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Wicking in twisted yarns

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Abstract

This paper investigated vertical wicking in twisted yarns. A mathematical model was developed based on a capillary penetration mechanism. By using a macroscopic force balance approach, the wicking time was derived as a function of the capillary rise of the liquid. Packing of fibers in the yarn was assumed to be uniform. In order to validate our model, a series of experiments was conducted on polyester yarns. The results showed a good agreement between the experimental data and the theoretical predictions. The influence of twist level of the yarn on the capillary flow was also investigated.

Graphical abstract

Wicking behavior of a liquid in a twisted yarn can be described by the theory developed in this paper.

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Introduction

Moisture/liquid transport in textile fabrics is one of the critical factors affecting physiological comfort. Fabrics that rapidly transport moisture/liquid away from the surface of the skin make wearers feel more comfortable by keeping the skin dry. In conditions where wearers sweat a lot (e.g., high level bodily activity), it is not only desirable for the fabric next to the skin to absorb liquid rapidly but also to transport it through the fabric promptly to avoid the discomfort of the fabric sticking to the skin. The comfort afforded by textile fabrics can be improved by understanding the liquid transport mechanism. Mathematical modeling of surface-tension-driven flow in yarns and fabrics can provide a way to develop such an understanding.

In capillary flow through textile fabrics, the constitute yarns are responsible for the main portion of the wicking action [1], [2], and therefore many researches have been conducted to study the wicking behavior in textile yarns. Among the extensive research in this field, textile yarns were treated either as porous media [3], [4], [5], the liquid transport through which can be described by Darcy's law [6], [7], or as capillary tubes [8], [9], [10], [11], [12], [13], [14], the liquid flow through which can be modeled by Lucas–Washburn kinetics [13], [14]. In the first case, however, the characteristic parameters, such as permeability, are difficult to quantify and are always obtained empirically. In the second case, similarly, the effective radius of the capillary tube, the effective contact angle, etc., are also determined by fitting the experimental data. An extensive literature review shows that although broad research has been carried out in this area, a comprehensive model to simulate capillary flow through textile yarns is still lacking. And a qualitative study on influence of twist level on the wicking behavior of liquid in twisted yarn is not available.

In this paper, capillary flow through twisted yarn was studied. A theoretical model was developed based on macroscopic force balance analysis of the liquid. A twist coefficient was introduced to consider the influence of twist of the yarn on the wicking mechanism. Experimental validation showed that our model can predict the capillary rise with a reasonable accuracy.

Section snippets

Model development

In the model for the wicking of a liquid in a vertically positioned twisted yarn, the following forces determine the movement of the liquid:capillary force:Fc,gravity: Fg=ρlgAL,viscous drag: Fv=λkLdLdt,inertia: Fi=ddt[ρlALdLdt], where ρl is density of liquid, g is gravitational acceleration, A is area available for liquid flow in yarn cross section, L is capillary rise and k is frictional coefficient. In order to take the twist of the yarn into account, a twist coefficient λ is introduced into

Experimental

In order to validate our theoretical model, a series of experiments on polyester yarn was conducted with distilled water as the wicking liquid. Radii of fibers were assumed to be identical. The experimental apparatus is shown in Fig. 2. In the apparatus, the lab jack was used to hoist the liquid reservoir containing the wicking liquid, and the steel ruler to measure the wicking height. In order to apply different tensions to the yarn, a pulley was employed with one big weight (e.g. 50 g)

Results and discussion

The experimental data is presented and discussed in this section. The average radius of the fiber rf¯ is 8.4 μm and the density of the polyester fiber ρf is 1.38 g/cm3. The yarn is composed of 37 fibers. All yarns have a length of 400 mm. Distilled water was used as the wicking liquid. Density of the distilled water ρl is 1000 kg/m3, surface tension γ72mN/m, and advancing contact angle θa75.75°.

In the fiber bundle test, the capillary rise of the liquid at equilibrium with different tensions

Conclusion

The wicking behavior of liquid in a twisted yarn was investigated in this paper. Using a macroscopic force balance method, wicking time was obtained as a function of capillary rise. In order to analyze the effect of twist on the wicking of the liquid, a twist coefficient was introduced into the viscous drag term. A piece of experimental apparatus was designed, and a series of experiments was conducted on it. Data analysis showed that the wicking flow can be accurately described by the model

Acknowledgement

The authors wish to express their thanks to funding support from The Hong Kong Polytechnic University.

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