre for socks because its superior ability to absorb vapour slows the build-up of liquid, and because, being less weakly wicking than most shoe uppers, it is more efficient at transferring moisture to the shoe.
Wearer trials
These experimental findings were confirmed in wearer trials conducted at 33?C and 60% relative humidity in which participants were asked to compare socks of various fabrics in the categories of dampness, warmth, slipperiness, compression and overall comfort. Participants revealed a clear preference for the merino socks over cotton, acrylic and polypropylene, citing in particular their freedom from slipperiness and dampness.
Summary
Merino is weakly wicking in its natural state, which makes it ideal for socks, a fact confirmed by CSIRO wearer trials. However, the wicking behaviour of merino can be increased or decreased for other applications through the use of various finishing treatments.
