Spreading of water microdroplets simulating human sweating in polyester yarns and fabrics

Thermo-physiological comfort of human body is highly influenced by the moisture transmission behaviour of the fabric. For active sportswear, intimate apparels and inner layer of extreme cold weather clothing, moisture transport is very crucial. The liquid moisture transport of fabrics is analysed using standardised test methods like vertical wicking, absorbency test and moisture management through moisture management tester. These test methods use large reservoir of liquid in contrast to human sweating behaviour. The human skin generates sweats in the form of microdroplets through sweating glands. Hence, the standard test procedures employed to assess the fabric's liquid moisture transport fail to capture preciously the sweat transport from human skin to the fabrics. Hence, in this work, the spreading behaviour of water microdroplets simulating human sweat through yarns which are the building block of fabrics and knitted fabrics that are widely used as sportswear or inner layer of extreme cold weather clothing are investigated. The purpose of this study is to understand how the microfluid move along the capillary channels in yarns (in the forms of network of yarns crossing over each other and fabric) by keeping yarn count and twist same for all yarns. A single micro drop representing accumulated sweat droplets and multiple drops showing continuous sweating through sweat glands were allowed to wick through the yarns made up of coarser and finer fibres to understand the role of fibre diameter on wicking kinetics. Results from the conventional wicking test were compared with microfluidic wicking. It is found that the initial wicking speed in coarser-fibres yarn is faster in comparison to finer-fibres yarn, but the final wicking height is longer in finer fibre yarn. Overall liquid spreading length found to be similar for both the yarns. Yarn to yarn liquid transfer at crossover junctions is faster with finer-fibre yarn compared to coarser-fibre yarn. Same trends are found in fabrics constructed with these yarns. The effect of yarn tortuosity on liquid spreading behaviour has also been studied. As the yarn path becomes torturous (straight yarn to fabric form) wicking length also decreases.