Characterizing the transplanar and in-plane water transport properties of fabrics under different sweat rate: Forced Flow Water Transport Tester

The water absorption and transport properties of fabrics are critical to wear comfort, especially for sportswear and protective clothing. A new testing apparatus, namely Forced Flow Water Transport Tester (FFWTT), was developed for characterizing the transplanar and in-plane wicking properties of fabrics based on gravimetric and image analysis technique. The uniqueness of this instrument is that the rate of water supply is adjustable to simulate varying sweat rates with reference to the specific end-use conditions ranging from sitting, walking, running to other strenuous activities. This instrument is versatile in terms of the types of fabrics that can be tested. Twenty four types of fabrics with varying constructions and surface finishes were tested. The results showed that FFWTT was highly sensitive and reproducible in differentiating these fabrics and it suggests that water absorption and transport properties of fabrics are sweat rate-dependent. Additionally, two graphic methods were proposed to map the direction of liquid transport and its relation to skin wetness, which provides easy and direct comparison among different fabrics. Correlation analysis showed that FFWTT results have strong correlation with subjective wetness sensation, implying validity and usefulness of the instrument.


Test methods
Testing principles Measurement parameters Spontaneous Uptake Water Transport Tester (SUWTT) 1 Water was supplied to the sample continuously which simulates profuse sweating. The mass of water supply depends on the absorbency of the sample. In 1-layer set up, fabric was placed onto the sample podium and its water absorption rate, spreading area and water content was recorded. In 3 layer set up, the test fabric was placed inbetween two filter papers and the distribution of water in each layer was measured. Gravimetric and image analysis technique were adopted for the measurement.
1-layer set up -Water absorption rate (g/s) -Spreading area of fabric (cm 2 ) -Water content of fabric 3-layer set up -Transplanar ratio -Mass of water absorbed by bottom filter paper (g) Wettability test (AATCC 79) 2 A drop of water was delivered from a fixed height onto the test sample. The time it takes for the drop of water to disappear was taken as a measure of the wettability of fabric and it was recorded by visual observation. The shorter the time, the much wettable the fabric is.
-Water absorption time (s) Vertical wicking test (AATCC 197) 3 A preconditioned strip of the specimen was suspended vertically with its lower end immersed in a reservoir of distilled water and the height of water reached in the fabric against gravity was visually observed and recorded after a fixed time. The initial and extended wicking rate, expressed in mm/s, indicates the average speed of water to reach 20 and 150 mm height, respectively.
-Initial wicking rate, 20 mm divided by time spent (mm/s) -Extended wicking rate, 150 mm divided by time spent (mm/s) Horizontal wicking test 4 A fixed amount of water was supplied at the bottom side of fabric at a constant rate (10 ml/h). A camera, standing on top of the set up, was utilized to capture the image of the wetted sample and the water spreading area was measured.
-Horizontal wicking area (cm 2 ) Moisture management tester (AATCC 195) 5 The sample was put in-between two sets of metal electrodes and a fixed quantity of liquid was dropped onto the back side of the fabric and the direction of water spread was traced automatically by the metal electrodes.
-Overall (liquid) moisture management capability (OMMC) Water absorption capacity test 4 Fabric was put into a tank of water and 5 minutes was allowed for it to sink completely into the water. The fabric was then taken out by tweezers and hung onto a rod vertically until there was no water dripping within a 30-second interval. The water gain in fabric was measured and it is expressed as mass of water gain per unit gram of fabric in percentage.
-Wet pick-up (%)  In general, a higher pressure loading may give highly reproducible results. However, a higher loading pressure may reduce the differences of testing results between different fabrics compared to a lower loading pressure.     Figure S3.

Accuracy of FFWTT
The correlation between FFWTT and SUWTT is shown in Supplementary Figure S3  Fabrics with higher water absorption capacity or faster absorption contribute to higher amount of water absorbed by fabric. Figure S3

Validity of FFWTT
In order to examine the validity of the instrument, the FFWTT results were correlated with the subjective wetness sensation and it was assessed in accordance with Tang et al.'s method 25 . In their study, water was applied to the fabric at a constant flow rate and the amount of water required to trigger wet sensation, which depends on wetting and wicking property of the fabric, was evaluated. The much the water required to trigger wet sensation, the more comfortable the fabric is. They found that human is not capable of differentiating between hydrophilic and hydrophobic fabrics, and either one (i.e. hydrophilic or hydrophobic groups) should be interpreted at a time.
Wettability test (AATCC 79) was performed to define the hydrophobicity of these fabrics. Fabrics whose water absorption time exceeds 60 seconds are defined as hydrophobic while hydrophilic fabric refers to absorbing substrate with water absorption time shorter than 60 seconds. As a result, these 24 fabrics were separated into two groups and hydrophobic fabrics were eliminated for further analysed.
Supplementary Figure S4  strongly and positively correlated, suggesting that FFWTT has high validity in estimating subjective wetness sensation.
Apart from the psychophysical measurement, the percentage of water left on skin after the subjective test (as calculated by equation (1)), an indirect physical measurement reflecting skin wetness, was correlated with FFWTT results.
Supplementary Figure S4(b) shows that the percentage of water left on skin has positive relationship with the fraction of water absorbed by bottom filter paper by FFWTT. The direction of correlation is rational and it implies that the bottom filter paper can simulate our skin condition quite well.
Percentage of residual water left on skin, P % Injection amount g Amount of water absorbed by fabric g Injection amount g 100% (1) show the data measured at 3 ml/h water flow while green triangles and purple squares denote the result measured at 10 ml/h and 40 ml/h water flow, respectively. (a): Absolute threshold amount of water required to trigger wet sensation was plotted against fraction of water absorbed by fabric layer in FFWTT. The data points were fitted with linear function while the hydrophobic fabrics (marked in hollow shape) were eliminated from the model. The reason for not including these samples into the model is that people cannot differentiate the wetness level between hydrophilic and hydrophobic samples. This phenomenon can attribute to the different mechanisms involved to evoke the wet sensation. For hydrophilic fabrics, water spreads widely, evaporates quickly (dissipates body heat) and sticks closely to the skin which stimulates the cold receptors and mechanoreceptors and so boosting the wet sensation. For hydrophobic fabrics, wet sensation may attribute to the formation of water film between fabric-skin interface or the detection of dipping water. (b): Percentage of water left on skin after subjective assessment was plotted against fraction of water absorbed by bottom filter paper in FFWTT. The data points were fitted with linear function. The amount of water left on skin depends on water absorption and transport properties of the fabrics. For those hydrophobic fabrics, water cannot be absorbed by the fabric and majority of water may leave on skin, contributing to higher percentage of water left on skin. Inversely, for the hydrophilic fabrics, water was readily absorbed by the fabric contributing to lower percentage of water left on skin.

Uncertainty of different measurement parameters
Some parameters such as water content and transplanar ratio cannot be measured in a single measurement. In fact, several physical factors are involved and the uncertainty of these parameters can be calculated from the uncertainty of each direct measurement 26 . For the fraction of water absorbed by each layer, the water absorption amount by a specific layer is divided by and interrelated with the total absorption amount of the three layers, so the uncertainty of a specific layer is calculated by the sum of fractional uncertainties in two direct measurements, as shown in equation (2). For the transplanar ratio, the water absorption amount by top filter paper is divided by and interrelates with the water absorption amount in the bottom filter paper, so the uncertainty of transplanar ratio is calculated similarly by equation (3). The calculation of uncertainty of water content is based on the assumption that all errors are independent and random, and it is computed by the quadratic sum according to equation (4). The average uncertainty of each parameter is summarised in Supplementary Table S9. When considering the hydrophobic fabrics, the uncertainty of these parameters is particularly high. It might be due to the little amount of water absorbed by the fabric as well as by the top filter paper.     Fabric saturation is considered, but the initial wetting performance is not examined.
The initial water absorption rate cannot be measured. Spectroscopic method (e.g. MRI and NMR) Expensive equipment and complicated handling 