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Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

etting behaviour and surface properties of technical bamboo fibres

.A. Fuentesa,∗, L.Q.N. Trana, C. Dupont-Gillainb, W. Vanderlindenc, S. De Feyterc,

.W. Van Vuurea, I. Verpoesta

Department of Metallurgy and Materials Engineering (MTM), Katholieke Universiteit Leuven, Kasteelpark Arenberg 44, B-3001, Leuven, BelgiumInstitute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Louvain-la-Neuve, BelgiumDivision of Molecular and Nanomaterials, Katholieke Universiteit Leuven, Leuven, Belgium

r t i c l e i n f o

rticle history:eceived 29 December 2010eceived in revised form 15 February 2011ccepted 18 February 2011vailable online 2 March 2011

eywords:

a b s t r a c t

Bamboo fibres recently attracted interest as a sustainable reinforcement fibre in (polymer) compos-ite materials, due to specific mechanical properties which are comparable to glass fibres. To achievegood wetting and adhesion of the bamboo fibre with different polymers, the fibre surface needs to becharacterized. The wetting behaviour of technical bamboo fibres is studied experimentally by using theWilhelmy technique, and the results are modelled using the molecular-kinetic theory. A novel proce-dure, based on an autoclave treatment, allows stable and reproducible advancing contact angles to be

atural fibreambooilhelmyetting

ontact lineolecular kinetic theory

measured. In this way, meaningful information on interfacial interactions can be obtained, allowingimprovement of the bamboo-polymer interface. Additionally, for comparison, the wetting behaviour ofsynthetic poly(ethylene terephthalate) (PET) fibre is studied. This article aims at contributing to a betterunderstanding of the complex phenomena occurring during wetting of natural fibres. The results indi-cate that the high concentration of lignin on the surface of bamboo fibres is responsible for their wettingproperties, whereas typical phenomena affecting wetting experiments on plant fibres can be minimized.

. Introduction

Among the many different kinds of natural fibres used in com-osite materials, bamboo is deemed to have one of the mostavourable combinations of low density and good mechanical prop-rties: the specific strength and stiffness of bamboo fibres areomparable to those of glass fibres [1]. However, many natu-al fibres have several disadvantages such as poor wettability,ncompatibility with some polymeric matrices and high moisturebsorption by the fibres [2].

A major difficulty is related to the fibre–matrix adhesion. Bond-ng between the reinforcing fibre and the matrix has a significantffect on the properties of the composite since stress transfer andoad distribution efficiency at the interface is determined by theegree of adhesion between the components. Using the experimen-al data obtained from wetting measurements, fibres and matricesan be examined and matched in terms of their surface components

n order to improve the interfacial properties; predicting and ver-fying their compatibility allows more suitable combinations andherefore better composites to be made.

∗ Corresponding author.E-mail address: Carlos.Fuentes@mtm.kuleuven.be (C.A. Fuentes).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.02.032

© 2011 Elsevier B.V. All rights reserved.

There are a variety of techniques for measuring wetting of sin-gle fibres. The most common methods include both the Wilhelmytechnique and fluid geometry analysis. The former consists in ameasurement of the liquid weight lifted in the meniscus by thespreading of the liquid upwards on a fibre, while the latter is con-cerned with the profile determination of a barrel-shaped volumewhere the fibre is wetted by a finite volume of liquid (a droplet)[3–5] or of a meniscus in the case of fibre wetting by an infi-nite reservoir [6,7]. In the case of natural fibres, both methodsare hardly applicable due to surface irregularities and perimetervariation. To avoid these complications, the characterization ofthe wetting behaviour of natural fibres has been reported throughthe use of the modified Washburn or capillary-rise method [8].However, this method does not allow the influence of wettingvelocity on the determined contact angle to be studied. If surfaceirregularities are minimized, the Wilhelmy technique represents agood option to study the wetting of solids at different immersionvelocities.

The interpretation of experimental wetting data depends onwetting theories. These have been derived to describe wetting on

an ideal surface wherein complexities in relation to their wettingbehaviour such as the viscoelastic response of a polymer surface toa wetting liquid [9], contact angle hysteresis due to surface irregu-larities or chemical heterogeneity [10,11] are assumed to be absent.Therefore, wetting phenomena can be modelled with some success

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or synthetic materials, in which the phenomena mentioned abovere not expected to play a major role [12].

In order to predict the wetting properties of solids, several the-retical models were developed. Among them, two approachesocus on the location of energy dissipation during the wettingf a solid by a liquid: viscous dissipation in the bulk of the liq-id (hydrodynamic model), and dissipation in the close vicinity ofhe solid near the wetting line (molecular-kinetic theory) [13,14].hese models obtained for synthetic materials revealed the depen-ency of dynamic contact angle on wetting velocity (dependingn both speed and direction of displacement). As wetting veloc-ty approaches zero, the wetting quasi-equilibrium parameters arebtained and may refer to either an advancing (wetting) or a reced-ng (dewetting) contact angle. The difference between advancingnd receding contact angles is called hysteresis.

However, similar approaches to model wetting behaviourannot normally be applied to natural fibres due to complex phe-omena at their surface. Barsberg and Thygeseny [12] argued thatlant fibres may give rise to various complex phenomena duringetting experiments which are typically not found for syntheticbres: liquid sorption/diffusion into the surface layers, diffusion of

ow-molecular-weight compounds (extractives) from the surfaceayers into the liquid, different “glassy” behaviour of the chemicalonstituents of the surface layers or viscoelastic response of theurface layers to the liquid. As a consequence, the influence of suchhenomena on the wetting behaviour of natural fibres may be aossible source of invalidation of typical experimental techniquesor measuring wetting.

Moreover, the difficulties in the characterization of the wettingroperties of natural fibres are increased as a result of the com-lexity of their overall microstructure which by far exceeds that ofynthetic materials. This complexity is due to the natural fibre hier-rchical organisation at different length scales and the presence ofifferent materials in variable proportions such as cellulose, hemi-ellulose, lignin and pectin [15,16]. It is claimed that some liquidsan penetrate the natural fibre structure [12,17], allowing wettingroperties of natural fibres to be affected by sorption and diffu-ion. For instance, technical bamboo fibre consists of bundles ofore than one hundred elementary fibres. An elementary fibre con-

ists of several layers where crystallized cellulose nano-fibrils areligned with different angles with respect to the longitudinal fibrexis and are bound together with hemi-cellulose and lignin [18–20].schematic illustration of the complexity of bamboo fibre micro-

nd nano-structure is shown in Fig. 1.The wetting properties of natural fibres (and solids in gen-

ral) are determined by molecular interactions between theirurface and liquids. If these molecular interactions can be evalu-ted by means of well described advancing and receding contactngles (quasi-equilibrium parameters), it is then possible to eval-ate surface energy components by means of theories such aswens–Wendt and acid–base approaches, which are based on the

heoretical Young contact angle, assuming that an equilibrium statean be reached.

The surface condition and surface constitution of technical bam-oo fibres play an imperative role in the interfacial strength of theiromposites. While information about bamboo fibre microstructurelready exists, as mentioned above, the nature of the surface ofechnical bamboo fibres (used as reinforcement fibre in compos-tes) is still unknown due to the fact that topography, chemicalonstituents and constituents distribution (mainly lignin and cel-ulose) are affected by the method of extraction such as steam

xplosion, retting, chemical extraction, or mechanical processes21–25].

The aim of the present work is to examine whether technicalamboo fibres can be considered a well defined system for whicheproducible and stable advancing contact angles can be measured.

icochem. Eng. Aspects 380 (2011) 89–99

Treatment of the fibres was proposed in such a way that typicalphenomena known to affect wetting experiments in plant fibresmay play a limited role. In this manner, meaningful information oninterfacial interactions can be obtained, allowing improvement ofthe bamboo composite interface.

2. Theoretical basis

2.1. Molecular-kinetic theory

The molecular-kinetic theory was developed by Blake [14] toexplain the wetting properties of solids. It employs Frenkel andEyring’s explanation of the process that takes place during themomentum transport of liquids, viewed as the movement of amolecule from one local energy minimum to another [26].

This theory in its basic form discards the explicit energy dissipa-tion due to viscous flow and focuses instead on energy dissipationoccurring in the immediate vicinity of the moving contact line dueto the process of attachment or detachment of fluid molecules fromthe solid surface [13]. The macroscopic behaviour of the wettingline is explained by the individual molecular displacements occur-ring within the three-phases contact line [27]. These displacementsoccur randomly but progressively in the direction of the movingcontact line [28].

According to Blake [14], during spontaneous spreading, a liquiddrop exhibits a dynamic contact angle � that depends on the instan-taneous contact line velocity � and decreases progressively towarda static contact angle �0 at � = 0. In forced wetting, the substrate ismoved at constant speed to drive the contact line at a given velocity,forming a stable dynamic contact angle � (v). The displacement ofthe contact line depends on the frequency of forward and backwardmolecular displacements within the three phases zone, K+ and K−,respectively. At equilibrium, � = 0 and the net rate of displacementis zero, so that K+ = K− = K0, where K0 is the equilibrium displace-ment frequency. According to this theory, energy is dissipated bydynamic friction associated with the moving contact line.

If the driving force for wetting is taken to be the out-of-balancesurface tension force � (cos �0 − cos �), then the relationshipbetween � and � is given by:

� = 2K0� sin h

[��2

2kT(cos �0 − cos �)

](1)

where � is the average length of each molecular displacement, kis the Boltzmann constant, T the absolute temperature, and � thesurface tension of the liquid. The complete derivation of this modelis presented by Blake [14].

3. Materials and methods

3.1. Materials

Technical fibres (bundles of elementary fibres, see Fig. 1) weremechanically extracted from Guadua angustifolia bamboo culms inthe Department of Metallurgy and Materials Engineering (MTM)at KULeuven. Polyethylene terephthalate (PET) monofilaments(diameter 800 �m) from Goodfellow were used to compare thewetting behaviour of synthetic and bamboo fibres. Ultrapure water(18.2 � cm resistivity, Millipore Direct Q-3 UV) was used to studythe fibres’ wetting behaviour. Lignin powder Protobind 1000 wassupplied by Granit from Switzerland.

3.2. Fibre preparation

The technical bamboo fibres that were examined underwent thefollowing preparation procedure. After being selected (by means

C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99 91

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ig. 1. Schematic diagram of bamboo fibre structure: (A) transverse section from bhe culm presents an irregular form and consists of bundles of elementary fibres. Thrranged in a honeycomb pattern [18], (C) model of the polylamellate structure ofith different fibrillar orientation [18], (D) nano-fibrils are bound together with he

f an optical microscope), the fibres were cleaned, first with warmater for 1 h (90 ◦C), then wiped with ethanol with a piece of cotton

issue before being dried in a vacuum oven at 80 ◦C for 1 h. With theim to smooth the lignin at the fibre surface, a group of technicalamboo fibres was also further put in an autoclave under 3 barsf pressure at 150 ◦C for 1 h. Finally, all the fibres were wiped withexane and then conserved under silicagel.

To obtain a clean surface for the PET fibres, these were washedith a detergent (RBS-35 from Chemical Products) at a concen-

ration of 4% (v/v) in water during 1 h under magnetic stirring toemove organic residues, and next rinsed in ultrapure water at 90 ◦Cor 1 h. The cleaned fibres were then dried under vacuum at 90 ◦Cor 2 h and then conserved under silicagel.

.3. Contact angle measurements

Dynamic contact angles were measured with a Krüss K100 ten-iometer using the Wilhelmy technique. This method is based onhe Laplace equation that describes the pressure exerted by the

eniscus. Initially developed for plates, the method was also con-erted to be used with fibres replacing the perimeter of the platey the perimeter of a cylinder.

The fibre is immersed into the liquid and the microbalanceetects a force (Fmeasured), being the sum of the wetting forceFwetting), the weight of the fibre (G) and the buoyancy forceFbuoyancy):

measured = Fwetting + G − Fbuoyancy = p� cos � + mg − �gAd (2)

here p is the fibre perimeter, m the fibre mass, g the acceleration ofravity, � the liquid surface tension, � the liquid density, A the fibreross-sectional area and d the immersion depth. When the weightf the probe is measured beforehand and set to zero on the balancend the force is extrapolated back to zero immersion depth, only

o internodes [16], (B) a typical cross section of the technical bamboo fibres insides-section of these elementary fibres is either pentagonal or hexagonal and they arementary bamboo fibre. It consists of thick and thin layers of cellulose nano fibrils

llulose and lignin [19].

the wetting force remains:

Fmeasured = p� cos � (3)

The method applied to determine the fibre perimeter is basedon the same principle as the tensiometric measurement that wasdiscussed above. In this case, however, instead of the contact anglethe perimeter is sought. When a liquid with 0◦ contact angle is used,Eq. (3) becomes:

Fmeasured = p� (4)

At relatively low speeds, hexane is assumed to have a contactangle of 0◦ with virtually all substrates. To evaluate the accuracyof this assertion, perimeters of technical bamboo fibres were mea-sured using hexane at 1.5 mm/min before they were put in a resin(to prevent some damage during the cutting procedure) and thencut into four pieces of three millimetres to obtain various crosssections along the fibre length. Subsequently, the perimeters weremeasured at each cross-section using the scanning electron micro-scope and the mean values were compared to those of the hexanemethod.

The reproducibility of the contact angle determination fortechnical bamboo fibres was examined by performing duplicatemeasurements on the same fibres. After the first measurement,the fibres were dried in a vacuum oven at 80 ◦C for 1 h and thenconserved under silicagel. The time interval between two measure-ments of the same sample was set to one week.

Dynamic contact angle measurements at a given speed wereperformed with a 0.05 mm data sampling step. Accordingly, the

data for a single 10 mm length fibre represents 200 values. Theaverage and standard deviation values reported in this study werecalculated from the data of all the fibres measured at a given speed;so e.g. for 10 evaluated fibres at a certain speed, this means that2000 data-points were averaged.

9 : Physicochem. Eng. Aspects 380 (2011) 89–99

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.4. Absorption measurement

Technical bamboo fibres, treated or not in an autoclave at 150 ◦C,ere immersed in ultrapure water. Before the sorption test, the

pecimens were subjected to the same cleaning and condition-ng procedure used for contact angle measurement samples. Tovaluate the effect of absorption phenomena during the Wilhelmyxperiment, the content of water absorbed by the sample was cal-ulated by the weight difference between the weight of the fibreefore immersion, and the weight of the fibre after it was removedt a velocity of 1.5 mm/min from water. Weight was measured withhe electronic microbalance option (Krüss K100) to a resolution of�g.

.5. Fitting procedure

The parameters of the wetting kinetics are obtained by curvetting from the correlation plot of experimental dynamic contactngle values and wetting velocity. The procedure followed by Vegat al. [29] was adapted in fitting the data using the molecular-inetic theory. Accordingly, Eq. (1) can be simplified to:

= A sin h[B(C − cos �)] (5)

here A = 2 K0�, B = ��2/2kT, and C = cos �0 are the independentarameters. Eq. (5) was used as the regression model to fit each setf experimental data, maximizing the coefficient of multiple deter-ination by adjusting the independent parameters. The starting

alues were randomly chosen, and were adjusted during the fittingrocess to bring the curve close to the data points.

.6. Surface characterization

.6.1. Atomic force microscopy imagingImaging was carried out in Tapping ModeTM, with a

ultimodeTM system (Veeco) operating with a Nanoscope IVTM

ontroller (Veeco) and a type E scanner. Silicon RTESPA cantileversVeeco) with resonance frequencies of about 275 kHz were used.mages were collected at scan frequencies of 0.5–1 Hz and a resolu-ion of 512 × 512 pixels. Images were corrected for sample tilt andnalyzed with Scanning Probe Imaging Processor software (Imageetrology, AS).

.6.2. X-ray photoelectron spectroscopy (XPS)XPS analyses were performed on a Kratos Axis Ultra spec-

rometer (Kratos Analytical, Manchester, UK) equipped with aonochromatized aluminium X-ray source (powered at 10 mA and

5 kV).One single fibre was cantilevered fixed on a flat stainless steel

rough with a piece of double sided isolative tape. This way ofounting insures that the fibre surface only was analyzed but not

ts surrounding. The troughs, holding each one fibre sample, werehen inserted in the multispecimen holder.

The pressure in the analysis chamber was about 10−6 Pa. Thengle between the normal to the sample surface and the direc-ion of photoelectrons collection was about 0◦. Analyses wereerformed in the hybrid lens mode with the slot aperture andhe iris drive position set at 0.5, the resulting analyzed area was00 �m × 300 �m. The pass energy of the hemispherical analyseras set at 160 eV for the survey scan and 40 eV for narrow scans.

n the latter conditions, the full width at half maximum (FWHM) of

he Ag3d5/2 peak of a standard silver sample was about 0.9 eV.

Charge stabilisation was achieved by using the Kratos Axis Ultraevice. The electron source was operated with a filament currentetween 1.9 and 2.1 A and a bias of −1.1 eV. The charge balancelate was set between −3.3 and −3.9 V.

Fig. 2. Advancing dynamic contact angle versus fibre position for water on bamboofibres that were not autoclaved at 150 ◦C.

The following sequence of spectra was recorded: survey spec-trum, C1s, O1s, N1s, Ca2p, Si2p, Na1s, P2p and C1s again to checkfor charge stability as a function of time and the absence of degra-dation of the sample during the analysis. The C–(C,H) componentof the C1s peak of carbon was fixed to 284.8 eV to set the bindingenergy scale.

The data treatment was performed with the CasaXPS program(Casa Software Ltd., UK). Mole fractions were calculated using peakareas normalised on the basis of acquisition parameters after alinear background subtraction, and consideration of experimentalsensitivity factors and transmission factors (depending on kineticenergy, analyser pass energy and lens combination) provided bythe manufacturer.

C1s spectra were decomposed with a Gaussian/Lorentzian(70/30) product function, by constraining the FWHMs of all com-ponents to be equal.

4. Results and discussion

4.1. Dynamic contact angle

Fig. 2 shows examples of contact angle tests in water for techni-cal bamboo fibre samples that were not autoclaved at 150 ◦C. Theexperimental data show a big scatter for the non-autoclave treatedfibres, showing values of advancing contact angle from 60◦ to 100◦.These results are in agreement with literature reviews attributingthis large fluctuation to the influence of both chemical and topo-graphical heterogeneity of the fibre surface [12,30–32]; the formerdeals with the difference in wetting between natural fibre con-stituents such as lignin, cellulose and hemicelluloses, while thelatter is concerned with surface roughness and differences in fibreperimeter.

The results of duplicate measurements (after one week) on thesame fibres that were not autoclaved at 150 ◦C are shown in Table 1(left), allowing the reproducibility of contact angle measurementsto be evaluated. The unpredictable large variation of contact anglevalues seems to confirm the statement that plant fibres, and bam-boo in particular, do not constitute a well-defined system amenableto wetting studies, as reported before for sisal and coir fibres [12].

However, after autoclave treatment at 150 ◦C, technical bamboofibres exhibit stable contact angles. As revealed in Table 1 (right),the standard deviation of the contact angle average is reduced toaround 3◦ for the first measurement. Furthermore, the duplicate

contact angles show small differences, confirming that autoclavetreatment allows a better reproducibility to be achieved.

The results indicate a reduction of hysteresis caused mainly byfibre surface irregularities reduction at different length scales dueto the autoclave treatment, as will be demonstrated further on.

C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99 93

Table 1Variation between two different contact angle measurements on the same bamboo fibres.

Non-autoclave treated fibres (1.5 mm/min) Autoclave treated fibres (1.5 mm/min)

Fibre no. Contact angle (◦) Variation % Fibre no. Contact angle (◦) Variation %

Firstmeasurement

Secondmeasurement

Firstmeasurement

Secondmeasurement

1 72.6 ± 7.2 95.1 ± 7.6 23.7 11 67.4 ± 1.1 66.1 ± 2.3 1.92 97.0 ± 8.2 84.2 ± 3.8 15.3 12 69.3 ± 1.7 67.3 ± 2.2 2.93 98.0 ± 4.3 81.5 ± 7.4 20.2 13 66.3 ± 2.2 65.6 ± 1.4 1.14 101.0 ± 2.1 105.0 ± 3.7 3.8 14 71.8 ± 3.4 67.1 ± 2.7 6.55 72.8 ± 9.1 92.3 ± 6.6 21.1 15 68.9 ± 1.3 66.0 ± 1.9 4.26 83.3 ± 2.4 72.0 ± 3.8 15.7 16 68.8 ± 2.3 66.4 ± 3.8 3.57 98.6 ± 6.7 80.0 ± 4.3 23.0 17 67.6 ± 3.4 68.0 ± 1.7 0.68 99.9 ± 7.1 89.0 ± 9.1 12.2 18 69.8 ± 3.8 69.3 ± 2.4 0.7

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Average 87.7 ± 14.7 88.5 ± 10.4

in [16] has studied the lignification process of bamboo stems bybserving cross-sections and using fluorescence microscopy andt was observed that lignin surrounds the bamboo technical fibre.ence, it is possible that a layer of lignin remains on the surface ofechanically extracted bamboo fibres. Lignin can be softened, as

tated by Tejado [33], who reported glass transition temperaturesTg) between 90 and 180 ◦C among different underivatized ligninamples; consequently, lignin can be put under pressure to even thebre surface. Moreover, contact angle values may suggest lignin ashe main component at the technical bamboo fibre surface. Indeed,

aximova [34] studied the wetting behaviour of lignin on the sur-ace of cellulose fibres, reporting a water advancing contact anglealue just under 70◦ for a cellulose fibre saturated with adsorbedignin. In the same fashion, Liukkonen [35] reports a contact anglealue of 67◦ for lignin by observing water microscopic drops in thenvironmental scanning electron microscope (ESEM). These val-es are similar to the average advancing water contact angle of9◦ obtained for autoclaved bamboo fibres in this study, and, inontrast, far from other natural fibre constituents’ contact anglesreviously reported in the literature: cellulose 30◦ [36] and alphaellulose 14◦ [37]. However, determination of the surface chemi-al composition is needed in order to prove the hypothesis of highignin concentration on the surface of bamboo technical fibres. This

as done using XPS.

.2. Surface chemical composition: XPS

In Fig. 3, the decomposed C1s spectra for lignin from Granit andnon-autoclave treated technical bamboo fibre sample are com-ared. The C1s peak intensity at ∼285 eV is related to the presencef lignin [38]. As reported by Johansson [39], cellulose is ideally

Fig. 3. XPS high resolution spectra from carbon C1s region: (A) lignin from

9 70.2 ± 4.1 68.9 ± 4.4 1.90 67.9 ± 2.1 69.1 ± 1.2 1.8

verage 68.8 ± 3.2 67.1 ± 2.3

devoid of aliphatic carbon-carbon bonds (designated as C1) becauseof its polysaccharide structure; however, in milled wood lignin, 49%of the carbon atoms are C1 type, as shown by Shchukarev [38]. Sim-ilarly, the measured Protobind Granit lignin shows 64% of C1 typecarbon atoms. The bamboo results give an average of 57% of C1type carbon among the 10 tested samples. This indicates that theremay be various types of lignin which are chemically different orthat other compounds containing aliphatic-carbon may be presentas well. The differences between the various lignin samples maybe explained by the use of different lignin isolation processes inwhich extractives and other chemical components are removed or,at least, their molecular structure is changed [40]. If we hypothesizethat only cellulose and lignin are present, then lignin is certainlypredominant on the surface of our technical bamboo fibres.

Fig. 4 shows the results regarding surface chemical constituentsof both autoclave-treated and non-treated technical bamboo fibresobtained from the decomposition of the high resolution carbon 1sspectrum for each fibre. Lignin content on the fibre surface was ana-lyzed by determining the oxygen-to-carbon atomic ratio, and therelative concentration of the C1 component. The first aspect dealswith the fact that oxygen-to-carbon atomic ratios are different forcellulose and lignin, while the second aspect is concerned with thelack of C1 bonds in chemically pure cellulose within the C1s spec-trum. The references consist of theoretical values for pure celluloseand milled wood lignin [39], and measured data of lignin powder(Protobind 1000, Granit).

The results clearly indicate that technical bamboo surface con-stituents for the ten tested samples are close to our references forlignin, indicating that bamboo technical fibres may be homoge-neously covered with lignin and possibly some other molecules, butnot with cellulose. Since spectra are enriched in C1 links more than

Granit and (B) non-autoclave treated technical bamboo fibre surface.

94 C.A. Fuentes et al. / Colloids and Surfaces A: Phys

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ig. 4. A correlation graph depicting the percentage of C1/C ratio versus O/C ratioor chemical groups at the surface of non-autoclave treated technical bamboo fibres,he surface of autoclave treated technical bamboo fibres, lignin from Granit, andheoretical values for cellulose and lignin according to Shchukarev [38].

xpected for milled wood lignin, the presence of other compoundsuch as lipids cannot be excluded.

Furthermore, the difference in constituents between fibres auto-laved at 150 ◦C or not is imperceptible within the measured depthf 5–10 nm. This does indicate that the autoclave treatment doesot change the chemical structure and composition of the technicalamboo fibre surface. Hence, it is presented as a reliable method totabilize contact angle measurements in technical bamboo fibresy reducing surface irregularities. The irregular lignin material onhe technical bamboo fibres’ surface would be smoothened out (seeurther).

.3. Perimeter and absorption evaluation

The perimeters of bamboo fibres, autoclaved at 150 ◦C or not,ere determined by wetting the fibres with hexane and measuring

he wetting force. Afterwards, these values were compared withhe perimeters measured on SEM images at 4 cross-sections alonghe fibre (see Section 3). As can be seen in Fig. 5, a typical crossection of a technical bamboo fibre presents an irregular form andonsists of elementary fibres compactly arranged in a honeycomb

attern (with very small lumens), joined by a thin wall of mainly

ignin [16,18]. The results, presented in Table 2, indicate a goodgreement between both methods with relative errors less than 8%or both autoclaved and non-autoclaved fibres, and thus confirm

ig. 5. SEM image of a bamboo fibre cross-section with contour perimeter line.

icochem. Eng. Aspects 380 (2011) 89–99

that it is possible to measure the technical bamboo fibre perimeterusing hexane with reasonable accuracy.

However, it is necessary to study the influence of the perimetervariation on the determination of contact angle values as a pos-sible source of invalidation of plant fibres wetting studies. If it isassumed that only the perimeter slightly varies during the contactangle measurement for a bamboo–water system, then it is possi-ble to evaluate the contact angle � obtained as a function of theperimeter deviation, as can be seen in Fig. 6A (using 69◦ as theaverage advancing contact angle). Eq. (3) was transformed to:

� = arccos(

Fmeasured

p�

)= arccos

(cos 69◦

a

)(6)

where a varies from 0.85 to 1.15, and represents the perimeter vari-ation between −0.15p and +0.15p. The analysis of Eq. (6) showsthat the contact angle varies with less than 2.5◦ for a perimetervariation of 10% in the bamboo–water system. As can be seen fromthe results, the large fluctuation of contact angles values obtainedfor bamboo in water cannot be explained solely on the basis ofperimeter variation.

Another difficulty in the characterization of the wetting prop-erties of natural fibres is related to liquid absorption. Since theWilhelmy method is based on measuring the wetting force, the liq-uid absorbed by the fibre can distort contact angle measurements.Liquids may penetrate the structure of natural fibres, modifying theforce value and so the calculated dynamic contact angle.

To evaluate the effect of absorption phenomena during the Wil-helmy experiment, the content of water absorbed by the samplewas calculated by the weight difference between the weight of thefibre before immersion, and the weight of the fibre after it wasremoved at a velocity of 1.5 mm/min. This mass value was thenexpressed in force units and evaluated as percentage of the wet-ting force for each specific measurement in order to evaluate thevariation of the measured force due to the influence of absorbedwater. Table 3 shows the results of contact angle variation for bothautoclave and non-autoclave treated fibres.

The contact angle fluctuation due to water absorption is ana-lyzed by evaluating the variation in the contact angle � for abamboo–water system as a function of water absorption (Fig. 6B).The effect is analyzed as gain of weight only. As in the case ofperimeter variation that was discussed earlier, Eq. (3) was trans-formed to:

� = arccos(

Fmeasured

p�

)= arccos (cos 69◦ × b) (7)

In this case, the variation of the measured force is evaluated upto 0.15F, hence b varies from 1.00 to 1.15. The results show thatthe effect of water absorption is small and may not significantlyalter contact angle measurements on bamboo fibres; the averageweight effect of the water absorbed by the autoclave treated fibre(6.1%, see Table 3) represents a small variation of less than 1.5◦,as can be seen in Fig. 6B. For the case of non-autoclave treatedfibres, the effect of water absorbed by the fibre is larger (14.0%,see Table 3), however it only represents a contact angle variationof about 3◦. As it was already presented, technical bamboo fibresare composed of several elementary fibres which are joined withlignin. The mechanical process of extraction can disjoin some ele-mentary fibres (see Fig. 7A), facilitating the penetration of waterinto the fibre structure. The autoclave treatment at 150 ◦C is notonly smoothening the fibre surface, it is also compacting the wholefibre structure, resulting in a diminution of water absorption.

Eqs. (6) and (7) are not considering the effect of the structureand stability of the three-phase contact line during its movementthrough different activation energy barriers represented by thedifferent cross section along the fibre length [41], or the effectof the spontaneous diffusion of liquid molecules into the surface

C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99 95

Table 2Perimeter evaluation of technical bamboo fibres, based on scanning electron microscopy (SEM) images and on wetting measurements in hexane.

Fibre Fibreno.

Methods

Wettinganalysis (�m)

Image analysis(�m)

Relativeerror (%)

Non-autoclave treated bamboo 1 1022 ± 24 1030 ± 38 0.82 1032 ± 30 1108 ± 42 7.43 1087 ± 38 1122 ± 71 3.24 998 ± 18 1005 ± 54 0.75 1124 ± 25 1201 ± 78 6.9

Autoclave treated bamboo 1 1042 ± 29 1095 ± 35 5.12 887 ± 23 949 ± 36 7.03 1205 ± 26 1199 ± 25 0.54 1083 ± 22 1159 ± 28 7.05 1033 ± 34 1077 ± 63 4.3

Fig. 6. Influence of perimeter (A) and force (B) variation due to water absorption on the variation of the calculated contact angle. The contact angle variation is presented asthe absolute value of the difference between the average advancing contact angle obtained for a bamboo–water system (69◦) and the contact angle obtained from Eqs. (6)and (7).

Table 3Measured values of water absorption (in force units) for non-autoclave treated and autoclave treated bamboo fibres.

Non -autoclave treated fibres Autoclave treated fibres

Fibreno.

Absorbedwater (mN)

Wettingforce (mN)

% Fibreno.

Absorbedwater (mN)

Wettingforce (mN)

%

1 4.22 27.8 ± 2.8 15.2 1 1.51 24.4 ± 1.8 6.22 1.40 11.2 ± 1.2 12.5 2 2.15 30.2 ± 3.1 7.13 0.60 7.1 ± 1.4 8.4 3 1.88 31.9 ± 2.1 5.94 4.55 30.9 ± 1.7 14.7 4 1.20 25.0 ± 1.7 4.85 4.46 27.0 ± 3.6 16.5 5 2.26 31.3 ± 1.6 7.26 1.37 8.9 ± 1.1 15.4 6 1.86 27.4 ± 2.8 6.87 4.63 30.3 ± 4.3 15.3 7 1.76 30.3 ± 1.3 5.88 1.93 13.7 ± 0.7 14.1 8 1.33 26.1 ± 2.1 5.1

Average 14.0 Average 6.1

Fig. 7. (A) Optical microscopy of non-autoclave treated bamboo fibre and SEM image of disjointed elementary fibres, the scale bar in (a) is 10 �m. (B) Optical microscopyimage of altered PET fibre.

96 C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99

0

20

40

60

80

100

120

1086420

Conta

ct An

gle (°

)

Position (mm)Smooth PET Altered PET Bamboo

60

70

80

90

100

110

1086420

Conta

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gle (°

)

Position (mm)Smooth PET Altered PET 1 Altered PET 2

A B

F uremec oclavec e case

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is clear from the results presented that the scatter of contact angleson PET fibres increases with rise of waviness (or surface defects thatprovoke a change in the force direction). Furthermore, the effect onthe receding contact angle seems to be comparable to the wetting

ig. 8. (A) Advancing contact angle of smooth and altered PET fibres: (1) first measontact angles of smooth and altered PET fibres in comparison with a typical autontact angle of PET fibres is evident, showing a similar behaviour as observed in th

ayers decreasing the solid–liquid interface with time [12]. Thesehenomena are related to perimeter variation and water absorp-ion respectively, and can affect the contact angle measurement.

e try to make clear that the big scatter of contact angles valuesbtained for bamboo in water cannot be explained by means ofither perimeter variation or absorption without any relation tohe stability of the contact line.

In order to evaluate the wetting behaviour of an impermeableolid and compare it with bamboo fibre wetting behaviour, artificialurface defects were introduced in PET fibres using a sharp blade toreate perimeter variation (waviness) along the fibre (Fig. 7B), try-ng to reproduce the wavy surface of a bamboo fibre. The wavinessariation of the PET fibres used was random with poor reproducibil-ty, and thus, a detailed connection of the experimental data withhe geometry profile of the fibre surface is not possible. A goal of thisontribution is to identify possible factors related to waviness orhatever surface irregularity with spacing greater than roughness,hich can be affecting contact angle measurements.

Similarly to what was found for the untreated bamboo fibres,he results on altered PET fibres indicate that the wetting behaviourecomes unstable when waviness plays a role (Fig. 8A and B). Asevealed by Table 4, the smooth PET fibres present an averagedvancing contact angle of 87◦ with a small standard deviation of.5◦; such steady wetting behaviour has been reported for syntheticaterials where complexities leading to non-equilibrium can be

eglected or minimized [12,42]. In contrast, the altered fibres shown average advancing contact angle of 93.6◦ and a standard devia-

ion of 9.8◦ (Table 4). The latter is about seven times the deviationf an ordinary PET fibre with normal surface defects created at theime of extrusion. In order for the contact angle to reach 94◦ from7◦, the measured wetting force is not only being reduced several

able 4easured dynamic contact angles for PET fibres with different surface topography.

Fibre no. Contact angle (◦) Average contactangle (◦)

PET (normal)1 87.1 ± 1.3 87.0 ± 1.52 87.4 ± 1.43 86.6 ± 1.44 86.9 ± 1.55 86.9 ± 1.1PET (altered perimeter)1 96.6 ± 6.2 93.6 ± 9.82 101.0 ± 8.23 89.2 ± 9.14 87.1 ± 7.35 94.4 ± 6.4

nt and (2) second measurement of the same fibre, and (B) advancing and recedingtreated bamboo fibre. The effect of surface topography variation in the recedingof a representative bamboo fibre (instability of receding contact angle).

times but becoming negative as well. This effect can be explainedas a consequence of the introduced surface waviness.

Surface tension is a tensor that acts perpendicularly to the con-tact line, in the plane of the surface [43]. While the three-phasecontact line is moving, the wetting force position is constantlychanging and so its direction, depending on the radii of curvatureat the wetting perimeter position (see Fig. 9). The microbalanceis only detecting the forces parallel to the fibre immersion direc-tion. A similar behaviour is described by Czachor [44,45] when hestudied the contact angle variation in a wavy capillary geometry;by means of a mathematical model of meniscus movement in asinusoidally shaped capillary, it was concluded that the calculatedcontact angle is a strongly increasing function of wall waviness. It

Fig. 9. The radius of curvature of the fibre surface is constantly changing during themovement of the meniscus over the fibre body, and so the direction of the wettingforce, r–curvature radius, Flv–liquid–vapour interfacial force. Position (1): smoothfibre and position (2): fibre with surface irregularities.

C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99 97

faces:

biiw

itbeffiotbwiT

4

wfpoa

rw(otsBmam

a[tt[tW

Fig. 10. AFM images of technical bamboo fibre sur

ehaviour of bamboo fibres during the receding process, resultingn a situation of irregular or zero receding contact angles (Fig. 8B);t is hypothesised that water remains in the spaces between surface

aves.Due to the waviness effect or macro-defects (micro-roughness

s still not being considered), the measured force is affected andhe calculated contact angle using the Wilhelmy formula cannote related to the real contact angle value. The obtained appar-nt contact angles (even bigger than 90◦, which involves negativeorces) for non-autoclave treated bamboo fibres and altered PETbres can be explained by their wavy surface (Fig. 7A and B), sincether phenomena like diffusion or absorption provoke the reduc-ion of contact angles values. Wensel and Cassie models cannote applied in this macro scenario, since they may become validith small-wavelength roughness [46] and a larger scale of surface

rregularities (waviness of fibre surface) is what is relevant here.he micro-roughness effect will be addressed next.

.4. Fibre micro-roughness and absorbing material

A representative AFM image of a technical bamboo fibre thatas not autoclaved at 150 ◦C is shown in Fig. 10A where its sur-

ace exposes a micro rugged topography. Roughness analysis afterlane correction yields a root mean square (RMS) roughness valuef 481 nm. In contrast, Fig. 10B shows a far less rugged surface forn autoclave treated fibre with a RMS roughness of 64 nm.

So, the bamboo fibres without autoclave treatment appear to beough (see Fig. 10A). This seems to be another reason of the higherater absorption values measured for non-autoclave treated fibres

disjoining of elementary fibres was discussed before). The ligninn the surface is not well compacted and porous which makeshe liquid movement inside the surface easier. The area for thepontaneous diffusion of liquid molecules is also being increased.oth phenomena, absorption and diffusion affect the contact angleeasurement. For an absorbing material, the interfacial energy istime-dependent function which decreases from the instant theaterial is brought into contact with the liquid [12].In the case of autoclave treated fibres, absorption and diffusion

re being reduced since the surface is smoother and well compactedFig. 10B]. However, it should be noticed that neither the state nor

he geometry of the phase interfaces in the regions remote from thehree phase contact line has any direct effect on the contact angle46]. Diffusion must be occurring in autoclave and non-autoclavereated fibres but relatively far from the contact line. In the

ilhelmy technique, the wetting force is measured instanta-

(A) untreated and (B) autoclave treated at 150 ◦C.

neously and always on a new contact line (or surface area fraction[47]) during the continuous immersion in the fluid. The effect ofabsorption and diffusion remains as an increment of fibre weightonly, which was analyzed before, and was shown to represent justa small variation in the calculated advancing contact angle. How-ever, sorption and diffusion play a role in the irregularity of thereceding contact angle, and in this way do not allow a proper fullcharacterization of the wettability behaviour of the bamboo fibresurface.

The effect of roughness on contact angles has been welldescribed in literature [43,46,47]. Theories like Wenzel and Cassiepredict the apparent contact angle of advancing and receding frontsfor surfaces with uniform roughness where the contact line movesfrom one metastable state to another on representative contactareas of the whole surface [46–48]. Hence, these theories arehardly applicable since the surface of non-autoclave treated fibrespresents a non-uniform roughness and contact areas are differ-ent along the fibre length. Moreover, this non-uniform roughnessand irregularities (much larger than the ones observed in treatedfibres) must be creating several energy barriers and hence differentmetastable states along the contact line, increasing the instabilityof the contact angle results (Fig. 2).

The waviness can explain the large variation of contact angles(even bigger than 90◦), but not the non-reproducibility of the val-ues. As can be seen in Fig. 8A, the contact angle profile of altered PETfibres is almost the same from one measurement to the next. Thenon-reproducibility of measurements on untreated bamboo fibrescan be a consequence of swelling phenomena. Natural fibres areknown to swell significantly in water, and a typical Wilhelmy mea-surement is rather slow, which would allow significant swelling totake place (far from the contact line). The dimensional changes thataccompany the shrinking and swelling of non-autoclave treatedsamples would provoke changes of the measured surface from onewetting measurement to the next, creating new and totally differ-ent energy barriers that the contact line needs to pass during a newmeasurement. This effect would be increased by the rough and non-compacted surface presented in non-autoclave treated fibres. Thelarge contact angle scatter found for non-autoclave treated fibresmust be a consequence of the combined effect of waviness (surfaceirregularities) and roughness.

Absorption phenomena, waviness, and roughness have beenreduced in autoclave treated fibres. However, they still have surfaceirregularities and some swelling might be happening since thereare still some changes in the profile of the duplicate wetting mea-surements (see Table 1) which leads to a slightly different contact

98 C.A. Fuentes et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 380 (2011) 89–99

Fig. 11. (A) Theoretical curve obtained by nonlinear regression of experimental data using Eq. (1) (each point represents the average contact angle of 10 PET fibres, them uredf t speeo fittingr e of 8

acrifstcpt

4

tt(butoafcem

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aximum standard deviation was 1.4◦ for a given velocity). The angles were measor water on bamboo technical fibre. The dynamic contact angles were measured abtained by nonlinear regression of experimental data using Eq. (1) according to theanging from 0.15 to 500 mm/min. (Each point represents the average contact angl

ngle in the second measurement. The micro-roughness of auto-lave treated fibres with a RMS of 64 nm is comparable with theoughness of a synthetic fibre [29]; however, non-uniform surfacerregularities remain and make it more difficult to study their sur-ace in order to apply some theories of contact angles on roughurfaces. For the purpose of this article we were satisfied withhe strong reduction in surface roughness achieved by the auto-lave treatment. However, further optimization of the autoclavearameters in future could be useful to further fine-tune the surfaceopography.

.5. Dynamic contact angle as a function of wetting velocity

In the case of natural fibres, a direct measurement of the con-act angle is problematic, and so their wetting behaviour is difficulto study. The Wilhelmy technique represents a reliable methodfor the reasons given before) to study the wetting behaviour ofamboo fibres at different immersion speeds. We believe that thenpredictable wetting behaviour of bamboo fibres is related moreo surface topography than to some kind of an unpredictable naturef bamboo fibre surface material. If surface waviness, roughnessnd liquid penetration are minimized, then the bamboo fibre sur-ace represents a well defined system and so its wetting behaviouran be studied and a meaningful interpretation of wetting data isnsured. For this to be the case, contact angles of bamboo fibresust show an expected dependence to immersion velocity.The molecular kinetic theory has already been used to interpret

he dynamic contact angle data and to model the wetting phe-omena for several synthetic fibres [14,27,49,50]. More specifically,he advancing contact angle as a function of wetting velocity wastudied previously by Blake [13] for water on PET. Fig. 11A showsetting experiments on PET fibres that were conducted here with

wo intentions: comparing the wetting behaviour of a syntheticbre with our natural bamboo fibre and having a reference materialo evaluate our experimental and fitting procedures.

Our experimental wetting data seem to conform well to the pre-iction of the MKT. The quality of the fit to Eq. (1) is good with2 = 0.94 (Fig. 11A). Furthermore, the value of 1.16 nm obtained forhe characteristic length � is similar to the value of 1.10 nm reportedreviously by Blake [13] for water on PET monofilament. How-ver, our equilibrium displacement frequency K0 of 1.6 × 105 s−1

s decreased by a factor of 1.7 in comparison with 2.5 × 105 s−1

btained by Blake [14]. Concerning this variation, the literature sug-ests that this value is less stable [14,29]. Blake [14] reported theifference between the values of K0 and � obtained with high and

ow speed for water on PET fibres; while � was almost in the same

at 11 different speeds, (B) dynamic contact angle as a function of wetting velocityds ranging from 0.15 to 500 mm/min. The theoretical curve through the data wasprocedure. � = 0.827 nm, K0 = 5.83 × 104 s−1. The angles were measured at 9 speeds

different technical bamboo fibres.)

order of magnitude, K0 was some 4 orders of magnitude biggerat high speed, and there is also the fact that different PET fibreswere used. Accordingly, the surface roughness, crystallinity, andchemical composition might be slightly different.

The theoretical curve calculated by inserting determined meanvalues of K0 and � for water on autoclave treated bamboo fibres intoEq. (1), reveals good agreement between the experimental and thecalculated values of the dynamic contact angle � over the entireexperimental speed range, as can be seen in Fig. 11B. The experi-mental data provide a satisfactory fit with R2 = 0.90, which is notas good as in case of PET with R2 = 0.94. This difference is probablya consequence of the still remaining irregularities on the technicalbamboo fibre surface. Although the autoclave treatment success-fully reduces the bamboo fibre irregularities, these are still biggerthan those present on a synthetic fibre.

The obtained jump frequency K0 = 5.83 × 104 s−1 is low if itis compared with published values for water on other materials[13,14,29]. While K0 would suggest a more polar surface (whichseems to fit in with the XPS results), the advancing contact angle is abit high suggesting a largely non-polar surface. Since the measuredcontact angles represent just the advancing front (which is possiblyunderestimating the polar content), the small K0 value would alsosuggest a small receding contact angle for a bamboo–water system,which could not be measured for the reasons given before.

The molecular-kinetic equation provides a reasonable fit tothe data with acceptable values of K0 and �, as shown inFig. 11B, confirming the expected immersion velocity dependence,reproducibility and stability of the advancing contact angle in abamboo–water system; indicating that the measured contact angleis the true advancing contact angle. However, as stated before,surface irregularities and probably diffusion contributes to theinstability of the receding contact angle.

5. Conclusions

The high concentration of lignin on the surface of technical bam-boo fibres, as concluded from XPS results, seems to be responsiblefor their wetting properties. Furthermore, the experimental resultsreveal that the large fluctuations during wetting between various

bamboo fibres of the same batch may be due more to the surfacetopography irregularities of the fibres than to any other type ofunpredictable phenomena. These irregularities are largely reducedby autoclave treatment and subsequent smoothening of the ligninsurface layer, as confirmed by AFM results.

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C.A. Fuentes et al. / Colloids and Surfaces A

Systematic experimental results describing the dynamic wet-ing properties of bamboo and PET fibres with water were analyzedy applying the molecular-kinetic theory of wetting. Accordingly,he wetting behaviour of bamboo fibres appears to conform wello the predictions of the molecular-kinetic theory. This may ensurehat the advancing contact angle of liquids on autoclave treatedamboo fibre is stable, reproducible, and immersion velocityependent, allowing meaningful information on interfacial inter-ctions to be deduced. The preparation of composite materialseinforced by bamboo fibres will thereby be facilitated.

cknowledgments

Our thanks to Vincent Janssens and Sylvie Derclaye for help withetting measurements, and Yasmine Adriaensen and Michel Genet

or help with XPS measurements. We would also like to acknowl-dge the financial support of K.U. Leuven (SBA Scholarship), FNRS,elSPO (IAP network 6/27), and BelSPO-Vietnam.

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