# Capillary properties of natural fibre assembly for evaporative purposes

Stéphanie O.L. Lacour, Véronique Aguié-Béghin, Brigitte Chabbert, Mario Scheel, Damien Thomas, Pascal Clain

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## To cite this version:

Stéphanie O.L. Lacour, Véronique Aguié-Béghin, Brigitte Chabbert, Mario Scheel, Damien Thomas, et al.. Capillary properties of natural fibre assembly for evaporative purposes. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2025, 724, pp.137145. 10.1016/j.colSURFA.2025.137145. hal-05222049

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# Capillary properties of natural fibre assembly for evaporative purposes

Stephanie O.L. Lacour<sup>a, ID*</sup>, Véronique Aguié-Béghin<sup>b ID</sup>, Brigitte Chabbert<sup>b ID</sup>, Mario Scheel<sup>c ID</sup>, Damien Thomas<sup>a ID</sup>, Pascal Clain<sup>a,d ID</sup>

## HIGHLIGHTS

* Spinning effect on capillary rise and terminal height in yarns.

* Assembly warping during liquid ascension.

* Fibre movements related to competition between capillary and tensile forces.

* Plant fibre swelling with lumen opening under water pressure.

## GRAPHICAL ABSTRACT

### Capillary rise of water in natural fibre assemblies

#### Maximizing height for evaporative transfer

<figure><img src="image_5.png" /></figure>

## ARTICLE INFO

### Keywords:

Capillary rise in yarn

Water permeability of plant cell bundles

Capillary pressure forces

Water storage in plant wicks

## ABSTRACT

Natural fibres are well known for their high efficiency in evaporation and humidification applications, making them a promising solution for cooling processes during heat waves. Mimicking the evapotranspiration mechanisms of trees is essential for designing optimal fibre wicks in canopy-like devices. Such devices are designed by their capillary rising height and water storage capacity. This study examines the water retention properties and capillary rise height of various yarns: retention tests for surface properties and tomographies for topographical aspects are combined to explain observed water rising height in samples. Tomographic analyses document pore sizes, fibre diameters, and water distribution within the wicks. Water retention properties among the samples range from less than 10% to over 20% of dry mass, reflecting significant

<footnote><sup>a</sup> University of Paris-Saclay, INRAE-FRISE, 1 Rue P. Gilles de Gennes, 92761, Antony Cedex, France</footnote>

<footnote><sup>b</sup> University of Reims Champagne-Ardennes, INRAE-FARE UMRA164, 2 Esplanade de Rolland Garros, 51686, Reims, France</footnote>

<footnote><sup>c</sup> Synchrotron SOLEIL, L'Orme des Merisiers, 91190, Saint Aubin, France</footnote>

<footnote><sup>d</sup> De Vinci Higher Education, Research Center, 12 avenue Léonard de Vinci, 92916, Paris La Défense, France</footnote>

<footnote>* Corresponding author.
E-mail address: stephanie.lacour@inrae.fr (S.O.L. Lacour).</footnote>
wettability differences. However, the ultimate height of water ascension primarily depends on the structural yarn properties. Repeated tomographic scans during the rise reveal that water initially ascends in fibre-dense regions. The competition between capillary and tensile forces causes displacements, redistributing fibre densities. In plant fibres, swelling amplifies these displacements and propagates them along the wick axis. Water envelops individual fibres or bundles, penetrating into the lumens. The size of wet lumens increases due to internal capillary pressure, which is influenced by the convexity of the internal walls. This pressure drives a creeping progression of the water front along the fibre axis. The study finds that a high number of strings with intermediate porosities, which accommodate swelling, enhances the capillary rise height. However, further research is needed to fully understand the role of lumens. While they provide additional water storage capacity and facilitate capillary action through small diameters, they may also reduce the flow rate of water ascent.

# 1. Introduction

Process cooling requirements are increased by climate change and evaporative cooling is a method studied to facilitate heat rejection into warm ambient air. With high heat dissipation fluxes, evaporative transfer helps to cool down processes but has some adverse effects. Water films on heat sources must be controlled to prevent film rupture, fouling or clogging. Drainage is sought to control water film and can be operated with a porous medium to achieve an evaporative transfer similar to plant evapotranspiration. Such concepts were described, for example in [1] or [2] for heat sources and are under development, such as the tree inspired evaporator in [3]. Evaporation occurs at air/water interface, whereas capillary suction is ruled by the liquid/solid interface. In tree concepts, one has to design porous wicks, whose drying in the surrounding air is compensated by an internal capillary circulation of liquid. Such wicks were studied by Dai et al. in [4] by using copper micro-membranes. However, their membranes diffuses liquid in the horizontal direction and this limits the cooling effect of evaporation near ground, as does coating treatments. With tree concepts, transfer surface is vertically increased, because capillary pressure can drive liquid in large vertical extent, as for trees. Then, the evaporative surface is increased in proportion with the rising height, which becomes a key factor of transfer efficiency [5,6].

Capillary flows in porous wicks dedicated to heat and mass transfer were studied by many authors at macroscale. Zhao et al. [7] have compared the strengths and weaknesses of many materials like wood, ceramic or metals, estimating costs, shaping ability, contamination risks and efficiency for heat and mass transfer. Cellulosic pads (pack beds) are often used at ambient temperature to humidify or cool down air, thanks to their high transfer efficiency [8–11]. Other investigations were conducted on bio-sourced cellulosic materials for economic and ecological motivations: Al-Sulaiman et al. [12] studied some natural fibres, like jute, palm and luffa. Jain et al. [13] added efficiency assessments about aspen, coconuts, palash and khus fibres and Surajan-Salins et al. [14] documented wood shaving and coconuts fibres, showing comparable humidification efficiency of natural fibres, with respect to a commercial cellulosic pad, used as reference. Cellulosic materials are always interesting candidates for evaporative applications, but their rising characteristics for tree concepts are less documented, as macroscale parameters, like the biporosity defined in [15], are needed to describe flows, as well as heat and mass transfer, in such porous media with edge effects.

In porous media, Darcy's equation estimates the flowrate, by drawing an analogy between hydraulic and electrical conductivity. The Carman–Kozeny equation refines this description by introducing geometric features to describe an average topographical effect in the permeability estimation [16]. Porosity, particle/pore radius or specific surface are introduced in the flowrate equation, making the Carman–Kozeny permeability a widely used model for fibres [17–19]. Further studies on sphere and cylinder packing, in [20,21] have established theoretical bounds for porosity and the corresponding permeability of packed structures. The Carman–Kozeny approach is often combined with the Lucas–Washburn model describing the dynamic penetration of

fluid in the porous medium. Based on the momentum equation, Lucas–Washburn equation describes rising height with time, being controlled by a friction/capillary pressure at start and converging towards Jurin's weight/capillary balance at the end. Bosanquet added the inertial component in the movement equation [22,23] and evaporation contribution seems not significant according to Duan et al. [6]. Whatever the way of expressing permeability or force balance in the momentum equation, the terminal height is always sized by the contact angle and capillary radius.

By measuring the rising height and mass over time, the Washburn model is often used to evaluate the contact angle of a material. Cellulose, in particular, exhibits complex wettability properties and can be either hydrophilic or hydrophobic, depending on its crystalline properties after chemical treatments, as discussed by many authors [24–26]. Chemical heterogeneities at the nanocrystal or cell scale may explain some difficulties in expressing the contact angle or surface free energy of cellulosic material [27]. These prevent the development of standardized methods for characterizing hydrophilic properties [28], perhaps because of dynamic interactions between cellulosic solids and liquids. Hence, authors sometimes uses the Lucas–Washburn model to get the contact angles. For example, Volkov et al. in [29] assessed reproducible contact angles for cellulosic fibres, using final equilibrium height of capillary ascension and initial dynamic quantities to express a so-called equivalent capillary radius in fibres. It was also used on microcrystalline cellulose [30]. The contact angle corresponds to a thermodynamic equilibrium [31], mechanically and chemically achieved at terminal height. Pucci et al. in [19] used Washburn technique to monitor the sinking height, and found this theory appropriate for carbon fibres but not for flax fibres: this was attributed to swelling and authors established a relationship between swelling and water retention properties. Vo et al. in [32] measured the swelling rate of unitary fibres at various moisture contents and introduced fibre radius growth into the equation. This led to satisfactory results, once the capillary radius growth was accurately fitted. With biosourced fibres, accurately determining both the cellulosic contact angle and the capillary radius remains challenging, making it difficult to predict a terminal height with respect with a design: indeed, the capillary radius comes out from fitting rise measurements and not from studying the geometry.

Theoretical permeability is also still inappropriate for dynamic ascension in synthetic fabrics for other reasons. According to Fisher et al. [18], some stochastic bursts occur in rising and are related to pore sizes. Heterogeneity in pore sizes enhances height and rising rate and leads to a stepwise ascension [22]. Constrictions [33] and non-circular shape [34] are well known for their major role in capillary flows, like those in capillary rising. Moreover, the rise dynamic has to deal with the swelling behaviour of cellulose, which exhibits additional 3D-dimensional changes in topography induced by water [27]. Cellulosic material are known for their swelling properties, of which Garat et al. [35,36] measured the radius evolution of many natural fibres, with increasing water vapour or after immersion in liquid. Van der Velde et al. [37] studied the behaviour of solvents droplet on a swelling polymer, including constraints, stress analysis, and observed de-swelling phenomena. Heterogeneities in size, non-circular shapes,
water induced changes in topographies, uncertain wettability properties, a lot of parameters are involved when looking for rising height, which is always measured rather than controlled by design.

Capillary systems made of yarns can effectively pump water films in evaporative applications but investigations are necessary to thoroughly describe their surface and volume storage, explaining their high efficiency in evaporative transfer. This work aims to explore the design for achieving superwickling yarns, in which the capillary rise height determines the extent of the evaporative surface. The capillary process involves both surface properties, related to wettability and structural features such as capillary diameter, typically associated with porosity and fibre diameter. Therefore, measurements of surface properties, topographic distribution of diameters and the related rising heights were conducted on various samples made of natural fibres. Morphologies of yarns were studied thanks to tomography in wet and dry conditions, using two resolution scales and a dynamic observation. The dynamic results and multiple scale approaches help identify the needs for accurate capillary rise models and contribute to the design of superwickling devices. After fibre sizing in dry conditions, sorption results reveals significant discrepancies in the wettability properties of samples. From the rising results, some macroscale parameters discriminates the terminal height of ascent. Fibre displacements observed during the rise process indicate that fibre mobility affects porosity and yarn volume. Swelling effect is studied at the cell scale for plant fibres: this intermediate scale of porosities, related to lumens, is documented. Swelling and capillary pressure also modifies yarn geometry, as demonstrated and discussed in the final section. Water flows radially coming from small porosities, within plant cells on the one hand, and within larger scale porosities on the other. These results suggest that wicking capacity is not solely governed by the fibre diameter distribution, but must also take into account multiscale wet porosity when defining the effective capillary diameter.

## 2. Material and method

Capillary rise and imbibition are often considered key advantages of bio-sourced materials. In this context, natural samples were selected to compare plant fibres with a wool sample. This analysis includes a broad set of biobased materials, whose main characteristics are described in the first subsection. Furthermore, sorption and desorption isotherms provide insights into wettability. These curves are influenced by the surface properties of the solid, notably through the contact angle. Sorption tests are presented and discussed in the second subsection. Capillary rise was measured using a custom-designed device to assess variability in capillary ascent. In the study of [32], tomography and optical microscopy were employed to evaluate both wet and dry porosity, as well as the initial average capillary radius. Similarly, in the present work, dual-scale tomography was used to determine porosity and fibre dimensions, capturing the dual-scale nature of swelling, as described in [32]. In addition, a dynamic tomographic test was performed to monitor morphological changes over time. Tomography settings are detailed in the last subsection.

### 2.1. Yarn description at macroscale

A collection of commercial hemp yarns, twines and rope was tested in the present study. They were provided by the same manufacturer and covered a wide range of diameters and structures with similar composition. Other samples of plant fibres, with different structures and compositions, (jute, sisal, cotton and flax) were also studied. One sample of animal fibres (wool) was added to compared various bio-sourced materials. All are commercially available samples, made of pure natural fibres and may have been exposed to unknown chemical treatment before or after spinning. Yarn diameters and densities are detailed in Table 1. For most samples, the overall diameter was measured with a

caliper or optical measurements but for small samples, like hemp fil1, cotton and wool.

Yarn structures vary depending on the amount of unitary fibres involved, tension and twist imposed during spinning. The spin structure is studied by unravelling yarns to establish the number of strings down to unitary fibres. For hemp, the smallest yarn, named 'fil1', has the smallest diameter and a Z-spin: this single thread yarn is intended for textile fabrics. It is also used to make more elaborate threads, from fil3 up to fil12, which are composed of 3 up to 12 'fil1', assembled by an S-shaped spin. Other textile fibres, namely wool and cotton, also present two levels of spinning: wool is spun using 3 strings, and cotton with 5 strings, as described in Table 1. Hemp twines are composed of threads of bigger diameters: the smallest 'fic1' is made of two strings, whereas 'fic2' and 'fic3' are made of two by two, respectively three by two 'fic1' yarns. There is one more spin level in the rope 'cd03'. For other materials (jute, flax, sisal), fibres are more rigid with fewer spin levels, compared to hemp samples: one spin level for sisal and two for jute (2 strings) and flax (3 strings). Tex was measured by weighting appropriate lengths, from 50 cm to 3 m, depending on the yarn width and the capacity of laboratory balance.

### 2.2. DVS method

The water sorption-desorption isotherms of samples (100 mg) were measured using a gravimetric sorption high speed analyser (SPSx-1μ, ProUmid, Germany). The samples were placed in a chamber maintained at 25 °C and relative humidity (0% RH to 90% RH) with a laminar air flow consisting of a controlled mixture of dry air and water vapour. A microbalance (resolution 1 µg) weights the absorption and desorption of water vapour on the sample. Each kinetic point represents the steady-state mass $meq(t)$, reached by the sample after several hours for each relative humidity step. Equilibrium was reached when the slope of the linear regression of the points over 90 min less than 0.02% in the mass. Each value measured at equilibrium between the vapour in the atmosphere and the water in the sample was taken as a data point for plotting the water sorption/desorption isotherm curve. The water content ($Cw, \%$) was calculated according to the following equation:

$$Cw(\%) = 100 \times \frac{m_{eq}(t) - m_{d}}{m_{d}} \qquad (1)$$

where $m_d$ is the mass of the dry sample, which was measured after a drying sequence, consisting in a heating step at 40 °C under a dry flow of air for 24 h.

### 2.3. Rise measurements

Water ascension was studied using a brace monitored by an infra-red camera for water and a conventional camera for colorimetric tests. Yarns were fixed on the brace, shown in Fig. 1(a), and plunged into a vessel filled with water to get pictures like 1(b) : the same protocol as in [38] was used to calibrate the method using the final static height of rise, compared with colorimetric ones. Rise dynamic was also monitored for a few samples using regular shooting and infra-red pictures were analysed using Python treatments.

### 2.4. Tomography measurements and further definitions

In this work, tomography was used to characterize fibre diameters and porosity. A special device was 3D-printed to hold yarns vertically with a relative constant but not controlled tension inside a tube, by fixing the bottom of strings at a docking port and fixing it with adhesive at top port (1(c, d)). Bottoms of the samples are plunged in a vessel, which is empty for dry samples or filled with water for wet samples. For all materials, wet samples were prepared one day before the tomography experiment, to ensure a stable height for liquid rise. All samples were stored at 25 °C and 55% humidity.
<figure><img src="image_6.png" /><figcaption>Fig. 1. Devices = brace (a) used to measure rising height of water by infra-red camera (b) for 6 yarns of samples and pot for the tomography (c, d) of wet and dry samples.</figcaption></figure>

<table><caption>**Table 1**

Yarn characteristics: material, name, structure, string dry diameter, wet on dry diameter ratio, tex, dry porosity and capillary diameters.</caption><thead><tr><th>Type of fibre</th><th>Material</th><th>Name</th><th>Strings</th><th>Diameter mm</th><th>Ratio d<sub>w</sub>/d<sub>d</sub></th><th>Weight g/km</th><th>Porosity %</th><th>d<sub>i</sub> µ.m</th></tr></thead><tbody><tr><td rowspan="10">Dissoc.</td><td>Hemp</td><td>fil1</td><td>1</td><td>0.3</td><td>1.03</td><td>100</td><td>39</td><td>4.2</td></tr><tr><td>Hemp</td><td>fil3</td><td>3</td><td>0.95</td><td>1.1</td><td>315</td><td>53</td><td>4.8</td></tr><tr><td>Hemp</td><td>fil6</td><td>6</td><td>1.1</td><td>1.36</td><td>580</td><td>37</td><td>3.8</td></tr><tr><td>Hemp</td><td>fil12</td><td>12</td><td>1.7</td><td>1.</td><td>1310</td><td>36</td><td>3.6</td></tr><tr><td>Hemp</td><td>fic1</td><td>2</td><td>0.9</td><td>-</td><td>700</td><td>-</td><td>-</td></tr><tr><td>Hemp</td><td>fic2</td><td>2*2</td><td>1.8</td><td>1.09</td><td>2200</td><td>27</td><td>3.3</td></tr><tr><td>Hemp</td><td>fic3</td><td>3*2</td><td>3.6</td><td>-</td><td>7300</td><td>-</td><td>-</td></tr><tr><td>Hemp</td><td>cd03</td><td>3*3*2</td><td>3.3</td><td>1.03</td><td>7200</td><td>22</td><td>1.5</td></tr><tr><td>Wool</td><td>wool</td><td>3</td><td>1.45</td><td>0.88</td><td>470</td><td>63</td><td>19</td></tr><tr><td>Cotton</td><td>cotton</td><td>5</td><td>1.2</td><td>1.42</td><td>320</td><td>52</td><td>6</td></tr><tr><td rowspan="3">Bundles</td><td>Jute</td><td>jute</td><td>2</td><td>2.2</td><td>1.18</td><td>2300</td><td>47</td><td>17</td></tr><tr><td>Sisal</td><td>sisal</td><td>1</td><td>1.6</td><td>1.10</td><td>1500</td><td>27</td><td>12</td></tr><tr><td>Flax</td><td>flax</td><td>3</td><td>3.4</td><td>1.07</td><td>3700</td><td>24</td><td>13</td></tr></tbody></table>

<table><caption>**Table 2**

Parameters related to tomography settings and sizes.</caption><thead><tr><th>Magnification</th><th>Pixel size µm</th><th>Mean X-ray energy keV</th><th>Distance mm</th><th>Exposure time s</th><th>Number N projections</th><th>Total time s</th></tr></thead><tbody><tr><td>2X</td><td>3.082</td><td>15</td><td>290</td><td>0.05</td><td>2000</td><td>130</td></tr><tr><td>10X</td><td>0.654</td><td>17.8</td><td>39</td><td>0.1</td><td>2000</td><td>240</td></tr><tr><td>10X (binning)</td><td>1.309</td><td>20.4</td><td>39</td><td>0.01</td><td>300</td><td>6.32</td></tr></tbody></table>

Yarns were fixed in a tube of the tomography holder (see Figure 1c) which allows fluid to be added at the base. Wet or dry yarns were imaged with the microtomography setup on the ANATOMIX beamline [39]. Three configurations were used and their corresponding settings are given in Table 2. At low resolution, the so called ‘2X’ provides a field of view 6.3 mm × 6.3 mm and the high resolution, called ‘10x’ in text covers 1.3 mm × 1.3 mm. The collected data at different heights were reconstructed with the standard reconstruction pipeline at ANATOMIX using PyHST2 [40]. The pagarin length was adapted for optimal segmentation of the wet yarns. All samples were scanned at 2X where the whole yarn section fits into the field of view. At 10X, only the finest yarns (fil1, fil03, cotton) fits fully into the field of view. For other samples, 10X tomographies are used to study unitary fibres, whose concept needs further explanations.

Tomography images show that some samples are made of aggregated fibres: each plant cell (unitary fibre) appears as a cell wall surrounding a hole which corresponds to the lumen. Cells are stuck together in so-called bundles, as shown in Fig. 2(b). Bundles were observed for jute, flax and sisal. For hemp, cotton or wool, unitary fibres are more or less free from the others and present smaller and roughly circular shapes. These latter are therefore called dissociated

fibres in the following. Lumens are not present in wool, which is not a plant fibre. In dry conditions, lumens of plant dissociated fibres are not clearly seen at the 10X magnification but they become more apparent in wet samples, as shown on Fig. 2(c).

## 3. Results

The structure of the yarn samples, specifically their porosity and fibre dimensions, is characterized using X-ray tomography and subsequently discussed. Water retention properties and the final height of capillary rise are then examined. The dynamics of water ascent are presented thereafter: fibre displacements are described and affect the morphology of samples during rise. These changes, related to swelling phenomena, are described at two distinct scales: first at the cell level, and then at the yarn scale.

### 3.1. Fibre sizing in dry conditions

Python codes were developed in order to assess porosity and size distributions for aggregates, fibres and micropores, i.e. lumens. The core size of yarns is in agreement with caliper measurements. Once the core selected, porosity is assessed for dry samples and reported in Table 1. This porosity corresponds to a macroscale porosity, which includes macropores between strings and main gaps between fibres.

For jute, sisal and flax, bundles were sized using the 10X magnification. Size distribution of bundles was truncated in order to exclude small and large particles (noise and incompleted particles on edges for small particles and multiple detections for large particle). The results are somewhat sensitive to this truncation step: bimodal distributions were obtained for flax and sisal. The highest mode may be related to slanting fibres and was filtered with criteria on the sphericity. After filtering, distributions were in good agreement at 2X and 10X,
<figure><img src="image_7.png" /><figcaption>**Fig. 2.** Horizontal slices of the tomography volume at 10X magnification: (a) hemp fibres in fill12 are dissociated and (b) jute has aggregated fibres (bundle). Fibre swelling is observed on wet samples on hemp (c) and jute (d): white and black belts indicate air bubbles. The scale bar represents 100 µm.</figcaption></figure>

<table><caption>**Table 3**
Diameters for unitary fibres, bundles and pores (lumens) in dry and some wet samples.</caption><thead><tr><th>Unitary fibres</th><th>Mean</th><th>sd</th><th>Aggregated fibres</th><th>Mean</th><th>sd</th><th>Lumens</th><th>Mean</th><th>sd</th></tr></thead><tbody><tr><td colspan="9">Dry diameter (microns)</td></tr><tr><td>Wool</td><td>34</td><td>8</td><td>Sisal</td><td>100</td><td>30</td><td>Sisal (12%)</td><td>6.8</td><td>4.4</td></tr><tr><td>Cotton</td><td>16</td><td>5</td><td>Flax</td><td>140</td><td>68</td><td>Flax (13%)</td><td>7</td><td>4.3</td></tr><tr><td>Hemp (all filx)</td><td>19</td><td>5</td><td>Jute</td><td>56</td><td>23</td><td>Jute (4.3%)</td><td>5.1</td><td>3</td></tr><tr><td colspan="9">Wet diameter (microns)</td></tr><tr><td>Wool</td><td>35</td><td>8</td><td>Sisal</td><td>117</td><td>35</td><td>Sisal</td><td>7</td><td>3</td></tr><tr><td>Cotton</td><td>26</td><td>9</td><td>Flax</td><td></td><td></td><td>Flax</td><td>-</td><td>-</td></tr><tr><td>Hemp (all filx)</td><td>-</td><td>-</td><td>Jute</td><td>58</td><td>26</td><td>Jute (7.2%)</td><td>6.6</td><td>3</td></tr></tbody></table>

showing that the small control volume at 10X adequately represents the whole yarn particles. Due to their small sizes, dissociated fibres were measured using only the 10X images. Most porous samples were used in the case of hemp, because cells were too close to be distinguishable in dense samples.

With plant fibres, there is a so-called internal porosity, corresponding to lumens, which are not seen with the 2X resolution. Lumens are very small in dry conditions. Thus, it was not possible to assess their sizes for dissociated fibres (cotton and hemp). In bundles, a two-step treatment is applied: first, aggregates are detected and used as masks on the original picture to detect holes. This internal porosity (porosity of bundles), is given in parenthesis in Table 3: pore density is about 0.0021 pore/square micrometer of aggregate for jute, 0.0031 for sisal and 0.0033 for flax in dry conditions. The image analysis often fails in wet conditions, because it becomes difficult to detect bundles in the first step of treatment. Water penetrates inside the cristallin network of plant tissue and is therefore everywhere, lowering contrast in dense fibre or bundle area. Hence, no data are reported for flax in wet conditions.

Measurements are expressed in diameters, computed from areas as if particles were fully circular. Fibre sizes are in agreement with literature data for wool and cotton. For hemp, fibre sizes are slightly lower than those of stems [41] and closer to those after extraction [42]. It is more difficult to compare bundle sizes, which are strongly influenced by their industrial treatment before spinning. Sisal results are however in the range found in [36,43,44]. Lumen porosity varies inside the usual range

of 5%–15% for dry bundles [45,46] and increases in wet conditions. By using two resolution scales, tomography enables the measurement of fibre dimensions, from bundles to lumen diameters, and provides insights into the size distribution around their mean values. However, we were unable to quantify the number of fibres in samples due to their high density in string cores.

## 3.2. Water retention ability

The water sorption capacity was clearly contrasted between studied fibres across the entire RH range (Fig. 3(a)). From the cotton fibre with the lowest water sorption capacity to the wool fibre with the highest one, there are dissociated fibres (hemp) followed by the bundled fibres (sisal, jute and flax). The chemical treatment, which might have been applied to the cotton, reduces its hygroscopic properties (water content inferior to 10% at 90% RH), in comparison to pure cellulose nanofibres [47,48]. Regardless of the structure of hemp fibre assemblies (fill, 3, 6, 12, fic2 and cd03), similar water capacity properties were observed: water sorption is poorly related to the porosity, although isotherms are more or less corresponding to type II (macroporous media, see [49]).

Hysteresis, calculated from Eq. (2) and shown in Fig. 3(b), is related to the ability of the porous fibre network to retain water molecules adsorbed on hydrophilic sites on the surface of polymer chains and into the material's opening nano- and micropores [50].

$$H_w = C_{w,d} - C_{w,s} \qquad (2)$$
<figure><img src="image_8.png" /><figcaption>Fig. 3. (a) Water sorption/desorption isotherms as a function of relative humidity (RH, %) of commercial fibres and (b) respective hysteresis measured by dynamic vapour sorption at 25 °C.</figcaption></figure>

where C<sub>w,d</sub> and C<sub>w,s</sub> are water content, expressed in g of water per 100 g of the dry sample, determined during the desorption and sorption cycles respectively.

Type IV Hysteresis [49] indicates a narrow slit-like porosity, corresponding well to fibre assembly. This water retention ability is higher for bundled fibres than for dissociated fibres. H<sub>w</sub> value increases regularly from 5% RH to 60% RH, until it reaches a plateau at around 1% for hemp fibres and 2% for bundled ones. For wool, individual fibres are saturated with water and then no longer able to retain free water after 60% RH. Contrasted retention values were observed, revealing contrasted wettability properties among samples. These differences are attributed to their chemical properties rather than their porosities [45]. Indeed, the close results obtained for hemp would give very similar pore sizes with the BET, although their scattered densities. Consequently, no attempts were made to assess porosity from these tests.

### 3.3. Equilibrium height of water rise

After one night of stabilization, the terminal height is measured thanks to the infra-red camera. Results are shown in 4. The first nine samples are made of hemp. They exhibit equilibrium heights over a large interval of [2–12] cm: even if their water retention ability are closed, assembly properties are dominant and their equivalent capillary diameters differ significantly. High rises are observed for the textile yarns with many strings: the more strings, the higher rise. Terminal height is even higher for twines (fic2 and fic3). The most dense rope cd03 has however an intermediate height. This suggests an optimum for fibre density to achieve a significant terminal height.

Heights are not correlated with the unitary fibre diameters: flax or sisal (df > 100 µm) have an intermediate rise within 5–7 cm, like hemp rope (df = 20 µm). Terminal height in jute (df = 60 µm) is far above others. A high rising height can be achieved with large bundle diameters. Cotton (df = 15 µm) has a slightly higher rise than wool (df = 30 µm). Its water retention ability is one third of that of wool. Hemp yarns also have very different terminal height although similar retention abilities: the water retention test gives a poor indication on water rise in yarns.

Pucci et al. [19] considered an ideal arrangement of cylinders of d<sub>f</sub> diameter and regularly spaced with a distance d<sub>i</sub> between them. For a large number of cylinders, a relationship gives the porosity φ from d<sub>f</sub> and d<sub>i</sub>. Here, it is inverted to get an estimate of d<sub>i</sub>, distance between cylinder, which should sizes the capillary pressure:

$$d_i = d_f \left( \sqrt{\frac{\pi}{2\sqrt{3}(1-\phi)}} - 1 \right) \quad (3)$$

Estimate of d<sub>i</sub>, which involves both porosity and fibre diameter, are given in Table 1 using dry values. Capillary pressure and hence, terminal height, should vary inversely with the capillary diameter. This

<figure><img src="image_9.png" /><figcaption>Fig. 4. Equilibrium height of water rise in samples: crosses are for average and ticks for the standard deviation interval.</figcaption></figure>

was not obtained. For hemp, a very dense yarn, like cd03, has a very small distance between fibres (estimate < 2 microns) while it is above 4 µm for a light yarn like fil3: both equilibrium heights are close. If the distance between fibres is somewhat correlated with terminal height for hemp yarns, it fails when comparing other yarns: for example, jute and wool have the biggest distance between particles (of about 17–20 microns), high retention abilities (above 20% of mass at RH = 100%), but contrasted terminal height (8 cm for wool, 16 cm for jute). Even if the distance were computed in wet conditions, distances would not differ sufficiently to explain these contrasting terminal heights.

Capillary diameters are not solely related to unitary fibre diameters: high rises are observed for intermediate porosity and assembly geometry has a dominant effect on rising, suggesting a role for major voids in yarns.

### 3.4. Dynamic rise of water in yarn

Water rises spontaneously in yarns after plunging their bottom in the vessels. The front of liquid is diffuse and the rising height is defined by the point where the spread of the liquid becomes as wide as the diameter of the yarn in camera pictures. On Fig. 5(a), the rising height is reported for 6 pieces of fil6. It reveals the scattering of ascension, which is slightly reduced with an improved control for tension, but remains high due to the variability of such natural materials.

Dynamic conditions were studied using tomography: a conventional tomography was repeated 20 times at the same height on a sample just after plunging it in water. For imaging the dynamic rising of fluid, faster and less resolved scans at magnification of 10X (binning 2 × 2, 1.3 µm) were recorded in 6.3 s and the repetition rate was every minute. The
<figure><img src="image_10.png" /><figcaption>**Fig. 5.** (a) Rising height of water according to time in 6 samples of fil6 monitored with the IR camera, (b) Schematic macropore junction at related contact line crossing and (c, d, e, f) Tomographies at different times during the rise of water on right – scale bar represents 200 microns.</figcaption></figure>

height was chosen at 4–6 cm height, where water slows down. The time $t = 0$ corresponds to the beginning of repeated scans. On horizontal slices of the volume (see Fig. 5(b)), the 6 strings of fil6 are clearly seen. Strings are deformed by twist: inner strings are compressed by peripheral ones, which are wider and elongated in a ellipsoidal shape. A contact line is formed between pairs of strings: here, fibre density is near the one observed in core of strings. Some so-called macropores appears mostly at junctions of three strings, or in peripheral regions of yarns. Same patterns were observed on other samples and shows how the amount of strings rules macropores.

Water arrival in sample is observed thanks to changes in texture in Fig. 5(c-f): it marks the imbibition process and appears first in dense areas. After 300 s, all strings contain water. Macropores are progressively filled afterwards. The same phenomena is observed at the top of rising: liquid remains in dense area but macropores and the periphery are dried.

This is why an irregular interface is observed around the terminal height: the final height is an equilibrium between the ascensional capillary forces in dense areas and some dispersive effects related to wider gaps found inside yarn and around its edges.

### 3.5. Mobility of fibres during rise

A second test was made at a higher height of the hemp fil 6 sample: water flows through the scanned volume after 740 s in this second test, after what all strings are wet. Displacements during the sampling period are described for some recognizable fibres (fibres of interest: FOI) located at the yarn edge. Their shape were recomputed for each of the 20 tomographies with a connectivity algorithm. FOI trajectories are represented on Fig. 6(a). Small movements are observed at the beginning of scan, resulting from the propagation of motion coming from the bottom. Water arrival in the volume is marked by a sudden change in the trajectory orientation: at this moment, the string core has entered in our subvolume. Central fibres swell and push outwards peripheral fibres with sticking them: hence, clipping was needed to separate FOI from core. This prevents a proper assessment of barycenter displacement. The reorientation is associated with the radial swelling of string, which repels radially the FOI. Among FOI, the forked fibre, with

its protrusion, reveals also a rotational movement, probably impart to spinning twist (Fig. 6(b)). Vertical movements are difficult to detect for vertical fibres but are observed for the slanting fibre in the corner (Fig. 6(c)).

Movements are present throughout the observation period but are more pronounced at water crossing. The velocity of fibre centre of mass reaches 0.2–0.4 microns/s at their maximum. The displacements are on the order of ten microns in all directions between the start and the end. The volume of fibres also increases over time and the representativeness of changes was founded insufficient for providing a more detailed assessment. Similar movements, observed with a camera on a pair of artificial fibres under swelling conditions, were described by Van der Velde et al. [37]. During water rising, first vertical displacements are related to deformation propagated by tension above the water front. Swelling is a rapid phenomena, appearing in dense areas and inducing further displacements after water passage. It reduces distance between fibres, with respect to their degree of freedom to move radially. Macropore filling is facilitated by swelling and the associated fibre merging.

### 3.6. Swelling and imbibition at the cell scale

Swelling modifies porosities and fibre sizes. With an animal origin, wool is the unitary material, whose fibre diameters are easily measured and found to remain constant in wet and dry samples. In the case of plant fibres, swelling induces significant changes in yarn topography. Lumens might also play a role. Indeed, water sheathes fibres and penetrates inside cells, for both dissociated or bundled plant fibres: this was shown in hemp and jute sections in Fig. 2 and with volume in hemp and sisal volumes in Fig. 7. Wet lumens become more visible for all plant fibres, including cotton which has a poor water retention value. Plant fibres are biporous, as Semenic and Catton wishes for their high performance cooling wicks [15].

From volume reconstruction, it appears that liquid water easily crosses cellulosic wall: The cristalline network of cellulose is like wire lattice rather than a solid. Lumens in bundles appear fully filled with water, as are gaps between bundles (see Fig. 7(b)). Water comes from the cell exterior and sheathes fibre walls. The capillary forces have
<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Fibre positions during dynamic rise of water in hemp fil6, for (a): cut of five recognizable fibres of interest (FOI) at yarn periphery, (b) another view of the forked FOI, showing the rotational displacement and (c) another view for the slanting FOI in corner, showing vertical displacement. In violet, *t* = 0 s and in yellow, *t* = 1200 s, grid ticks are in voxels, with 1.3 µm per voxel.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 7.** Top views of 3D rendering of segmented volumes in the phases of water (blue), fibres (brawn), and air. Units are in voxel. (a): contact line between strings in fil3 with the central macropore free of water in front. (b) three sisal bundles at yarn edge are sheated, filled with water, and a water bridge between neighbouring bundles appears (c) sliding droplets on detached peripheral fibres at hemp fil6 edge.</figcaption></figure>

their main component along the fibre axis and a small component oriented inwards, due to the concave cylindrical shape of wall. Conicity and curvature are well known for their role on capillary velocity of flows [51]. When water penetrates, the lumen pressure becomes high, due to the high convex wall curvature of lumens: lumen capillary forces are oriented outwards. Outwards forces could explain the lumen opening observed with electron microscopy by Chatron-Michaud et al. [52]. This could also explain how water propagates from pore to pore in bundles, until the latest is full. When the sample is fully wet, most of lumens are also filled with water: less than 5% of lumens for sisal, 5% for jute (see Fig. 2) and 20% for flax contains air bubbles. At the top of the liquid column, both lumens and largest macropores become empty together with increasing height. Water bridges, which are water volumes more likely stored between 3 cells or 3 bundles, become increasingly disrupted and reduce to a thin sheath around cell walls at the column top. As water inside lumens comes from the outside, the secondary and so-called internal porosity, related to lumens, may consume the flow rate and depletes the outside capillary pressure, rather than enhancing rising through its small dimensions. Pressure might be also depleted by peripheral fibres: droplets are seen on some isolated fibres at yarn edges, suggesting water-sliding flow downwards in the periphery, shown on Fig. 7(c). As suggested in [32], both fibre density and internal porosity are involved in capillary rise.

Due to the small thickness of cell walls, measuring sizes of water bridge was challenging. It was difficult to find out a reliable threshold without dissolving some solid. With the phase contrast method, a high contrast value represents “air/other” interface, but other is either water or fibre. On Fig. 7(b) for example, the air/water interface appears like a “fibre sheet” inside the water bridge in the front of the volume. In addition, fibres becomes very close to the surrounding ones. Thus,

swelling size changes are given only for cotton, jute and sisal, which presents most regular fibre density and provide more reliable trends with various image treatment (ImageJ, Python). For jute and sisal, both pore size and porosity increase in wet conditions, compared to dry ones (at 50% of RH). Assuming a constant number of pores per bundles, jute bundle surface increases (+7%), mainly because of the pore size increase (+70% for lumen opening) and only slightly due to a wall thickness increase (+4%). On the contrary, increase of sisal bundles (37%) is mainly related to wall thickness increase (41%) rather than pore size increase (7%); but, the change between dry and wet sizes is not significant for sisal (p-value = 0.49), whereas it is clearly significant for jute (p-values < 0.001): a net lumen opening is observed for jute, which is in accordance with surface growth in [52]. Hydroexpansion of cell tissues is attributed to bond water within cell walls and the middle lamella, according to Garat et al. [35]: it seems predominant for sisal. At bundle size, hydroexpansion rates were found higher than those measured on one bundle by Laser [35]. Differences in protocols and pressure inside lumens during rising might explain some of the differences. Moreover, difficulties in sizing pores and bundles, along with averaging, lead to uncertainties and large scattering in our results.

On average, the swelling of cells should enhance capillary rise, as it reduces distance between fibres. But lumens have an ambiguous role: too many lumens might dissipate flow rate and explain poor rises obtained for the most dense yarns. Hence, an optimal porosity might be specific to plant fibres.

### 3.7. Swelling and tension at yarn scale

Assembly properties are important and spin tension affects the displacements of fibres during water ascension. Hence, yarn scale was
<figure><img src="image_13.png" /><figcaption>Fig. 8. Radial average profile.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 9. Mean radial contrast for wool, cotton and hemp fil6 on left and local contrast in dry and wet sections.</figcaption></figure>

also studied. Hence, positive contrast of tomographies was averaged over the whole scan box height: the contrast is used as marker for fibre densities. Height averages are expressed as function of radius, with respect to the slice barycenter. In this way, the rotational effect related to spinning twist is smoothed. Profiles are represented in Fig. 8 with corresponding local sections in wet and dry conditions in Fig. 9. Wool, cotton and hemp fil6 were chosen for their contrasting behaviour when switching from dry to wet conditions.

For wool, fibres are more or less uniformly distributed and regularly spaced in dry conditions. Hence, strings are less visible on slices. Macropores are more likely observed near the periphery of the yarn (see Fig. 9(a)). Wet, unitary fibres do not swell but are displaced by capillary

forces. Indeed, the capillary force is related to the distance between fibres, which sizes the curvature of the liquid/gaz interface and the related capillary pressure. When this *interfibre* distance increases at yarn periphery, it induces a radial gradient of capillary pressure, between dense fibre region and periphery. This gradient is responsible for fibre movements. A density deficit also appears in the contrast profile at yarn centre: a central macropore is created. Yarn radius decreases from 1060 to 880 µm because spinning tension prevents radial displacement at the borders.

For cotton, unitary fibres swelling is not visible at yarn scale. In dry conditions, a macropore is found in the central region of the yarn, where the contrast is reduced and around which cotton fibres are wound. As some fibres are inside, the contrast in macropore is two third of the maximum one, found at core string locations. Wet, the central pore is still visible. However, the area of maximum contrast is larger, indicating the spread of the high density regions: the whole yarn also swells and its radius moves from 520 to 620 µm. The wet section loses its circular shape and fibres are still dispersed, even in dense fibre regions.

For hemp fil6 dry, macropores at junctions are observed in the dry section (see Fig. 9(e)) but disappear from profiles when integrating on height: this indicates that macropore heights are small, thanks to spinning with many strings. Fibre density decreases slowly at yarn edges due to peripheral pores. In wet conditions, fibres swell and yarn radius increases from 610 to 770 µm. Unitary fibre swelling makes yarn swelling: distance between particles are reduced, tending near zero and macropores are also reduced (see Fig. 9(f). The terminal height for hemp fil6, of 6 cm, is somewhat lower than those of previous samples (8.9 cm for wool and 10.2 cm for cotton). Swelling, like water retention properties, is not sufficient to obtain a good rise.

Yarn radius increases in wet conditions for all samples made of plant fibres (see Table 1 for ratios): distance between fibres and macropore sizes are reduced. Yarn radius increases of less than 10 percent for monostrings (sisal, fill) and ropes (flax, cd03). This might be related to high twist tension, which may affect fibre mobility and prevent swelling. But the low porosity of dense samples (cd03, sisal, flax) might also limit rise due to a poor amount of water available for filling internal porosities. Vo et al. [32] found necessary to account for fibre radius growing (low fibre density) and added the internal porosity of flax in assessment for rising dynamics in high density samples. In addition, it is shown here how fibre displacement has an impact on fibre density and macroporosity: the morphology of yarns can changed during the capillary rising of water. Changes depend upon spinning method which needs to be further described. However, intermediate porosity gives higher terminal height and is often associated with a larger yarn expansion.

## 4. Conclusion

This study provides an integrated framework for understanding and optimizing water transport through capillary-driven textile assemblies, with direct implications for the design of efficient wicks in spray cooling applications. By combining high-resolution and mesoscale X-ray tomography with capillary rise observations, the work characterizes how porosity, fibre swelling, and mechanical response interact across scales. Two levels of porosity, inter-yarn and intra-fibre (lumen) were successfully documented. In dry conditions, the distribution and density of fibres strongly influenced yarn porosity, while in wet conditions, capillary forces triggered complex deformation patterns. In wool, a non-swelling fibre, imbibition caused contraction of the yarn due to edge fibre adhesion and expansion of a central macropore. This restructuring altered the internal flow paths, illustrating the competition between capillary pressure and mechanical tension. In contrast, plant fibres exhibited swelling that led to lumen opening and progressive filling of internal porosity, accompanied by a reduction in external porosity. This dual-scale reconfiguration delayed the establishment of a stable capillary front and limited water ascent, especially in dense yarns.
A key finding is the importance of intermediate porosity: it has to allows sufficient fibre mobility and liquid retention without the drawbacks of macropore-driven reverse flows. Highly porous yarns showed rapid but inefficient rise due to edge leakage, while overly dense ones restricted flow through excessive swelling. Additionally, fibre displacements were observed before liquid arrival, suggesting axial transmission of deformation ahead of the wetting front, and a reorientation of movements radially once the control volume is reached. This delayed mechanical equilibrium, observed over extended durations, introduces a novel interpretation of capillary-driven flow in flexible porous media.

These results redefine the conditions under which Jurin's law can be applied to yarns, emphasizing the need to consider evolving pore size distributions and fibre mechanics. Water retention values were reported for contact angle assessments. The study also highlights the unique role of cell wall wettability and water storage in cellulosic fibres, potentially explaining their high evaporative efficiency. Finally, it opens promising directions for engineering yarns with tailored capillary properties by controlling fibre type, swelling capacity, twist, and tension, with potential applications in textile-based heat exchangers, passive cooling devices, and biomimetic systems. Modelling approaches should be further developed to account for the movement of permeable fibres in the assessment of capillary radius in yarns.

## CRediT authorship contribution statement

**Stephanie O.L. Lacour:** Writing – original draft, Data curation, Conceptualization. **Véronique Aguié-Béghin:** Writing – original draft, Validation, Formal analysis, Data curation. **Brigitte Chabbert:** Writing – original draft, Supervision, Resources, Methodology. **Mario Scheel:** Writing – original draft, Visualization, Methodology, Data curation, Conceptualization. **Damien Thomas:** Writing – original draft, Validation, Software, Formal analysis. **Pascal Clain:** Writing – original draft, Supervision, Software, Methodology, Investigation.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors would like to thank Timm Weitkamp and Camille Rivard for their invaluable assistance in designing the experimental project at Synchrotron, especially Timm for his insightful contribution to the development of the plans and devices. We also extend our gratitude to the Transform department of INRAE for its financial contribution to this work. This research would not have been possible without the support of the technical staff. Special thanks go to Seydina Ndoye, for printing the experimental devices and assisting during the campaign at Soleil, as well as to Miguel Pernes and Laurence Foulon for their extensive efforts in conducting the water retention tests using adverse setups. The authors would like to express their sincere gratitude to Bernard Kurek for the insightful and fruitful discussions that greatly contributed to the development of this work.

## Appendix A. Supplementary data

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.colsurfa.2025.137145.

## Data availability

Data will be made available on request.

## References

[1] T.D. Wheeler, A.D. Stroock, The transpiration of water at negative pressures in a synthetic tree, Nature 455 (7210) (2008) 208–212, http://dx.doi.org/10.1038/nature07226.

[2] Z.W. Huang, Y.H. Hwang, R. Radermacher, Review of nature-inspired heat exchanger technology, Int. J. Refrig.- Rev. Int. Froid 78 (2017) 1–17, http://dx.doi.org/10.1016/j.ijrefrig.2017.03.006.

[3] P. Wu, X. Wu, Y. Wang, H. Xu, G. Owens, A biomimetic interfacial solar evaporator for heavy metal soil remediation, Chem. Eng. J. 435 (2022) 134793, URL https://www.sciencedirect.com/science/article/pii/S138589472200300X.

[4] X. Dai, F. Yang, R. Yang, Y.-C. Lee, C. Li, Micromembrane-enhanced capillary evaporation, Int. J. Heat Mass Transfer 64 (2013) 1101–1108, URL https://www.sciencedirect.com/science/article/pii/S0017931013004201.

[5] N.-H. Kim, Residential humidifying elements having horizontal corrugated channels, Appl. Therm. Eng. 193 (2021) 117052, http://dx.doi.org/10.1016/j.applthermaleng.2021.117052, URL https://www.sciencedirect.com/science/article/pii/S1359431121004968.

[6] Z. Duan, M. Wang, X. Dong, J. Liu, X. Zhao, Experimental and numerical investigation of wicking and evaporation performance of fibrous materials for evaporative cooling, Energy Build. 255 (2022) 111675, http://dx.doi.org/10.1016/j.enbuild.2021.111675, URL https://www.sciencedirect.com/science/article/pii/S0378778821009592.

[7] X. Zhao, S. Liu, S.B. Riffat, Comparative study of heat and mass exchanging materials for indirect evaporative cooling systems, Build. Environ. 43 (11) (2008) 1902–1911, URL https://www.sciencedirect.com/science/article/pii/S0360132307002338.

[8] A. Malli, H.R. Seyf, M. Layeghi, S. Sharifian, H. Behravesh, Investigating the performance of cellulosic evaporative cooling pads, Energy Convers. Manage. 52 (7) (2011) 2598–2603, URL https://www.sciencedirect.com/science/article/pii/S0196890410005595.

[9] S. He, H. Gurgenciz, Z. Guan, X. Huang, M. Lucas, A review of wetted media with potential application in the pre-cooling of natural draft dry cooling towers, Renew. Sustain. Energy Rev. 44 (2015) 407–422, URL https://www.sciencedirect.com/science/article/pii/S1364032114010892.

[10] A. Tejero-González, A. Franco-Salas, Optimal operation of evaporative cooling pads: A review, Renew. Sustain. Energy Rev. 151 (2021) 111632, URL https://www.sciencedirect.com/science/article/pii/S1364032121009072.

[11] N.-H. Kim, Performance of an indirect evaporative cooler (IEC) made of PET/celulose composite sheet as wetting media, Appl. Therm. Eng. 186 (2021) 116492, URL https://www.sciencedirect.com/science/article/pii/S1359431120339673.

[12] F. Al-Sulaiman, Evaluation of the performance of local fibers in evaporative cooling, Energy Convers. Manage. 43 (16) (2002) 2267–2273, URL https://www.sciencedirect.com/science/article/pii/S0196890401001212.

[13] J.K. Jain, D.A. Hindoliya, Experimental performance of new evaporative cooling pad materials, Sustain. Cities Soc. 1 (4) (2011) 252–256, URL https://www.sciencedirect.com/science/article/pii/S2210670711000400.

[14] S. Surjanan Salins, S.K. Reddy, S. Kumar, Experimental investigation on use of alternative innovative materials for sustainable cooling applications, Int. J. Sustain. Eng. 14 (5) (2021) 1207–1217, http://dx.doi.org/10.1080/19397038.2021.1924894.

[15] T. Semenic, I. Catton, Experimental study of biporous wicks for high heat flux applications, Int. J. Heat Mass Transfer 52 (21) (2009) 5113–5121, http://dx.doi.org/10.1016/j.ijheatmasstransfer.2009.05.005, URL https://www.sciencedirect.com/science/article/pii/S0017931009003172.

[16] P. Carman, Fluid flow through granular beds, Chem. Eng. Res. Des. 15 (1997) S32–S48, http://dx.doi.org/10.1016/S0263-8762(97)80003-2, URL https://www.sciencedirect.com/science/article/pii/S0263876297800032; Trans. Inst. Chem. Eng. 15 (1937).

[17] G. Malvault, Détermination expérimentale de la distribution de taille de pores d'un milieu poreux par l'injection d'un fluide à seuil ou analyse harmonique (Ph.D. thesis), Ecole Nationale Supérieure d'Arts et Métiers, 2013, Thèse de doctorat Mécanique Paris, ENSAM 2013, URL http://www.theses.fr/2013ENAM0040/document.

[18] R. Fischer, C.M. Schleputz, J. Zhao, P. Boillat, D. Hegemann, R.M. Rossi, D. Derome, J. Carmeliet, Wicking dynamics in yarns, J. Colloid Interface Sci. 625 (2022) 1–11, http://dx.doi.org/10.1016/j.jcis.2022.04.060, URL https://www.sciencedirect.com/science/article/pii/S0021979722006166.

[19] M.F. Pucci, P.-J. Liotier, S. Drapier, Capillary wicking in flax fabrics – Effects of swelling in water, Colloids Surfaces A: Physicochem. Eng. Asp. 498 (2016) 176–184, URL https://www.sciencedirect.com/science/article/pii/S0927775716301923.

[20] A.G. Dixon, Correlations for wall and particle shape effects on fixed bed bulk voidage, Can. J. Chem. Eng. 66 (5) (1988) 705–708, http://dx.doi.org/10.1002/cje.5450660501, URL https://onlinelibrary.wiley.com/doi/abs/10.1002/cje.5450660501.

[21] J. von Seckendorff, N. Szesni, R. Fischer, O. Hinrichsen, Experimental characterization of random packed spheres, cylinders and rings, and their influence on pressure drop, Chem. Eng. Sci. 222 (2020) 115644, http://dx.doi.org/10.1016/j.ces.2020.115644, URL https://www.sciencedirect.com/science/article/pii/S0009250920301767.
[22] M. Borumand, T. Lee, G. Hwang, Enhanced wickability of single-columnar, non-uniform pore-size wick using Lattice Boltzmann Method, Comput. Fluids 238 (2022) 105376, http://dx.doi.org/10.1016/j.compfluid.2022.105376, URL https://www.sciencedirect.com/science/article/pii/S0045793022000548.

[23] C. Van Engeland, B. Haut, L. Spreutels, S. Sobac, Evaporation versus imbibition in a porous medium, J. Colloid Interface Sci. 576 (2020) 280-290, http://dx.doi.org/10.1016/j.jcis.2020.02.105, URL https://www.sciencedirect.com/science/article/pii/S0021979720302551.

[24] B. Ray, J. Anderson, J. Scholz, Wetting of polymer surfaces .1. contact angles of liquids on Starch, Amylose, Amylopectin, Cellulose and Polyvinyl Alcohol, J. Phys. Chem. 62 (10) (1958) 1220-1227, http://dx.doi.org/10.1021/j150568a015.

[25] H. Modaressi, G. Garnier, Mechanism of wetting and absorption of water droplets on sized paper: Effects of chemical and physical heterogeneity, Langmuir 18 (2002) 642-649, URL https://api.semanticscholar.org/CorpusID:95222918.

[26] C. Zou, L. Lao, Q. Chen, J. Fan, D. Shou, Nature-inspired moisture management fabric for unidirectional liquid transport and surface repellence and resistance, Energy Build. 248 (2021) 111203, http://dx.doi.org/10.1016/j.enbuild.2021.111203, URL https://www.sciencedirect.com/science/article/pii/S0378778821004874.

[27] M.A. Hubbe, D.J. Gardner, W. Shen, Contact angles and wettability of cellulosic surfaces: a review of proposed mechanisms and test strategies, BioResources 10 (4) (2015) 8657-8749, http://dx.doi.org/10.15376/biores.10.4.8657-8749.

[28] V. Fernandez, M. Khayet, Evaluation of the surface free energy of plant surfaces: toward standardizing the procedure, Front. Plant Sci. 6 (2015) 510, http://dx.doi.org/10.3389/fpls.2015.00510.

[29] V.A. Volkov, B.V. Bulushev, A.A. Ageev, Determination of the capillary size and contact angle of fibers from the kinetics of liquid rise along the vertical samples of fabrics and nonwoven materials, Colloid J. 65 (4) (2003) 523-525, http://dx.doi.org/10.1023/A:1025141523629.

[30] M.V. Hammes, A.H. Englert, C.P.Z. Noreña, N.S.M. Cardozo, Effect of water activity and gaseous phase relative humidity on microcrystalline cellulose water contact angle measured by the Washburn technique, Colloids Surfaces A: Physicochem. Eng. Asp. 500 (2016) 118-126, URL https://www.sciencedirect.com/science/article/pii/S0927775716302503.

[31] P.G. de Gennes, Wetting: statics and dynamics, Rev. Modern Phys. 57 (3) (1985) 827-863, http://dx.doi.org/10.1103/RevModPhys.57.827, URL https://link.aps.org/doi/10.1103/RevModPhys.57.827.

[32] H. Vo, M. Pucci, S. Corn, N. Le Moigne, W. Garat, S. Drapier, P. Liotier, Capillary wicking in bio-based reinforcements undergoing swelling - Dual scale consideration of porous medium, Compos. Part A: Appl. Sci. Manuf. 134 (2020) 105893, http://dx.doi.org/10.1016/j.compositesa.2020.105893, URL https://www.sciencedirect.com/science/article/pii/S1359835X20301317.

[33] A. Kovscek, C. Radke, Gas bubble snap-off under pressure-driven flow in constricted noncircular capillaries, Colloids Surfaces A: Physicochem. Eng. Asp. 117 (1) (1996) 55-76, http://dx.doi.org/10.1016/0927-7757(96)03637-0, URL https://www.sciencedirect.com/science/article/pii/S0927775796036370.

[34] T. Ransohoff, C. Radke, Laminar flow of a wetting liquid along the corners of a predominantly gas-occupied noncircular pore, J. Colloid Interface Sci. 121 (2) (1988) 392-401, http://dx.doi.org/10.1016/0021-9797(88)90442-0, URL https://www.sciencedirect.com/science/article/pii/0021979788904420.

[35] W. Garat, N.L. Moigne, S. Corn, J. Beaugrand, A. Bergeret, Swelling of natural fibre bundles under hygro- and hydrothermal conditions: Determination of hydric expansion coefficients by automated laser scanning, Compos. Part A: Appl. Sci. Manuf. 131 (2020) 105803, http://dx.doi.org/10.1016/j.compositesa.2020.105803, URL https://www.sciencedirect.com/science/article/pii/S1359835X20300415.

[36] W. Garat, S. Corn, N.L. Moigne, J. Beaugrand, A. Bergeret, Analysis of the morphometric variations in natural fibres by automated laser scanning: Towards an efficient and reliable assessment of the cross-sectional area, Compos. Part A: Appl. Sci. Manuf. 108 (2018) 114-123, http://dx.doi.org/10.1016/j.compositesa.2018.02.018, URL https://www.sciencedirect.com/science/article/pii/S1359835X18300599.

[37] P. Van de Velde, J. Dervaux, C. Duprat, S. Protiére, Imbibition and collapse between swelling fibres, J. Fluid Mech. 978 (2024) A2, http://dx.doi.org/10.1017/jfm.2023.976.

[38] R. Nemcokova, V. Glombikova, P. Komarkova, Study on liquid moisture transport of knitted fabrics by means of mmt, thermography and microtomography systems, AUTEX Res. J. 15 (4) (2015) 233-242, http://dx.doi.org/10.1515/aut-2015-0022.

[39] T. T Weitkamp, M. Scheel, J. Perrin, G. Daniel, A. King, V. Le Roux, J. Giorgetta, A. Carcy, F. Langlois, K. Desjardins, Microtomography on the ANATOMIX beamline at synchrotron SOLEIL, J. Phys. 2380 (7210) (2021) http://dx.doi.org/10.1088/1742-6596/2380/1/012122.

[40] A. Mirone, E. Brun, E. Guillart, P. Tafforeau, J. Kieffer, The PyHST2 hybrid distributed code for high speed tomographic reconstruction with iterative reconstruction and a priori knowledge capabilities, Nucl. Instruments Methods Phys. Res. Sect. B: Beam Interactions Mater. Atoms 324 (2014) 41-48, http://dx.doi.org/10.1016/j.nimb.2013.09.030, 1st International Conference on Tomography of Materials and Structures, URL https://www.sciencedirect.com/science/article/pii/S0168583X14000251.

[41] P. Amarasinghe, C. Pierre, M. Moussavi, A. Geremew, S. Woldesenbet, A. Weerasooriya, The morphological and anatomical variability of the stems of an industrial hemp collection and the properties of its fibres, HELIYON 8 (4) (2022) http://dx.doi.org/10.1016/j.heliyon.2022.e09276, e09276-APR 2022.

[42] M. Grégoire, M. Bar, E. De Luycker, S. Musio, S. Amaducci, X. Gabrion, V. Placet, P. Ouagne, Comparing flax and hemp fibres yield and mechanical properties after scutching/hacking processing, Ind. Crop. Prod. 172 (2021) 114045, URL https://www.sciencedirect.com/science/article/pii/S0926669021008104.

[43] W.P. Inacio, F. Perissé Duarte Lopes, S. Monteiro, Diameter dependence of tensile strength by Weibull analysis: Part III sisal fiber, Matéria (Rio de Janeiro) 15 (2010) 124-130, http://dx.doi.org/10.1590/S1517-70762010000200006.

[44] J. Morán, V. Alvarez, V. Cyras, A. Vázquez, Extraction of cellulose and preparation of nanocellulose from sisal fibers, Cellulose 15 (2008) 149-159, http://dx.doi.org/10.1007/s10570-007-9145-9.

[45] A. Bourmaud, D. Siniscalco, L. Foucat, C. Goudenhooft, X. Falourd, B. Pontoire, O. Arnould, J. Beaugrand, C. Bailey, Evolution of flax cell wall ultrastructure and mechanical properties during the retting step, Carbohydr. Polymers 206 (2019) 48-56, http://dx.doi.org/10.1016/j.carbpol.2018.10.065, URL https://www.sciencedirect.com/science/article/pii/S0144861718312542.

[46] B. Chabbert, J. Padovani, C. Djemiel, J. Ossemond, A. Lemaître, A. Yoshinaga, S. Hawkins, S. Grec, J. Beaugrand, B. Kurek, Multimodal assessment of flax dew retting and its functional impact on fibers and natural fiber composites, Ind. Crop. Prod. 148 (2020) 112255, http://dx.doi.org/10.1016/j.indcrop.2020.112255, URL https://www.sciencedirect.com/science/article/pii/S0926669020301710.

[47] L. Muraille, M. Pernes, A. Habrant, R. Serimaa, M. Molinari, V. Aguié-Béghin, B. Chabbert, Impact of lignin on water sorption properties of bioinspired self-assemblies of lignocellulosic polymers, Eur. Polym. J. 64 (2015) 21-35, http://dx.doi.org/10.1016/j.eurpolymj.2014.11.040, URL https://www.sciencedirect.com/science/article/pii/S001430571400425X.

[48] X. Falourd, C. Rondeau-Mouro, M. Cambert, M. Lahaye, B. Chabbert, V. Aguié-Béghin, Assessing the complementarity of time domain NMR, solid-state NMR and dynamic vapor sorption in the characterization of polysaccharide-water interactions, Carbohydr. Polymers 326 (2024) 121579, http://dx.doi.org/10.1016/j.carbpol.2023.121579, URL https://www.sciencedirect.com/science/article/pii/S0144861723010445.

[49] K.S.W. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 54 (11) (1982) 2201-2218, http://dx.doi.org/10.1351/pac198254112201.

[50] Ö. Ceylan, L. Van Landuyt, F. Meulewaeter, K. De Clerck, Moisture sorption in developing cotton fibers, Cellulose 19 (5) (2012) 1517-1526, http://dx.doi.org/10.1007/s10570-012-9737-x.

[51] B. Hu, Z. Duan, B. Xu, K. Zhang, Z. Tang, C. Lu, M. He, L. Jiang, H. Liu, Ultrafast self-propelled directional liquid transport on the pyramid-structured fibers with concave curved surfaces, J. Am. Chem. Soc. 142 (13) (2020) 6111-6116, http://dx.doi.org/10.1021/jacs.9b13286.

[52] P. Chatron-Michaud, V. Trillaud, B. Lesaint, J. Adrien, J. Beaugrand, P. Steyer, Towards dynamic in situ observation of flax fibres under varying water contents through environmental SEM and X-ray tomography, Ind. Crop. Prod. 215 (2024) 118654, http://dx.doi.org/10.1016/j.indcrop.2024.118654, URL https://www.sciencedirect.com/science/article/pii/S0926669024006319.