Accepted Manuscript Tribological and Mechanical Properties of Brazilian Hair
A. Elzubair, N.F. de Oliveira, F. Munhoz, C. Flor, F. Fiat, N. Baghdadli, S.S. Camargo, G.S. Luengo PII: DOI: Reference:
S2352-5738(17)30005-7 doi: 10.1016/j.biotri.2017.06.001 BIOTRI 64
To appear in:
Biotribology
Revised date: Accepted date:
26 May 2017 12 June 2017
Please cite this article as: A. Elzubair, N.F. de Oliveira, F. Munhoz, C. Flor, F. Fiat, N. Baghdadli, S.S. Camargo, G.S. Luengo , Tribological and Mechanical Properties of Brazilian Hair. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Biotri(2017), doi: 10.1016/j.biotri.2017.06.001
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ACCEPTED MANUSCRIPT Tribological and mechanical properties of Brazilian hair A. Elzubair*a, N. F. de Oliveirac, F. Munhozc, C. Florc, F. Fiatd, N. Baghdadlid, S. S. Camargo Jr.a,b and G. S. Luengo*d a
Programa de Engenharia Metalúrgica e de Materiais, COPPE, Universidade Federal de
Programa de Engenharia de Nanotecnologia, COPPE, Universidade Federal do Rio de
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Rio de Janeiro, Ilha do Fundão Cidade Universitária, CEP 21941-972, R J, Brazil
Janeir, Ilha do Fundão Cidade Universitária, CEP 21941-972, R J, Brazil
L’Oréal Research and Innovation, Av Eugène Schueller, 93600 Aulnay Sous Bois, France
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L’Oréal Research and Innovation, Rua São Bento, 08 – Centro, CEP 20090- RJ, Brazil
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Authors to whom correspondence should be addressed: *aAmal Elzubair (
[email protected]) *dGustavo S. Luengo (
[email protected]
ACCEPTED MANUSCRIPT Abstract Background - The high Brazilian ethnical mixture combined with the country climate conditions make Brazilian hair an uncommon category that is still scarcely studied. Methods - Brazilian hair of types II to V were investigated by nanoscratch and nanoindentation techniques. The results were statistically analyzed using ANOVA (p ≤
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0.05 level) and statistical correlation between the measured parameters was studied by
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linear regression.
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Results - Nanoscratch at low loads showed for hair types II and III a pronounced elastic recovery with little damage, while for types IV and V a more plastic and brittle behavior
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with higher friction in the direction from tip to root was observed. At high loads the tip reached the cortex and elastoplastic deformation, plowing, fracturing and chipping of
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cuticle cells occurred in all types. Quasi-static nanoindentation yielded average values H = 0.22 ± 0.06 GPa and E = 4.7 ± 0.8 GPa. Dynamic nanoindentation showed increasing H
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and E values when going from types II to V.
Discussion - The static H and E values of Brazilian hair are consistent with the high
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Brazilian ethnical mixture. Nanoscratch failure mechanisms can be explained based on the dynamic H and E values. The scratch resistance in the direction from root to tip is
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associated to the cuticle mechanical properties, but in the opposite direction increased mechanical property values lead to increased damage. The surface friction behavior is
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determined by roughness of the fibers, however, when the tip goes into the cortex friction decreases due to its softer nature.
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Keywords: Brazilian hair; nanotribological properties; nanoindentation; differential friction effect.
ACCEPTED MANUSCRIPT 1. Introduction Brazil is a country with a very high ethnical mixture [1] and, as a result, has a great diversity of hair types. This ethnic diversity, combined with aggressive, tropical climate conditions, a high incidence of solar radiation, high temperature and humidity levels, make
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Brazilian hair an uncommon category that is still scarcely studied.
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Hairs of the world are classified into eight types (STAM classification) depending on the degree of curliness, i.e. increasing with the curl diameter, number of waves and twists [2].
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According to this scale, hair type I is completely straight, while type VIII is completely kinky hair. Asian hairs are mostly types I and II, African hairs tend to be between types V
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and VIII while Caucasian’s show between II and IV [3]. In Brazil, we can find the eight
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hair types, with predominance of types I to V. Typically, the curlier hair has more twists and is more difficult to manage since more efforts are needed during combing, for example.
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Hair fiber mechanical and tribological properties can be affected by the degree of curliness
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[4].
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The hair fiber is composed of proteins, lipids, melanin, water and traces of mineral elements. The main components are the cystine rich keratins and associated proteins (60-
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95% of the fiber composition). The disulfide bonds of cystine are found being responsible for the mechanical strength of hair, particularly in the wet state [3, 5-6]. The hair fiber consists of three distinct compartments: cuticle, cortex and medulla. The outer part is the cuticle, which consists of flat overlapping cells (scales) and a cell membrane complex (CMC). CMC consists of cell membranes and adhesive material that binds or “glues” the cuticle and cortical cells together in keratin fibers [4]. The thickness of each scale is about 0.3 - 0.5 µm, and the cuticle thickness is made of 5 - 10 scales (1.5 - 5 µm) [5]. The shape
ACCEPTED MANUSCRIPT and orientation of cuticle cells are responsible for the differential friction effect (DFE) in hair, where the frictional force required to slide from ‘tip’ to ‘root’ direction is higher than that along the opposite direction for a given load [7]. The main inner compartment of hair is the cortex, which makes the major part of the fiber mass (70 - 90%) and is the core
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contributor to the overall fiber properties. The cortical cells are composed of macrofibrils,
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containing the highly-organized keratin intermediate filaments (made of helices), and the
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amorphous matrix of the keratin associated proteins (KAP). Finally, either completely
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absent or highly variable in size [6], the medulla is the central, empty part of the hair fiber. For all types of materials, including hair fibers, the mechanical properties (elastic modulus,
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hardness) are known to strongly affect their tribological performance (coefficient of friction, wear) [8]. Nanoindentation and nanoscratch techniques record the load and
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penetration of a hard probe (tip) on a surface either vertically or laterally (shearing),
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respectively. They are very useful methods to assess mechanical and tribological properties
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of solid materials [9].
In the case of hair, the increase in hardness and elastic modulus measured by Pavan et al.
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[10] and others [11] using instrumental nanoindentation at the surface of the hair fiber,
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could be explained by the higher amount of cross- linked disulfide bonds in the proteins of the hair cuticle, since it has been shown to contain a higher percentage of cystine than the whole fiber [12]. This was observed in hairs from the three most studied ethnic groups, i.e. Caucasian, Asian and African. However, nanoindentation measurements in African hair showed a higher dispersion because of their curled and elliptical shape [5]. Wei and Bhushan [13] also studied the effect of the scales and direction dependence of tribological properties of hair using a conical diamond tip (radius about l µm). They observed that when
ACCEPTED MANUSCRIPT going in the tip to root direction the tip collides and has to climb up the scale edges, resulting in a high friction signal. So, the magnitude of friction when going up the steps is larger than the friction when the tip is going down the steps, bringing out a direction effect. Franbourg et al. [14] observed that African hair was mechanically more fragile than
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Caucasian and Asian hairs (earlier breakage at lower extension and stress requirements
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during tensile tests) even though no differences in structural or chemical composition were
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observed between the three types of hair. This mechanical characteristic of African hair
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could come from its particular morphology and geometry, that is, the natural constrictions along the fibers, twisted shape of the fibers, and presence of microcracks or fractures in the
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fiber surface [15-18]. Finally, African hair is often perceived as fragile by the consumer [19], very likely as a result of intrinsic structural factors (i.e. curliness) combined with the
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particular consumer’s grooming practices (strong combing, use of relaxers, etc.).
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This work aimed at studying the mechanical and tribological properties of Brazilian hair,
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and exploring to which extent these differences deviate from common characteristics of African, Caucasian and Asian hair types, due to the high level of ethnic mix in the country.
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For this purpose, hair collected from young women with different hair types, according to
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their degree of curliness, were evaluated using nanoscratch and nanoindentation techniques.
2. Materials and Methods In this study, virgin hair samples (defined as hair that has not been chemically treated), with different curl types: II, III, IV and V were cut trim to the scalp from two young female individuals for each type, their age range from 17-28 years, living in Rio de Janeiro and São Paulo, Brazil (see Figure 1). The hair samples, after being weighed, were washed with 2%
ACCEPTED MANUSCRIPT sodium dodecyl sulfate (SDS) for 10 minutes (at least 1 ml for each gram of hair), rinsed
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with tap water for 5 min. then with pure water (Milli-Q) and air dried.
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Figure 1 Eight volunteers (two for each hair type) from which the Brazilian hair samples
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were obtained.
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Samples of length about 2 cm were cut at a distance ~5 cm away from the root area to be
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tested by nanoscratch and nanoindentation techniques. All measurements were performed in this region. The hair fibers were stretched carefully and fixed with glue on a 2x2 cm2
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glass slide, care was taken so as not submerge the fibers in the glue. Both tests were
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performed using Agilent G200 Nanoindenter (Santa Clara, USA), which has a displacement resolution <0.01 nm, and force resolution of 50 nN. An optical microscope with a
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magnification of 40x attached to the nanoindenter was used to locate the scratch and indentation positions on the hair surface. Nanoscratch test was performed by sliding a Berkovich diamond tip of 20 nm diameter along the lateral surface of the hair samples. Linear ramped loading with maximum normal loads of 10 and 50 mN, scratch velocity of 0.5 µm/s, scratch angle at 0o (orientation of the Berkovich tip), and scratch length of 100 µm were used, so that during one nanoscratch, multiple cuticle cells of the hair sample were embraced. The test was performed with the tip in the face forward direction, at least five
ACCEPTED MANUSCRIPT times on the same hair for data reproducibility. All the nanoscratch tests in this work were carried out in the direction along the hair fiber axis from root to tip and from tip to root, independently. The nanoscratch test was performed at three steps: first, the original profile or topography
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of the surface before the nanoscratch process (pre-scan) was obtained by scanning the
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sample using a very low load of about 0.05 mN; then the nanoscratch scan was carried out
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by translating the hair sample along the same path while ramping from the starting load of
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0.05 mN to the maximum load (10 mN and 50 mN); finally, the residual depth (residual deformation) post scratch (post-scan) was measured using again a low load of 0.05 mN.
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Coefficient of friction and nanoscratch resistance were evaluated. The optical microscope was used to define the positions of the test and to image the samples after the test. Scanning
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electron microscopy (SEM-FEG) was used to image some samples after the tests in
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environmental mode with a voltage of 10 keV.
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Conventional nanoindentation measurements were carried out using a basic hardness and elastic modulus with calibrated tip method, employing a normal load of 4 mN; loading a
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single cycle in 15 s, peak hold for 2 s. The Poisson’s ratio of hair was assumed to be 0.35
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for the purposes of the analysis. More than 30 indentations were performed for each hair type; at least 6 different hair fibers of the same sample were tested. All the positions for testing the hair fiber surfaces were chosen in clean well- focused cuticle cells. Areas with too much surface irregularity were very difficult to focus on, impeding the indenter to perform the tests. Hardness and elastic modulus were obtained using the method of Oliver and Pharr [20].
ACCEPTED MANUSCRIPT Nanoindentation using the continuous stiffness measurement (CSM) enables stiffness S to be continuously measured during loading, so hardness and elastic modulus can be obtained as a continuous function of surface penetration [20, 21]. In CSM, in addition to the semistatic load, a small oscillation is superimposed to the force signal and the resulting response
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of the system is analyzed by a lock- in amplifier. CSM was performed to maximum depth of
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1500 nm, applying excitation frequency of 32 Hz and a constant 0.01s-1 strain rate, which is
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defined as the indention velocity divided by the depth: (dh/dt)/h [20]. The excitation amplitude was controlled such that the harmonic displacement amplitude remained constant
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at 2 nm. All nanoscratch and nanoindentation tests were carried out at a temperature of
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~22o C and humidity of 50±5%.
All data obtained from nanoscratch and nanoindentation tests were statistically analyzed
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using ANOVA (p ≤ 0.05 level) and Tukey test, using the software Origin 8.5. Statistical
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correlation between all the measured parameters was studied by linear regression.
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3. Theory
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Bowden and Tabor proposed that the frictional force in scratch experiment can be
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expressed by a two-component model, such that the tangential force FT is the sum of the shearing and the plowing terms, as follows [22]: FT = sAV + pAH
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where s is the shear strength, related to the frictional adhesion and defined as the force per unit area required to shear interfacial junctions, and p is the scratch resistance defined as the mean contact pressure required to shift materials from the front of a scratch probe. AH and AV represent the projected contact area between the probe and the sample surface along the
ACCEPTED MANUSCRIPT horizontal and the vertical directions, respectively [23]. The two-component model refers to two distinct energy dissipation processes to account for the frictional work. The shear component is the result of the shear of adhesive junctions and involves the energy loss processes in regions immediately adjacent to the interface. The other component,
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commonly termed the plowing component, results from subsurface energy losses caused by
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the penetration of the probe into the adjacent body [24]. Depending on the surface
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topography, the frictional force can show different behavior upon scratching on opposite
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directions; this is called differential friction effect (DFE).
Many theories were proposed to explain the cause of the DFE [24]: Martin [25] supposed
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that it is due to the fact that the magnitude of the interface shear stress depends upon the sliding direction. The model is called the asymmetric field theory (ASF). Lincoln [26] and
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Grosberg [7] proposed that the real area of contact is a function of direction; this is called
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the theory of Elastic Anisotropy of Scale (EAS theory). Both arguments provide the
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required result that the friction in the ‘with’ direction should be less than that in the ‘against’ direction. Flanagan [27] considered that local topography might allow the
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tangential and normal forces to interact. Makinson [28] considered that the frictional anisotropy is caused by a plowing component. This component operates only in the
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‘against’ direction and hence is the source of frictional anisotropy.
4. Results and Discussion 4.1. Nanoscratch tests Figure 2 shows representative displacement into surface versus nanoscratch distance curves and images of nanoscratch tests performed with 10 mN maximum load in the two directions
ACCEPTED MANUSCRIPT of the hair fiber; from root to tip (with cuticle cells direction) and tip to root (against cuticle cells) for hair types II and V. For each experiment, Figure 2 shows the curves corresponding to the pre-scratch original profile (blue traces); the ramp load nanoscratch (green traces) and the post-scratch or residual deformation (orange traces) are shown.
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The actual depth during scratching can be considered as the difference between the original
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profile (blue trace) depth and the nanoscratch depth measured during scratching (green
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trace). The residual deformation can be obtained as the difference between the original
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profile and the post-scan (orange trace) and provides information about the amount of permanent (plastic) deformation or damage, while the difference between the post-scratch
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scan and the nanoscratch scan can be related to elastic recovery of the hair fiber after removal of the normal load. The pre-scratch scans show the topography and surface
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irregularities associated to the scales borders as high as 500 nm. For all hair fibers tested on
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both directions, only a small amount of permanent deformation and a high elastic recovery
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are observed. Nevertheless, the optical images in Figure 2 clearly show a shallow track in the hair surface. Fractured and chipped cuticle cells are observed in some of the
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micrographs as the nanoscratch load increases.
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In the root to tip direction (Figure 2a and 2b) both types II and V presented pronounced elastic recovery and very small permanent deformation. This is evident when observing that the post nanoscratch curve (orange) is almost superimposed the original profile (blue), but with a larger elastic recovery for type II. However, the micrographs show little damage occurs in case of type II, while type V shows chipped cuticle fragments piling up at the sides of the grooves. On the other hand, in type II the tip plows through the fiber due to their soft nature and the removed material, for some of the samples, accumulate at the end
ACCEPTED MANUSCRIPT of the groove. Less damage is observed in this case. When scratching in the opposite direction, from tip to root, complete elastic recovery is observed only for type II, but not for type V which is permanently deformed. However, the micrographs show that cuticle fragments are removed by the tip in both cases. Qualitatively, type III fibers behaved
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similarly to type II, while type IV in spite the large variation of the results seems to behave
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in between types III and V.
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The chipped fragments result from shearing of surface asperities that originate from the
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natural surface roughness due to the highly-curved fibers, local topographical features such as lifted cuticle cells and damage at the cuticle edges. Therefore, are more likely to occur in
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the tip to root direction when the tip is plowing in the direction against to scales. In African natural hair, the lifted cuticles are likely to be caused by mechanical damage due to
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physical processes such as aggressive brushing, back combing, and other grooming
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techniques that put a lot of physical stress upon the hair fiber leading the cuticle to flake
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and sometimes strip away. In addition, curly hair fibers present substantial changes in the cross-section shape of the fiber in the regions of twist that involve flattening or even
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collapse of the fiber structure. Such changes in cross-section within relatively short
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distances (as seen in type V and IV) can lead to stress concentration during plastic deformation and, consequently, to failure [6]. These factors may critically modify the loading at some contact points and hence influence processes such as adhesive frictional shear and plowing [29].
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Figure 2 Typical displacement into surface vs scratch distance curves and scratch images obtained for 10 mN maximum load from root to tip on type II (a) and type V (b) and from tip to root on type II (c) and type V (d) Brazilian hair fibers. The curves corresponding to the pre-scratch or original profile (blue trace); the ramp load nanoscratch (green trace) and the post-scratch or residual deformation (orange trace) are presented.
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Figure 3 Extent of the damage caused by nanoscratch tests with 10 mN maximum load on a
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type V hair fiber: from root to tip (a) and from tip to root (b).
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Figure 4 Average coefficient of friction (a) and depth at maximum load (b) obtained with
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10 mN maximum load for the four types of Brazilian hair fibers.
The damages resulting from nanoscratch test with 10 mN load on type V hair fiber are shown in Figure 3 in the two directions. In the root to tip direction (a), there is a permanent deformation but no piling up is evident, only some lifted cuticle cells are chipped, whereas in the tip to root direction (b), the cuticle cells suffer plowing, shear and chipping.
ACCEPTED MANUSCRIPT The average coefficient of friction and depth at 10mN maximum load, in the two directions are presented in Figure 4. Friction coefficient varies from 0.28 to 0.50; those of types IV and V in the tip to root direction are higher than in the root to tip direction. This differential friction effect (DFE) is associated with the cuticle structure [7] and occurs when the tip
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scratches the hair surface against cuticle cells (scales), 0.3 – 0.5 µm high cuticle cells edges
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resist the tip movement, leading to higher coefficient of friction [13]. The DFE observed in
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the coefficient of friction data appears to obey the theories of Martin, Flanagan and Makinson [25, 27-28]. It can be attributed to the interface shear stress dependence upon the
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sliding direction; the local topography and the assumption that the frictional anisotropy is
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caused by plowing component. It is evident that the two components of Bowden theory [22] are present. The penetration depth at 10 mN maximum load (Figure 4b) varies in the
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range 1.6 - 2.1 µm, with average value of 1.9 ± 0.1 µm, which corresponds to a depth of 4
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to 6 scales.
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When scratching with 50 mN load, three types of behavior and damage can be observed in all samples, as shown in Figure 5: the elastoplastic deformation, plowing, fracturing and
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chipping of cuticle cells. When the applied normal load increased, the hydrostatic pressure
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increased significantly leading to fractured cuticle cells which piles up on the sides of the grooves [29].
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Figure 5 Typical displacement into surface vs scratch distance curves and scratch images obtained with 50 mN maximum load from root to tip on type II (a) and type V (b), and from tip to root on type II (c) and type V (d) Brazilian hair fibers. The curves corresponding to the pre-scratch or original profile (blue trace); the ramp load nanoscratch (green trace) and the post-scratch or residual deformation (orange trace) are presented.
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The examination of the curves and optical micrographs presented in Figure 5 shows that strong plastic deformation accompanied by fracture events occur at early stages of the test and continues with increasing load. The permanent deformation increases significantly as
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can be observed from the significant difference between the original topography (blue) and
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the residual deformation (orange) scans. It is interesting to note that the cuticle edges can
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still be seen near to the end of the tests showing that little material removal occurs.
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However, the comparison of the pre-scratch and post-scratch scans shows that the cuticle
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edges have been flattened by the passage of the tip.
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Figure 6 The shear and extent of damage due to scratching with 50 mN maximum load in: type III hair fiber from root to tip (a) and type V hair from tip to root (b).
Figure 6 shows the extent of damage in two types of Brazilian hair scratched with 50 mN maximum load. In type III hair fiber (from root to tip direction; Figure 6a) shear and plowing of the tip through the cuticle cells is observed with no piling up. On the other hand,
ACCEPTED MANUSCRIPT type V hair fiber (from tip to root direction; Figure 6b), as pointed above, all types of deformation, plowing and fracture of cuticle scales with high piling up on the side of the
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groove are present.
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Figure 7 Average coefficient of friction (a) and depth at maximum load (b) obtained with
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50 mN maximum load for the four types of Brazilian hair fibers.
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Average coefficient of friction and depth at 50 mN maximum load are shown in Figure 7
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for the four types of Brazilian hair. The coefficient of friction values is in the range of 0.40 - 0.48, so are higher than in the previous case (see Figure 4). The depth at maximum load
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varies between 4.75 and 5.75 µm, with average value of 5.2 ± 0.5 µm, suggesting that the tip reached the soft cortex. The cortical cells topography, morphology and concentration of cystine were different from that of the cuticle cells; therefore, it is reasonable that the differential friction effect was almost absent. 4.2. Nanoindentation
ACCEPTED MANUSCRIPT Nanoindentation experiments were performed to investigate the mechanical properties of Brazilian hair fibers and to give additional support to understand the results of nanoscratch
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tests.
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Figure 8 Conventional nanoindentation load-displacement curves represented by Brazilian
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hair type II obtained with 4 mN maximum load (a) and indentation marks on the hair
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surface (b).
Figure 8 shows representative load-displacement curves and indentation marks of multiple indentations on Brazilian hair type II. The curves show the variation of the maximum penetration depth in the range 700 – 1300 nm for a maximum load of 4 mN, which is indicative of the heterogeneity of mechanical properties of hair fiber. The obtained average maximum penetration depth is 1.10 ± 0.15 µm, which indicates that only 2 - 4 scales were
ACCEPTED MANUSCRIPT reached by the tip. This penetration depth is located in the layers of high content of cystine which contribute to the mechanical strength of hair fibers [3, 5-6]. From these curves, hardness and elastic modulus values were obtained. The average hardness obtained for all types of Brazilian hair fibers was 0.22 ± 0.06 GPa, while the
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average elastic modulus was 4.7 ± 0.8 GPa. In order to compare those values with the
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literature data, results at a penetration depth of 1.0 µm were taken from the work by Wei et
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al. [5]. The highest hardness was found in Asian hair, i.e. 0.34 ± 0.05 GPa and elastic modulus of 6.1 ± 0.8 GPa, followed by Caucasian hair with hardness of 0.27 ± 0.02 GPa
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and elastic modulus of 5.6 ± 0.2 GPa, whilst African hair showed the lowest hardness and
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elastic modulus, i.e. 0.18 ± 0.04 GPa and 3.7 ± 0.2 GPa, respectively. From this comparison, we can conclude that Brazilian hairs present mechanical properties that are
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approximately half way between those of the Caucasian and African hairs, which is
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reasonable taking into account the mixing of different ethnic groups in the country.
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As hair fibers present viscoelastic properties, we used nanoindentation continuous stiffness measurements (CSM) in order to gain a deeper understanding of the mechanical properties
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of Brazilian hairs.
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Figure 9 CSM nanoindentation on Brazilian hair type II, load-displacement curves.
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Figure 9 shows representative load-displacement curves obtained by this method on Brazilian hair type II. Measurements were performed down to a maximum depth of 1.5 –
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1.8 µm for all hair fibers which is approximately the maximum depth achieved during the
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nanoscratch tests performed with a maximum load of 10 mN. For reaching such penetration depths, type II hair required the lowest maximum loads (~ 8 mN) while type V hair needed
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the highest one (~ 12 mN) among the studied hair types. These demonstrate that type V is
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harder than type II. For all the curves of this dynamic test, the step seen at about 0.5 mN in the unloading curve (see Figure 9) indicates the viscoelastic recovery of the hair, where the thermal drift correction is not applicable. Example of the variation of the mechanical properties (hardness and elastic modulus) of Brazilian hair, as a function of depth, in the range 200 - 1500 nm, is illustrated in Figure 10. Reliable results could not be obtained at shallower depths due to the surface roughness of the hair fibers. The hardness and elastic modulus values are approximately constant over the entire range 200 - 1500 nm. A similar
ACCEPTED MANUSCRIPT result was observed with all investigated hair types (data not shown). In contrast to our findings, the results obtained by Wei et al. using conventional nanoindentation measurements showed strongly decreasing values of hardness and elastic modulus when the
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tip went from the surface down to penetration depths of 2 – 5 µm [5].
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Figure 10 Mechanical properties as a function of depth obtained by CSM on Brazilian hair
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type II: hardness (a) and elastic modulus (b).
The average values of hardness and elastic modulus at depth range of 200 - 1500 nm, with
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their standard deviations, obtained from CSM measurements for the four types of Brazilian hairs are shown in Figure 11. These values are smaller than those obtained from conventional nanoindentation measurements at similar depths. The possible reasons for such behavior include difficulties of finding good estimates of the contact point and uncertainties in tip area calibration function as well as uncertainties caused by hair surface roughness. Nevertheless, the results of Figure 11 show that both hardness and elastic modulus increase from type II to V, suggesting that types II and III are the softer and types
ACCEPTED MANUSCRIPT IV and V are the harder ones. Therefore, our results indicate that the mechanical property values of Brazilian hair increase with the degree of curliness. This result can be associated with the findings of nanoscratch tests, where types IV and V hair fibers were more readily fractured; indicating that their nanoscratch wear behavior may be related to their hardness
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and brittleness. It is noteworthy to mention that it was established that the hardness from
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nanoscratch and nanoindentation tests are nearly mathematically equivalent [30].
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Figure 11 Average values and standard deviations of the mechanical properties of Brazilian
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hair fibers obtained by nanoindentation tests in the CSM mode from the four different hair
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types: hardness (a) and elastic modulus (b).
4.3. Discussion of the results All data obtained from nanoscratch and nanoindentation tests are presented in Table 1 as means and standard deviations. Means labeled with the same superscript are statistically different at p ≤ 0.05 level.
ACCEPTED MANUSCRIPT The mean values of hardness and elastic modulus showed a tendency to increase with the degree of curliness, when going from type II to type V, as shown in Figure 11. From Table 1 we can observe that this trend is confirmed, except for the fact that type IV is indistinguishable from types III and V. In general, a considerable dispersion of the results
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for each of the tests parameters was obtained. This dispersion comes not only from the
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surface topography and irregularities of the cuticle surface, which directly affect the
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measurements, but also from the variation among the different hair fibers of one individual
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and between individuals of the same type.
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Table 1 Nanoindentation and nanoscratch tests parameters mean values and their standard deviations. The means labeled with the same superscript are statistically different at p ≤
COF
At
Depth
At
10 mN
(µm)
10 mN
T-R
R-T
T-R
0.30
0.33AB
1.65
±0.02
±0.08
±0.10
H
Hair GPa
3.3ABC
0.12ABC
COF
At
Depth
At
50 mN
(µm)
50mN
R-T
T-R
R-T
T-R
2.00A
0.44A
0.44AB
5.00
5.25
±0.29
±0.71
±0.05
±0.04
±0.79
±0.50
3.7AD
AC
II
R-T
CE
GPa Type
PT
E
ED
M
0.05 level.
0.14AD
0.30
0.28CD
2.10
1.95
0.41AB
0.41AC
5.25
5.75
±0.5
±0.02
±0.08
±0.08
±0.57
±0.30
±0.04
±0.02
±0.79
±0.50
3.7B
0.15B
0.28AX
0.48ACX
2.00
2.00
0.45BX
0.40DX
5.25
5.50
±0.5
±0.03
±0.10
±0.08
±0.71
±0.71
±0.05
±0.02
±0.50
±0.50
±0.3
III
IV
ACCEPTED MANUSCRIPT 3.9CD
0.16CD
0.38AY
0.50BDY
1.85
1.60A
0.43Y
0.48BCD
±0.4
±0.03
±0.10
±0.10
±0.78
±0.28
±0.05
Y
4.75
5.00
±0.50
±0.50
V ±0.02
A more detailed study, with a larger number of individuals and experiments, would be
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necessary in order to separate the different contributions to the overall dispersion of the
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data. Therefore, we believe that at this moment it is not possible to conclude whether the
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indistinguishability of the mechanical properties of hairs types IV from types III, and V is a
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proper feature of this hair type (being a mix of African and Caucasian) or it is a limitation of the present set of measurements.
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The analysis of the parameters obtained from scratch tests is less clear. As shown in figures
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2 and 5, the surface irregularities on the cuticle surface associated with the scale edges are in the range of 500 nm. These irregularities directly affect the coefficient of friction and
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penetration depth measurements and introduce considerable scattering in the measured
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data. So, the standard deviation of the data obtained from scratch tests is very large, in some cases even larger than 30%. Therefore, as seen in Table 1, most of the parameters
CE
obtained from scratch tests are indistinguishable at p ≤ 0.05 level. No clear correlations can
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be obtained between the various scratch test parameters and the hair types at p ≤ 0.05 level, the only clear conclusion can be drawn is that the mean value of coefficient of friction (COF) measured in the tip to root direction with 10 mN for types IV and V is significantly different from that of types II and III, confirming our qualitative analysis of scratch test made above. Application of statistical analysis for the comparison of the data relative to the scratch directions showed that the coefficient of friction is statistically different at p ≤ 0.05 level
ACCEPTED MANUSCRIPT only for types IV and V, with higher values in the tip to root direction at maximum load of 10 mN. So, we conclude that during the scratching a DFE could still be observed in types IV and V, but not in types II and III. By performing linear regression analysis between the different test parameters, the present
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data allowed us to establish some correlations between the various properties of Brazilian
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hair fibers and to gain some more insight about their mechanical and tribological behavior.
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The results obtained for R-squared for each pair of parameters are presented in Table 2. The
AC
CE
PT
ED
M
AN
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+ and – signs between parenthesis stand for positive and negative correlations, respectively.
ACCEPTED MANUSCRIPT Table 2 Results of linear regression analysis between all the nanomechanical and nanoscratch tests parameters. Values of R-squared are presented for each pair of parameters. The + and – signs between parenthesis stand for positive and negative correlations, respectively. Values of R2 0.10 were replaced by letter x. Depth (µm)
x
H (GPa)
x
(µm) 10mN TR 50mN
RT COF
TR
(–)
0.16
x 0.32 (–)
x
(+)
x
0.82
0.84
(–)
(–)
(–)
(–)
(+)
x
10mN
x
x 0.39 (+)
x
IP
(+)
(+)
0.74 (–)
x
(+)
0.28
(+)
(–)
0.49
0.82
0.84
(–)
0.89
0.95
(–)
0.96
0.91
(+)
(–)
0.81
(–)
(–)
x
0.17
(+)
(+)
0.38
0.66
(+)
(–)
0.56 (–)
(GPa)
10mN
0.16
(–)
H
RT
50mN
0.56
0.19
0.79
10mN
0.53
x
0.16
50mN
50mN
x
(+)
50mN
0.99
AC
10mN
0.32
0.25
x
M
50mN
(+)
10mN
ED
Depth
0.50
PT
RT
(+)
CE
10mN
0.24
50mN
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E (GPa)
10mN
TR
AN
50mN
RT
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TR
T
COF
0.76 (+)
ACCEPTED MANUSCRIPT As can be observed in Table 2, the values of elastic modulus E and hardness H are well correlated (R2 > 0.95) to each other, meaning that the harder the hair is the higher is its elastic modulus, as expected. It must be recalled that the indentation penetration depth is around 1.0 µm, so these must be considered as mechanical properties of the cuticle.
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Scratch penetration depth in the root to tip (R-T) direction with 10 mN load is reasonably
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correlated (R2 ~ 0.80) to the surface mechanical properties (E, H). This suggests that under
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these conditions the scratch hardness may be related to the mechanical properties of the cuticle [30]. However, in the opposite direction (T-R) a weaker, but inverse correlation is
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observed. This is very interesting because it suggests that different mechanisms occur when
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scratching in the two directions. In the R-T direction, when E and H increase the surface becomes more resistant to penetration of the tip, but in the opposite direction when E and H
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are higher the surface becomes more fragile, it is more easily fractured so the tip penetrates
ED
deeper. This conclusion is in agreement with the findings of the scratch tests, including the deformations observed in the micrographs shown in Figures 3 and 6. Scratch penetration
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depths obtained with 50 mN, on the other hand, do not show correlation to the surface
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mechanical properties, which is reasonable since in this case the tip reaches the cortex of the fibers. The penetration depths for different directions showed much better correlation
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(R2 = 0.89) with each other for 50 mN than for 10 mN (R2 = 0.56), indicating that the influence of the direction of scratch is much more significant near the hair surface. Coefficient of friction (COF) showed a poor correlation (R2 < 0.50) with surface mechanical properties (E, H) for both loads and both directions, indicating that the mechanical properties of the surface are not determining to friction. Coefficient of friction shows several inverse correlations with the penetration depth, some with R-squared values
ACCEPTED MANUSCRIPT higher than 0.80 and some with R2 > 0.90. This indicates that the more the tip penetrates into the hair fiber the smaller is the force to scratch it, as the cortex of the hair fiber is softer than its surface. Coefficient of friction measured with different loads in the same direction are not correlated to each other, reinforcing the idea that different mechanisms are
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responsible for the friction force at the cuticle and at the cortex. Coefficients of friction for
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different directions (R-T and T-R) measured with the same maximum load are not
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correlated to each other, indicating that different mechanisms are responsible for the
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friction force in both directions. The same holds for both loads (10 mN and 50 mN).
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5. Conclusions
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The mechanical and tribological properties of Brazilian hair fibers of types II to V were investigated by nanoscratch and nanoindentation experiments. Nanoscratch tests performed
ED
with a load of 10 mN showed that for all samples the tip penetrated 3 - 5 cuticle cells. In
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case of hair fibers of types II and III, the tip plowed through the fiber with little damage and pronounced elastic recovery, while in case of types IV and V, the fibers presented residual
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plastic deformation and the cuticle cells were fractured during scratching, demonstrating a
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brittle behavior. The coefficient of friction in the direction from tip to root (against the scales) was higher than in the opposite direction only for hair types IV and V. Such differential friction effect (DFE) is probably originated from both adhesion and plowing components of the friction force. At higher loads (50 mN) elastoplastic deformation, plowing, fracturing and chipping of cuticle cells occurred in all types of Brazilian hair fibers, along with the fractured cuticle cells piled up on the sides of the grooves. In this case, the maximum penetration depth reached the cortex.
ACCEPTED MANUSCRIPT Brazilian hair presents average hardness and elastic modulus values approximately half way between those of Caucasian and African hairs, which is consistent with the idea of ethnic mixture of the Brazilian population. Hardness and elastic modulus obtained by nanoindentation by CSM showed increasing values when going from type II to V hairs.
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Therefore, the fracture behavior of types IV and V can be associated to their higher
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hardness and brittleness, while the softer types II and III presented pronounced elastic
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recovery and little damage due to their softer nature.
The statistical analysis of scratch test parameters did not yield any clear correlations
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between the tribological properties and Brazilian hair types, except for the higher friction
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coefficient of the curlier hair type V in comparison with other types. However, linear regression analysis allowed drawing correlations between some of the properties of
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Brazilian hair fibers. When scratching in the direction from root to tip, the scratch
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resistance as measured by the tip penetration depth inside the fiber increased with the cuticle mechanical properties, revealing increased scratch hardness. However, in the
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opposite direction an inverse correlation was observed suggesting that increased hardness
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resulted in more brittle and easily fractured surface. The lack of correlation between the coefficient of friction and surface hardness and elastic modulus showed that friction of hair
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fibers may be rather determined by surface topography and roughness than by mechanical properties. However, when the tip goes deep into the cortex the friction decreases due to the softer nature of this region. Acknowledgements The authors would like to thank L’Oréal Company, COPPETEC Foundation and CNPq for financial support.
ACCEPTED MANUSCRIPT References [1] Kehdy, F.S., Gouveia, M.H., Machado, M., Magalhães, W.C., Horimoto, A.R. et al. “Origin and dynamics of admixture in Brazilians and its effect on the pattern of deleterious mutations”. Proceedings of the National Academy of Sciences 112 (28) (2015) 8696–8701.
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[2] De la Mettrie, R., Saint-Leger, D., Loussouarn, G., Garcel, A.L. “Shape variability and
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[3] Loussouarn, G., Garcel, A.L., Lozano, I., Collaudin, C., Porter, C., Panhard, S., SaintLéger D., de La Mettrie, R. “Worldwide diversity of hair curliness: a new method of
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[4] Bouillon, C., Wilkinson, J. “The science of hair care”. Taylor & Francis, Boca Raton,
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[6] Kamath, Y.K., Hornby, S.B., Weigmann, H.D. “Mechanical and fractographic behavior
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[8] Rabinowicz, E. “Friction and wear of materials”. Wiley-Interscience; New York, 2nd Ed., 1995.
ACCEPTED MANUSCRIPT [9] Luengo, G.S., Galliano, A. Applications of scanning probe methods in cosmetic science. In: Scanning Probe Microscopy in Industrial Applications, pp. 270–286. John Wiley & Sons Inc., New York, 2013. [10] Pavan, S., Loubet, J.L., Potter, A., Luengo, G. “Nanoindentation of natural human hair
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[13] Wei, G., Bhushan, B. “Nanotribological and nanomechanical characterization of
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on ethnic hair”. Journal of the American Academy Dermatology 48 (2003) S115–S119.
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revealed by light and scanning electron microscopy and computer aided three-dimensional reconstruction”. Archives of Dermatology 124 (1988) 1359–1363. [16] Gamez-Garcia, M. “Plastic yielding and fracture of human cuticles by cyclical torsion stresses”. Journal of Cosmetic Science 50 (1999) 69–77. [17] Draelos, Z. D. “Understanding African-American hair”. Dermatology Nursing 9 (1997) 227–231.
ACCEPTED MANUSCRIPT [18] Kamath, Y.K., Hornby, S.B., Weigmann, H.D. “Effect of chemical and humectant treatments on the mechanical and fractographic behavior of Negroid hair”. Journal of the Society of Cosmetic Chemists 36 (1985) 39–52. [19) Bryant, H.; Porter, C.; Yang, G. “Curly hair: measured differences and contributions to
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[24] Adams, M.J., Briscoe, B.J., Wee, T.K. “The differential friction effect of keratin fibers”. Journal of Physics D: Applied Physics 23 (1990) 406–441. [25] Martin, A.J.P. “Observations on the theory of felting”. Journal Society Dyers and Colourists; 60 (1944) 325–328. [26] Lincoln, B. “The frictional properties of the wool fiber”. Journal of the Textile Institute 45 (1954) T92.
ACCEPTED MANUSCRIPT [27] Flanagan, G.F. “Felting and ratchet action of wool fibers”. Textile Research Journal 36 (1966) 55–65. [28] Makinson, K.R. “On the cause of the frictional difference of the wool fiber”. Transactions Faraday Society 44 (1948) 279–282; “Mechanisms involved in shrink
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Highlights Brazilian hair is an uncommon hair category that is still scarcely studied. Its mechanical property values are approximately half way between those of Caucasian and African hairs. The friction effect of the Brazilian hair fibers may rather be determined by surface topography and roughness than by mechanical properties.