Evaluation of water flow in cotton yarn and fabric assemblies for capillary evaporative cooling

Abstract

In cotton yarn bundles and fabric layers, wicking and rate are accounted as crucial indigenous liquid transportation properties, playing a significant role in temperature reduction on their surfaces and being used to extend the food and agricultural storage life. In this article, manual test methods are described to measure water wickability and wicking rate of cotton yarn bundles and plain weave fabric layers. These methods described the water flow through the in-plane surface of yarn bundles and fabric layers oriented in either vertical or horizontal lines without external force. The wicking lengths and wicking growth rates in both untreated and treated cotton yarn bundles and fabric samples in either in-plane vertical or horizontal orientations were compared. The highest to lowest wicking length and rate were found in the treated yarn bundles, treated fabric layers, untreated yarn bundles, and untreated fabric strips, respectively. The wicking height and length obtained in untreated yarn bundles and untreated fabric layers was lower than the Kraft paper. The higher wicking and rate values indicate a considerable potential for liquid water migration. The results indicated that treated yarn bundles and fabric layers that are oriented horizontal direction are the best options for constructing capillary evaporative cooling.

Keywords:

• Fabric layers
• moisture-wicking
• wicking growth rate
• wicking length
• wicking height
• yarn bundles

REVIEWING EDITOR:

• Ian Phillip Jones, University of Birmingham, United Kingdom

SUBJECTS:

• Thermodynamics
• Heat Transfer
• Heating Ventilation & Air Conditioning
• Fluid Mechanic
• Nanobiotechnology

1. Introduction

Temperature and humidity are the most important factors affecting the comfort of human beings and extending the storage life of food and agricultural products. Evaporative cooling is a simple option that could fulfill these requirements (Mekonen et al., 2023; Yeshiwas & Tadele, 2021; Delele et al., 2019).

There are known challenges that are associated with evaporative coolers. It has relatively low coefficient of performance (COP) and difficult to lower the air temperature below a certain limit due to the saturation of the air (Chaomuang et al., 2023; Laknizi et al., 2021). In addition, there were limitations, including mineral deposits and scale buildup, which are caused by hard water that causes rust and corrosion in metal coolers, and relatively high water consumption, which makes it inconvenient to use in areas with water scarcity. Such accumulations of rust and corrosion can ultimately shorten a cooler’s life by about 50 percent. Further, scale buildup on moist materials can cause uneven distribution of water, leading to hot spots on the wetting media and reduced cooling because of reduced airflow, which results in poor achievement of the minimum wet bulb temperature (Velasco-Gómez et al., 2020; Ogbuagu, 2019; Seweh et al., 2016; Xu et al., 2016).

Construction of evaporative coolers with sufficient heat and mass transfer characteristics could be possible from widely available material sources, such as yarn, metals, ceramics, carbon composites, and zeolites (Cui et al., 2020; Velasco-Gómez et al., 2020; Xu et al., 2016). From thermodynamic views of evaporative cooling, the performance of evaporative cooling depends mainly on its moist material characteristics (Fischer et al., 2022) and the air mass flow rate (Chaomuang et al., 2023; Elmsaad et al., 2023). As the air passes over or through the moist material of the porous parts, water evaporates, reducing the airflow temperature (Mekonen et al., 2023; Velasco-Gómez et al., 2020; Yang et al., 2019). The moist material surface type, surface area, air mass flow rates, the drying status of the supply air that passes over the moist material channels, and the water volume are the main factors that directly affect the evaporative cooling system’s efficiency. Therefore, the moist material surface media selection needs careful attention (Lv et al., 2021; Kumar & Arakeri, 2020; Xu et al., 2016). Cotton yarns or cotton fabrics have higher water molecules and vapor adsorption capacity (Li et al., 2023; Lv et al., 2021; Xu et al., 2016). Vapor adsorption-desorption along the cotton yarn bundles or fabric layers increases the cooling air vapor carrying capacity, and hence, they are potential construction materials for the development of capillary absorptive evaporative air coolers with better performance.

Replacing the imbibition-type conventional evaporative coolers with capillary absorptive coolers had lower water consumption, waste, low maintenance, and capital costs (Tejero-González & Franco-Salas, 2021; Velasco-Gómez et al., 2020; Verploegen et al., 2019). The effectiveness of the capillary evaporative cooler operated with direct imbibition could be minimized due to blockage of the airflow channel with mud, corrosion, and mold growth. Removing these barriers could increase the capillary action by increasing the evaporative surface area and the water evaporation rate (Lv et al., 2021; Xu et al., 2016).

Wicking from an infinite reservoir could be achieved through trans-planar wicking (the liquid transport through the fabric thickness), in-plane wicking (vertical and horizontal wicking), and immersion (Mallick & De, 2022). Longitudinal wicking is more important than transverse wicking due to the liquid transport mechanism along the length of the yarn bundle or fabric layer. The higher channel length with a suitable shape, airflow with a higher inlet temperature, lower inlet humidity, and low airflow velocity give higher efficiency and cooling effectiveness (Kapilan et al., 2023; Wilkins & Fumo, 2023).

However, the untreated yarn bundles and fabric strips could potentially impede the wetting and wicking process by blocking the capillary channel to the water molecules (Jain et al., 2021; Menezes & Choudhari, 2011). The untreated materials are rich with natural non-cellulosic materials (waxes, pectin, cuticle, and protein and organic acids) that can create a physical hydrophobic barrier during the development of cotton yarn for the specified applications. The removal of these can be done using appropriate de-sizing, scouring, teaching, and causticisation/mercerization processes, whether in independent or combined conditions (Abate, 2017; Harane & Adivarekar, 2017). By combined treatment and causticisation or mercerization, the cotton fibers could be diverted from twisted structural arrangements into circular (cylindrical) structures, and it increases in water absorption, i.e. rapid wettability, and capillary actions take place when the yarn bundles or fabric layers are in contact with the water.

The yarn bundles/fabric layers treatments improved wicking and water absorbency because such treatment operations decreased the compactness of the yarn bundle structure (Azeem et al., 2017; Cho et al., 2015), and increased the yarn bundle bulkiness. Besides, the pores size which leads to increase the wicking capacity (Mallick & De, 2022; Azeem et al., 2017). In addition, Increasing yarn or fabric openness (Mallick & De, 2022), and bulkiness of the yarn bundle or fabric layer after removing the natural and impurities results in more air gaps in the yarn bundle structure (Mallick & De, 2022; Mallick & De, 2021). This gives the correct capillary channel formation within the yarn bundle and fabric strip surface with the parallel yarn arrangement and increased wickability (Said et al., 2023; Mallick & De, 2022). A direct evaporative cooler constructed from Kraft paper as a wet medium of the evaporative cooling system from literature (Xu et al., 2016) was used for comparative purposes in this study.

Previous studies focused on the wetting and wicking properties of cotton yarn and fabric sample pieces based on static conditions. However, none of these studies investigated the wicking flow and rate properties of cotton yarn bundles and fabric layers oriented in in-plane vertical or horizontal directions. In addition, there is only limited information about the development of cotton yarn bundles and fabric layer-based capillary absorptive evaporative cooling systems for the production of cold air used for ventilation and storage applications. Therefore, the objective of this study was to evaluate the wicking flow and wicking rate of the yarn bundle and the plain woven fabric layer on its warp side (larger in length) as a test direction in both in-plane vertical and horizontal orientations. Therefore, the study focused on the behavior of wicking and the wicking growth rate of untreated/grey, and treated yarn bundles and fabric layers. The horizontal wicking and vertical wicking orientations were considered the two main methods used for the evaluation of water transfer and wicking rate behavior through these naturally absorbent cotton material types. The wicking distance and wicking rate were accounted for the most important parameters and contributes a crucial role in the selection of suitable natural absorbent materials used for appropriate industrial moist media construction for cooling and safe storage of agrarian produce.

2. Materials and methods

2.1. Materials

Grey cotton fabric (plain weave) layers and yarn bundles were collected for this test from the workshop of the Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University. Caustic soda (reagent grade), sodium perborate tetrahydrate (analytical reagent), textile wetting agent (commercial grade), and textile emulsifier (commercial grade) are sourced from Sigma Aldrich, India Plc.

2.2. Materials treatment

All de-sizing, scouring, and bleaching processes are lengthy and require large amounts of water, chemicals, energy, and time, which bring high costs and loss of productivity. Therefore, for this treatment, both the grey cotton yarn bundle and fabric strip samples were treated independently with reagent-grade caustic soda (14%), analytical grade sodium perborate (3%) at 0.5% intervals, and containing commercial grade textile wetting agent (0.1%). Since caustic soda dilution is an exothermic process, the caustic soda was diluted three hours before the experiment to bring the hot solution to room temperature. Techniques of a two-dip, two-nip padding operation were adopted, and a wood rod was used to roll the samples on it and then batched for 30 min at ambient temperature (20-25 °C). Then, keeping the material liquid ratio at 1:25, the batched samples were washed twice for 15 minutes each time with a commercial grade textile emulsifier (0.1%) at boiling temperature to make a stable oil-water emulsion through the reduction of the separation force of oil from water, resulting in a stable oil-water liquor solution finally that is easily washed away it from cotton materials. During the second wash, 3% of sodium perborate was added while washing the samples, then rinsed with hot and cold water and air-dried at last, as shown in Figure 1(a)–(e) (Wagaw, 2022; Abate, 2017; Wagaw & Chavan, 2013).

Figure 1. (a) untreated fabrics (raw), (b) treated (mercerized) fabrics, (c) untreated cotton yarn bundles (raw) (d) treated (mercerized) cotton yarn bundles after drying, (e) Causticisation process of raw cotton fabrics/yarn bundles.

2.3. Yarn bundle and fabric strip samples conditioning and preparation

The yarn bundle samples were 100 cm in length with their different diameters, and the plain weave fabric layer samples were also cut in both warp (100 cm length) and weft direction (with different short widths), and all the samples were conditioned for 24 hours under standard atmospheric conditions of temperature 20 ± 2 °C and relative humidity 65 ± 5% Yarn bundle samples of 0.5cmx100cm, 1cmx100cm, 2cmx100cm, and 2.5 cmx100cm each were cut in diameter and length from the conditioned sample for yarn bundle wicking tests of vertical and horizontal configurations. Plain-woven fabric strip samples of 0.5cmx100cm, 1cmx100cm, 2cmx100 cm, and 4cmx100cm each were cut in the width and length from the conditioned sample for fabric strip wicking tests of vertical and horizontal configurations. Then the samples were mounted on the transparent measuring cylinder for the vertical alignment in which the bottom end was immersed in the water reservoir. Samples for the horizontal wicking alignment were put inside the transparent cylinder stretched with a wood stick and put horizontally, with one of its ends immersed in the distilled water reservoir as shown in Figures 2–5, respectively 10 cm dipped into the infinite reservoir containing distilled water. After running the test, a ruler with millimeter divisions was used to measure the wicking height or length accurately.

Figure 2. Diagrammatic illustration of vertical wicking flow tests for the untreated/treated fabric layer samples.

Figure 3. Diagrammatic illustration of vertical wicking flow tests for the untreated/treated yarn bundle samples.

Figure 4. Diagrammatic illustration of horizontal wicking flow tests for untreated and treated fabric bundle samples.

Figure 5. Diagrammatic illustration of horizontal wicking flow tests for untreated/treated yarn bundle samples.

The previous research was mostly carried out on single yarns and woven fabrics with different weave units’ proportions, and they compared the weft side wicking vs. warp side wicking to either orientation evaluation concerning thermodynamic comfort in commercial textile garments in addition to their yarn or fabric samples’ dye uptake ability confirmation. They have also used the places of interlacement between warps and wefts constantly formed in horizontal and vertical directions as water flow split. Either warp or weft, depending on the test direction (horizontal yarns), served as a reservoir of water to wick further along the vertical yarns.

We prepared samples as a test specimen in the direction of the warp, which is 100 cm in length for testing in both samples, providing the weft side with different widths to serve as a reservoir for the vertical and horizontal orientations of the warp side layers based on previous studies in the literature. Therefore, we used the warp of the plain weave fabric sample either in the vertical or horizontal direction in the test direction, leaving the shortest side (the weft) of the plain weave fabric strip sample based on previous studies that stated that wicking in the warp direction of the plain weave fabrics is mostly greater than the weft side of the plain weave fabric under our study scope in the capillary absorptive evaporative cooling. Therefore, we accounted for the weft side as short as possible in different widths, which could serve as a micro-reservoir in which the capillary wicking begins to flow parallel to the warp direction of the test direction. Some figure lines in both vertical, horizontal wicking and rate of the untreated material sample displayed overlapping due to their own very low appreciable wicking height, length, and rate differences during the measurement process.

Fixed infinite cylindrical reservoir (volume = 0.0035 cu., radius = 0.08 m, and height = 0.174 m) was filled with an equal volume of distilled water throughout the test operation. The reservoir level was kept as constant as possible by adding the required amount of distilled water continuously throughout the test operation so as to prevent capillary suction that would affect the penetration length.

2.4. Test methods

2.4.1. Vertical wicking testing

The vertical wick tests were carried out by using untreated and chemically treated fiber bundle and fabric strip samples independently at room temperature to measure the wicking flow height from the distilled water reservoir. The vertical fabric layers (Figure 2) or yarn bundles (Figure 3) are held vertically up side and placed inside a suitable transparent cylinder for stretching the bundle assemblies or fabric layer with a wood stick. For both untreated and treated yarn bundles or fabric strips, the wicking heights from the distilled water reservoir at room temperature (22-24 °C) were measured for each test run by using a transparent ruler. The vertical wicking height measurements were taken at 120, 240, 360, and 480 minutes after the immersion of the yarn bundle and fabric layer in the distilled water reservoir.

The advancing liquid front height as a time function was measured visually using running ink observation at 120-minute intervals until maximum equilibrium height (wicking height) was approached. All vertical wicking and rate tests were developed from AATCC TM 197-2018 Textiles Vertical Wicking (Tarbuk et al., 2019). For the vertical wicking tests, the 100 cm length was taken as the maximum measuring distance in this study with different diameters and widths of the yarn bundle and fabric layers, respectively. Each group of untreated and treated material samples was tested four times, and the average values were taken. The results are drawn and shown in Figures 6–9.

Figure 6. Vertical (a) and horizontal (b) wicking tests of the untreated yarn bundle.

Figure 7. The treated yarn bundle wicking tests: (a) Vertical and (b) horizontal.

Figure 8. The untreated fabric layers wicking tests: (a) Vertical and (b) horizontal.

Figure 9. The treated fabric layers wicking tests: (a) Vertical direction and (b) horizontal direction.

2.4.2. Horizontal wicking testing

The horizontal wick tests were carried out by using untreated and chemically treated yarn bundle and fabric layer samples independently at room temperature to measure the wicking flow length from the distilled water reservoir. The fiber bundle or fabric layer assemblies for the horizontal setup are kept horizontally by wood stretching inside the transparent cylinder. Both untreated and treated fabric strips (Figure 4) or yarn bundles (Figure 5), wicking distances from the distilled water reservoir at room temperature (22-24 °C) were measured for each test run using a transparent ruler.

The advancing liquid front length as a time function was measured visually by using the running dye liquor observation at 60-minute intervals until the maximum length was approached while the wicking time ceased. Horizontal wicking distance measurements were taken at 120, 240, 360, and 480 minutes after the immersion of the yarn bundle, and fabric layer in the distilled water reservoir. Each group of untreated and treated samples was tested four time, and their average values were taken. The graphical results are drawn and shown in Figures 6–13. All horizontal wicking flow and rate tests were developed from AATCC TM 198-2018 Textiles Horizontal Wicking (Tarbuk et al., 2019). For the horizontal wicking test, a 100 cm length sample was adopted as the maximum measuring distance for this study with different diameters and widths of the yarn bundle and fabric layers, respectively.

Figure 10. The untreated yarn bundle wicking rates: (a) Vertical direction and (b) horizontal direction.

Figure 11. The treated yarn bundles’ wicking rates: (a) Vertical direction and (b) horizontal direction.

Figure 12. The untreated fabric layers wicking rates: (a) Vertical direction and (b) horizontal direction.

Figure 13. The treated fabric layers wicking rates: (a) Vertical direction and (b) horizontal direction.

2.4.3. Wicking rate testing

The vertical wicking rate tests in untreated, treated yarn bundles and fabric layers were carried out from the development of AATCC TM 197-2018 Textiles Vertical Wicking (Tarbuk et al., 2019). The horizontal wicking rate tests of untreated, treated yarn bundles and fabric layers were also carried out from the development of the AATCC 198-2012 Textiles Horizontal Wicking Standard Test Method (Tarbuk et al., 2019), applicable for textile comfort in industrial cooling applications. All the samples were conditioned for 24 h under standard atmospheric conditions (temperature 20 ± 2 °C and relative humidity 65 ± 5%). The investigation was carried out after the sample conditioning as follows. Water wicking height/length in vertical or horizontal migration is measured at every 60 minutes of 480 minutes test duration from 10 cm immersed in a water-dye liquor. For clarity cases, the vertical/horizontal distances only at 120, 240, 360, and 480 minutes were taken for evaluation of the vertical/horizontal wicking rates (distance covered in mm over time taken in minutes). All the measurement test results for each material sample type are shown in Figures 10–13.

2.4.3.1. Vertical wicking rate testing of untreated/treated fiber bundles

While designing of yarn bundle to measure the vertical wicking rate behavior of the yarn bundle with different diameters and with the same length of 100 cm each, a total of 8 marks were positioned and immersed in the distilled water as shown in Figure 3 including marking. Vertical height measurements due to water wicking at 60-minute intervals of 480 minutes of test duration were carried out for all yarn bundle samples. The marks are placed one above the other in a vertical order at an equal distance from each of the prepared samples for this test. This arrangement is capable of measuring and recording the vertical wicking rate for the 100 cm wicking samples. The vertical wicking rates were also evaluated in terms of distance covered in mm over wicking time in minutes, and the measurement results are shown in Figures 10(a) and 11(b), respectively.

2.4.3.2. Horizontal wicking rate testing of untreated/treated yarn bundles

The horizontal wicking flow tests for untreated and treated yarn bundle samples were used in the horizontal wicking rate testing of untreated and treated yarn bundles similarly to the horizontal flow, using a mark with equal distance intervals over 100 cm of sample material over a 60-minute interval for a 480-minute test duration. The progressive horizontal distances covered in each 60-minute interval were taken, and the horizontal wicking rates were evaluated in terms of distance covered in mm over wicking time in minutes. The test operation of the horizontal wicking rate in the yarn bundle layer is similar to the horizontal wicking tests used in Figure 5, including the mark, and the measurement results are displayed in Figures 10(b) and 11(b), respectively.

2.4.3.3. Vertical wicking rate testing of untreated/treated fabric layers

While designing fabric layers to measure the vertical wicking rate behavior of the fabric layers with different widths and the same length, a total of 8 marks were positioned similarly, as shown in Figure 2. A progressive distance record at 60, 120, 180, 240, 300, 360, 420, and 480 minutes at 60-minute intervals was carried out for different sizes but the same length of fabric layer samples after immersion in distilled water. The marks are placed one above the other in a vertical order at an equal distance from each of the prepared samples for this test. This arrangement is capable of measuring and recording the vertical wicking rate for the 100 cm wicking samples. The expanded arrangement of this test operation is similar to that described in Figure 2, including marking but not included for clarity, and the vertical wicking rate measurement results of untreated and treated fabric layers are shown in Figures 12(a) and 13(a), respectively.

2.4.3.4. Horizontal wicking rate testing of untreated and treated fabric layers

The testing procedures used for horizontal wicking of untreated and treated fabric layers were applied in this section of horizontal wicking rate testing of untreated and treated fabric layers to the horizontal flow arrangement. The testing operation was similar to the operation used to determine the horizontal wicking rate of untreated and treated fiber bundle layers. Samples of fabric layers with different sizes are cut to measure the horizontal wicking properties of fabrics. 10 cm from the bottom end of the fabric is marked and immersed in the distilled water. To protect the bottom end of the fabric, a small clamp is attached to the fabric. Timing begins using a stopwatch with 60-minute intervals of a progressive distance in the direction of 100 cm sample material and is recorded. The measurement results of untreated fabric samples and treated fabric samples are shown Figures 12(b) and 13(b), respectively.

3. Results and discussion

3.1. Wicking ability

3.1.1. Untreated yarn bundle wicking performance on vertical and horizontal orientation

Figures 6–9 display the wicking flow graphical representation of untreated and treated yarn bundles and fabric strips. The vertical and horizontal wicking lengths of untreated cotton yarn bundles with time are given in Figure 6(a) and (b), respectively. Yarn bundles continued wicking at a slow rate for 120-480 minutes until they reached an equilibrium point. After 8 hours, the maximum vertical wicking height and horizontal wicking length were 37 mm and 34 mm, respectively.

Due to the low sensitivity of the measurement technique used in the study, it was not possible to see the expected effect of gravity during the vertical wicking experiment of untreated cotton yarn bundle. In both orientations, wicking length increased with a decrease in bundle diameter due to a reduction of yarn overlapping and yarn packing (Zarandi & Pillai, 2018), The graphical results of Figure 6(a) and (b) showed the wicking behavior of untreated yarns which is similar to the study results that was reported by Mallick and De (2022). The absorbed water-wicking flow length in the yarn bundle samples oriented vertically slowly reached to an equilibrium position due to the effect of the hydrostatic head. Furthermore, the decrease in capillary continuity could be due to the fact that the random fiber arrangement has its own contribution to the rate of liquid transport in such yarn assemblies (Mallick & De, 2022). Moreover, a reduction in permeability due to the increase in flow resistance as a result of the reduction in capillary pore sizes with capillaries swelling, and capillary arrangement in the untreated yarn bundle structure (Rajan et al., 2019).

3.1.2. Treated yarn bundle wicking performance on vertical and horizontal orientation

Figure 7 shows the vertical and horizontal wicking lengths of treated yarn bundle samples. The samples’ wicking trend in the vertical wicking direction is not similar to the horizontal wicking trend. For all treated bundle diameters, the vertical wicking length was lower than the horizontal wicking length. The highest wicking length observed was 752 mm after 2 hours of immersion in horizontal wicking orientation. This reduction in wicking length was probably due to the effect of hydrostatic pressure and gravity effect (Mallick & De, 2022). For the treated samples, the effect of the two hydrostatic pressures and gravity was clear and as per expectation. The length tends to reach an apparent equilibrium height where the acting. The longest wicking length after 8 hours was observed for the horizontal orientation of the smallest bundle diameter (0.5 cm). For both horizontal and vertical orientations, there was no continuous increase in wicking height, and this could be a continuous loss of water because evaporation from the liquid front line could be more slowly than in a setup without evaporation (Li et al., 2023). And water evaporation out of surface pores could occur when the surrounding gas is not saturated with the liquid vapor (Lei et al., 2020b). The location of wicking heights and wicking lengths in Figure 7(a) and (b), respectively, showed the availability of evaporative cooling (Lei et al., 2020b). Other reasons could be considered in both wicking processes; the diffusion or flooding of water to the radial direction of the yarn bundle would be expected due to the effects of discontinuous fibers or overlapped fibers forming shielding effects in the capillary tubes; this could be taken as a cause of this event (Figure 7(b)), and hence, it took much time to cover the expected height (Mallick & De, 2021; Li et al., 2023).

The formation of cracked reservoirs during wicking could reduce, or sometimes stop, and flood over the material surface outside of the capillary channels (Cho et al., 2015), and hence, both wicking height and length could be highly reduced by this event. Another reason could be seen as the accessible pore space dissimilarity along the yarns due to the yarns crumpling and the non-uniformity of water filling over the yarns height, which could result in a decrease in the yarns wicking distance, plus external factors such as yarn extension, liquid temperature, and relative humidity (Cho et al., 2015).

3.1.3. Untreated fabric layers wicking performance on vertical and horizontal orientation

There was no appreciable wicking height/length difference in the vertical and horizontal directions of Figure 8 (a) and (b), respectively. The slow equilibrium point attainment was due to the slow spread of distilled water over a large fabric layer surface. The horizontal wicking of the untreated fabric sample test results revealed a similar wicking of the fabrics to the vertical direction, in which the wicking of the longitudinal or lengthwise warp yarns was less rapid compared to the infill/weft direction during the fabric wicking in the warp direction (Zhu et al., 2015). Even though the difference was small, the wicking height of the vertical arrangement was greater than the wicking length of the horizontal arrangement. This violates the expected gravitational force effect. The distance covered by the vertical wicking flow is expected to be low compared to the horizontal wicking due to gravity and the hydrostatic pressure effects on the vertical flow (Kumar & Arakeri, 2020; Mirzajanzadeh et al., 2019). When we compared the result from this study with other similar previous studies, contradictory results were observed due to the low sensitivity of the wicking length measurement technique to capture the small wicking height/length of the untreated fabric layers (Zhu & Takatera, 2014).

The capillary effect is slowing down or completely stopping due to fiber swelling, which could cause a higher imbalance in capillaries in cellulose fibers (Asfand & Basra, 2020). Low capillary pressure development by small capillaries could originate from their microreservoirs, and hence, the small pores produce high capillary pressure that would result in high capillary rise and more wicking (Asfand & Basra, 2020). But the capillary barrier effect could exist due to the capillary arrangement in the fabric structure (Mallick & De, 2022; Zhu & Takatera, 2013).

As the sample materials are chemically untreated (Abate, 2017), the fiber shape in the yarn or fabric structure could change the wicking rate, and the size and geometry of the capillary spaces between fibers could also affect it, and hence, the fabric structure could determine the fabric water absorption capacity and water wicking (Rajan et al., 2019). Therefore, the location of the wicking height or length in the material samples could not cover good evaporative cooling due to their poor saturation event, as shown in both Figure 8(a) and (b). A similar observation was reported by Elmsaad et al. (2023) and Lei et al. (2020b).

The plain woven sample in this case would reflect this type of reduction in wicking ability due to the yarns structure. The width of the interstices could be tight in the woven plain fabrics, and this could change the pore sizes, which would affect wicking ability (Rajan et al., 2019). Therefore, the most probable sources of these raw fabric structural variations could be blockage of the capillary channels or overlap of other channels that could form helical twists, which sometimes could leave no air spaces, hence, the wicking height could be decreased as the wicking water was not able to fill it. Such a situation could reduce the wicking rate and, hence, result in a lower evaporation rate (Lei et al., 2020a).

In addition, the expected equilibrium wicking heights in Figure 8(a) could not be achieved due to an increase in yarn density, and hence the untreated cotton fabric in this case could show a negative wick gain from the weft side (Fischer et al., 2022). This is because the water flow could be hindered as the water coming across would have trouble wetting the untreated warp yarns as they were wicked into and out of the fabric (Li et al., 2023).

3.1.4. Treated fabric layers wicking performance on vertical and horizontal orientation

Figure 9(a) and (b) show the results of treated fabric layer samples with vertical and horizontal wicking directions, respectively. Initially (after 2 hours of immersion), the wicking length was higher for the vertical orientation, particularly for the sample layer of 4 cm wide fabric. After 8 hours, the wicking length of the treated fabric strips that were oriented in the horizontal direction was higher than that of the vertical arrangement. Again, this was due to the effects of gravity and hydrostatic pressure in the vertical direction. Similar to the cotton yarn bundles, there was no continuous increase in wicking length with time. Due to the quick evaporation of the spreading water over the fabric surface, the flow of water in the horizontal direction finally ceased with time (Velasco-Gómez et al., 2020; Zhu & Takatera, 2014).

In the first 120-240 minutes, there was rapid wicking in the vertical direction, which was greater than the wicking in the horizontal direction. There was a variation in lift-off, while wicking begins at the start, next by the different wicking trends in both the vertical and horizontal directions. The higher wicking length was observed in the horizontal wicking length in a time gap of 240-360 minutes due to the absence of hydrostatic pressure and gravity.

In addition, fabrics with a loosened structure have more macropores fabrics with more floats, and hence they would have high water transfer speed (Lei et al., 2020a). This suggested that the horizontal wicking test conducted on woven fabrics with more floats had a higher water transfer rate because floats in treated fabrics would contain more air space than untreated fabric strips.

Other reasons could be the variation in available pore space along the yarn due to the twisting of the yarn. The water filling over the height of the yarn could be highly non-uniform. This could be pointed out that the equilibrium vertical wicking height of yarn decreases with an increase in yarn twist. The fabric extension, liquid temperature, and relative humidity could influence the vertical wicking performance, considering them as external factors (Wang et al., 2022).

During short wicking times, the liquid migration from warp yarns to weft yarns could not be completed. On the other hand, the weft yarns would not function as reservoirs, consequently, a negative gain in wicking rates could be observed. But for prolonged periods, migration could be completed, and the weft side yarns could function as reservoirs to achieve positive gains in the wicking heights of the warp side yarns (Fischer et al., 2022).

3.2. Wicking rate

3.2.1. Untreated yarn bundle wicking rate on vertical and horizontal orientation

Figures 10–13 show the graphical representation of untreated and treated yarn bundles and fabric layer wicking rates. Figure 10 shows the wicking rate variation with time for an untreated yarn bundle. The wicking rate was relatively higher initially but decreased with time, it was close to zero at the end of the test after 480 minutes. The wicking rate in either orientation is inversely proportional to the wicking time. Doakhan et al. (2016) pointed out that the yarn capillary heterogeneity, i.e. variation in the yarn cross-section, along the yarn, and especially the sheath components of the yarn attributes the variation in capillary wicking rate. The yarn structure heterogeneity leads to the yarn pores’ (capillaries) heterogeneity, which could affect the capillary behavior. Relatively lower effective capillary radius of untreated yarns results highly tortuous water path. The water would take a more tortuous path in highly twisted yarns since the fibers would be tightly packed, and less capillary radius could exist; hence, low wicking rates could be observed in these chemically untreated yarns (Fischer et al., 2022).

A decrease in capillary pressure would decrease wicking or water transport, and hence, the wicking rate due to the unavailability of floats in the untreated sample that should be wetted and wicked (Lv et al., 2021). The untreated close packed yarn bundle could reduce the capillary radius, and then, the wicking rate (Testoni et al., 2018).

Most of the capillaries in sized yarns are filled by size chemicals during sizing formulation, the wicking length in these yarns would not be considerable and it is relatively low (Jain et al., 2021). And also, sizing chemicals contain hydrophobic wax in composition, and hence, wetting these chemicals is not an easy process (Harane & Adivarekar, 2017). These could be accounted for as evidence for the observed results that the untreated cotton yarn bundles showed a very low, notable difference in working rate between the vertical and horizontal flow directions, and their difference slowly decreased with time.

3.2.2. Treated yarn bundle wicking rate on vertical and horizontal orientation

Figure 11 shows the capillary wicking growth rate variation of treated yarn bundle samples under vertical (a) and horizontal (b) directions, respectively. In the vertical wicking rate, there is a significant decrease in the liquid wicked compared to the wicking in the horizontal direction. Gravity and hydrostatic pressure in vertical orientations caused the wicking rate to be lower than in horizontal orientations. For all samples, a rapid wicking rate was observed in the initial 120 minutes, afterwards, there was a continuous decrease in the working rate. After 8 hours, the working rate in vertical flow was lower compared to horizontal.

The decrease in wicking rate could be due to an increase in hydrostatic pressure and gravity on it and the decrease in capillary pressure on vertical orientation, as wicking height is directly proportional to the capillary tube radius, the contact angle, the surface energies, the capillary action, and the porosity changes with swelling (Li et al., 2023; Zarandi & Pillai, 2018; Azeem et al., 2017; Cho et al., 2015). The yarns’ fiber damage would form a cracked reservoir wicking that could slow or sometimes stop the wicking process (Cho et al., 2015), and hence, it would reduce wicking length as well as wicking rate. A further reason could be the variation of the available pore space along the yarn length due to yarn twisting, and the water filling variation over the yarn height surface resulted in a decrease in yarn wicking height (Cho et al., 2015).

In addition to this, the pore structure of fibrous materials is complex and difficult to quantify, and the water or moisture absorption in the fibers would change the pore configurations, and fiber positioning changes could affect the yarn’s wicking property (Asfand & Basra, 2020).

From Figure 11(a) and (b), it can be shown that the highest wicking rate was observed in the chemically treated yarn bundles compared to the untreated yarn bundles, and the wicking rate was also relatively high in these treated yarns due to the formation of twist-less filament yarns with open structures (Mallick & De, 2022; Harane & Adivarekar, 2017).

Increasing liquid retention in the yarns from cellulosic fibers would take place at the expense of capillary liquid capacity in the inter-fiber pores, which would increase the probability of fiber swelling, which provides the pore structure complication (Azeem et al., 2017). Such fiber welling could be the bottlenecking cause of the capillaries closing off, which later slows or even stops the capillary flow and contributes to a drastic reduction in wicking rate (Testoni et al., 2018).

The wicking flow rate would vary with material porosity. The optimal material porosity that corresponds to the maximum wicking rate could be decreased when the capillary bead radius increases due to swelling or chemical treatments. The optimal material porosity is highly sensitive to the capillary pores and bead size. Increasing the wicking flow rate with porosity would be possible until the peak, beyond that point, the flow rate could be decreased up to zero. The water flow front would not reach the wick top after it has approached zero, and hence, the flow rate could be negative at some times (Beyhaghi et al., 2014).

3.2.3. Untreated fabric layers wicking rate on vertical and horizontal orientation

Figure 12 shows the capillary wicking growth rate variation of untreated fabric layers at vertical (a) and horizontal (b) orientations. When the untreated fabric layers were left to wick for 8 hours, there was no change in the position of the liquid edge, as the liquid advancement had already ceased. Among the fabric layer samples, the 2 cm width fabric layer of vertical wicking rate (VWR) (Figure 12(a)) exhibited a noticeable reduction in wicking growth rate with time compared to other untreated samples. The fabric layer sample with a 1 cm width of VWR shows a rapid wicking rate reduction in time, it also shows an equivalent wicking rate reduction with a 1 cm width in horizontal wicking rate (HWL). While the sample with a 0.5 cm width shows more or less no reduction in wicking growth rate with time compared to others. In untreated fabric layers, both vertical and horizontal orientations show similar wicking rate reduction in the first 120-240 minutes, and slowly reduced in the time gap of 240-480 minutes with some variations in the vertical wicking rates of 1 cm wide fabric sample that showed relatively higher wicking rates at the beginning and slowly reduced.

The no-change condition in the liquid edge position could exist most probably due to the random fiber arrangement in the yarns in the warp direction. The fibers in the yarns on the weft side were used as micro-reservoirs because of the short widths of the yarns used in the warp direction (Wang et al., 2022). This shows that the capillary channels in untreated fibres of yarn bundles could be blocked with natural impurities as well as inherited impurities during transportation and processing that are impermeable/hydrophobic nature to water (Jain et al., 2021). Increasing in twist level in yarns also would reduce the inter-yarn spaces that lead to a lower capillary radius, and this in turn resulted with lower wicking rate first with significant reduction towards the test periods (Mallick & De 2022).

3.2.4. Untreated fabric layers wicking rate on vertical and horizontal orientation

Figure 13 discloses the vertical (a) and horizontal (b) wicking rate tests of the treated fabric layers. The wicking rate for the horizontal orientation was slightly higher than the vertical one. In 120-240, both wicking flows showed relatively faster wicking rate reduction but as time progressed it was slowly reduced to lower values. It was behaved similarly to the cotton fiber bundle, there was a continuous decrease in the wicking rate with time, and finally, it reached a minimal value. The fabric wicking rate in the vertical direction showed a similar behavior to the wicking rate of the horizontal direction.

Such phenomena in vertical and horizontal wicking arise when the rate of wicking in the warp threads is sufficiently high at the water flow pattern when few weft yarns remain in the condition of incomplete filling. This exhibits a somewhat inefficient water migration process, and a very low wicking rate could be gained from such samples (Rajan et al., 2019). On the other side, due to the existence of very poor liquid migration between the warp side and the short weft-sided yarns, most remained with incomplete filling. The warp-sided yarns lose water to the weft-sided yarns, and this condition would stop any water from the weft yarns, and thus a negative gain in wicking rate could be observed in the warp-sided yarns of the fabric samples (Li et al., 2023). The material porosity could affect the wicking flow rate, which could decrease up to zero when the wicking front does not achieve the wick top; consequently, a negative wicking flow rate could arise (Beyhaghi et al., 2014).

3.3. Effect of cotton materials treatment on wicking performance characteristics

In treated cotton yarn bundles, the wicking length in both vertical and horizontal directions in Figure 7(a) and (b), respectively, was much higher than the untreated samples in Figure 6(a) and (b). The average wicking length increase for all treated samples was 10.4 and 17.2 times higher than untreated samples for the vertical and horizontal directions, respectively. This shows the potential of treated cotton yarn bundles to develop capillary evaporative cooling systems. Similar to the cotton yarn bundles, the treatment of the fabric layers increased the wicking length in both vertical and horizontal orientations (Figure 13(a) and (b)). On average, the wicking length for treated fabric layers was 9.2 and 13.1 times higher than that for untreated fabric strips for vertical and horizontal orientations, respectively. Due to the presence of natural materials and added impurities, the untreated fibers and fabrics in both horizontal and vertical orientations showed little wicking distance coverage compared to the treated yarn bundle and fabric layer.

Removing the natural barriers and impurities improves the wicking capacity of the fiber bundles/fabric layers (Amanuel, 2022). Causticisation treatment of cotton yarn bundles or fabric layers with caustic soda improves capillary water absorption and strength (Said et al., 2023; Abate, 2017). Similar to the wicking length, treatment of the yarn bundles and fabric layers increased the wicking rate in both horizontal and vertical orientations in Figures 11(a) (b) (a), and (b), respectively. After 2 hours of immersion in the water reservoir, the result indicated that the treatment of the cotton yarn bundle increased the average wicking rate by 21.1 and 38.5 times in the vertical and horizontal orientations, respectively. During the same period, the fabric layers experienced a 29.3 and 25.1 times increase in wicking rate due to the treatment in both vertical and horizontal directions. Said et al. (2023) observed an improvement in dyeing efficiency due to the treatment of sample material with pretreatment chemicals. Harane and Adivarekar (2017) observed an improvement in the reactivity of the sample material with a dye due to the opening up of sample material as a result of the pretreatment.

3.4. Effect of cotton yarn bundle treatment on water wicking rate

The water absorbency increases as the yarn bundles become coarser. This could be because the higher bending rigidity of coarser yarn bundles and the decreasing yarn number from the yarn bundle cross-section increased the bulkiness of the yarn bundle. Increasing water absorbency is also associated with an increase in pore volume in the yarn structures. The fabric water absorbency increases at first while the yarn coarseness increases, then declines due to a decrease in yarn structure compactness and an increase in yarn bulkiness, which increases the capillary pores’ availability in the yarn structure, and increases the water absorbency (Mallick & De, 2022).

Furthermore, the fabric’s water absorbency also increases while the yarn sheath content increases. This could be due to an increase in fabric openness and yarn bulkiness while removing the sheath, resulting in more capillary pores in the yarn structure, which increases the water absorbency. The higher horizontal wicking rate resulted from the fact that every intersection point acts as a routing point for the water flow, which in turn contributes to higher wicking rates as increasing porosity decreases the horizontal wicking rate.

3.5. Effect of cotton material type on wicking performance characteristics

On average, the treated yarn bundle samples showed pronounced wicking lengths when compared with the treated fabric layer samples. For horizontal orientation, after 8 hours of immersion, the wicking length in the cotton yarn bundle was 1.3 times that of the fiber layers. For vertical orientation, the difference between the two materials was minimal. The difference in wicking length and rate for untreated cotton fiber bundles and fabric strips was too small. From the literature, 62 mm was registered as the highest wicking distance covered by using Kraft paper from previous studies (Xu et al., 2016), and this was higher than the wicking length that was recorded for untreated cotton yarn bundles and fabric layers. This confirms that the untreated fiber bundle and fabric layer samples’ wicking ability with different widths and diameters indicates that water wicking in untreated cotton moist materials is too low compared to the treated textile materials.

3.6. Effect of cotton materials dimension on wicking characteristics

For the cotton yarn bundle samples, it was clear that maximum wicking length was obtained with a decrease in bundle diameter. With a decrease in bundle diameter, there is a decrease in porosity that increases the capillary wicking capacity of the bundle. For the transfer of moisture through fabric layers, the diameter of the yarn bundle has a role, a reduction in yarn bundle diameter increases the fabric-wicking properties with a reduction of air permeability.

4. Conclusion

The wicking ability and wicking rates of both untreated and treated fiber bundles and fabric strips were analyzed and compared. Fiber and fabric treatment increased both the wicking length and rate. The highest wicking length and wicking rate were observed in treated cotton fiber bundles that were oriented horizontally. Untreated fiber bundles and fabric layers showed very low wicking ability and wicking rates compared to both the treated fiber bundle and fabric layer, and even their working capacity was lower than that of Kraft paper due to the absence of intra-fiber channels and the presence of much fewer inter-fiber channels in the bundle and layer structures.

The study showed that untreated fiber bundles and fabric layers are not suitable materials for the development of a capillary absorptive evaporative cooling system. Treated cotton fiber bundles and fabric layers that are oriented horizontally showed a relatively higher wicking capacity in terms of wicking length and rate. This was due to the presence of fiber channels and the presence of many inter-fiber channels in the bundle and layer structures and due to the absence of gravity and hydrostatic pressure that is exerted on vertically oriented materials. Therefore, treated fiber bundles and fabric strips could provide great potential with superior properties for the construction of capillary absorptive evaporative cooling systems with better efficiency.

Author contribution

Biruk F. Abate: Conceived and designed the experimental set-up; performed the experimental testing; Analyzed and interpreted the result; Wrote draft of the paper. Mulugeta A. Delele: Advised the designs and the experimental set-up; Supervised the research; Revised the draft paper. Abdela S. Ahmmed: Performed the experimental testing; Revised the draft paper.

Citation information

Cite this article as Evaluation of Water Flow in yarn and Cotton Fabric Assemblies for Capillary Evaporative Cooling, Biruk A. Fenta, Abdella S. Ahmmed & Mulugeta A. Delele, Cogent Engineering (2023).

Data availability statment

The data presented in this study are available on request from the corresponding author.

Acknowledgements

Our appreciation goes to Dr. Abera Keche, former Scientific Director of the Ethiopian Institute of Textiles and Fashion, Bair Dar University (EiTEX, BDU), and staff for their continuous support in the laboratory work of this study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Biruk Abate Fenta

Biruk Abate Fenta, PhD Candidate in Food process Engineering and postharvest technology, Bahir Dar Institute of Technology, Bahir Dar University, Ethiopia.

Abdella Simegnaw Ahmmed

Abdella Simegnaw Ahmmed (PhD), Assistant professor in Smart and Functional Textiles, Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University, Ethiopia.

Mulugeta Admasu Delele

Mulugeta Admasu Delele (PhD), Professor in Food and Bioscience Engineering, Bahir Dar University, Ethiopia.