pubs.acs.org/Langmuir © 2010 American Chemical Society
Interactions between Crossed Hair Fibers at the Nanoscale Hiroyasu Mizuno,† Gustavo S. Luengo,‡ and Mark W. Rutland*,†,§ †
Surface and Corrosion Science, School of Chemical Science and Engineering, Royal Institute of Technology, Drottning Kristinas v€ ag 51, 100 44 Stockholm, Sweden, ‡L’Or eal Recherche, 1 Avenue Eug ene Schueller, 93601 ag 45, 114 86 Aulnay-Sous- Bois, France, and §YKI, Institute for Surface Chemistry, Drottning Kristinas v€ Stockholm, Sweden Received July 28, 2010. Revised Manuscript Received October 15, 2010 The atomic force microscope fiber probe is used to directly measure the forces and friction between two human hairs under various conditions. It is shown that the forces between the hair fibers in solution can be well explained by a DLVO interaction and that cationic surfactant modifies the interactions in a manner entirely consistent with current views of adsorption behavior. A Coulombic attraction occurs between the crossed hair fibers in air due to the heterogeneity of the surface, and at shorter separations a clear dispersion interaction is observed. Exposure of the hair to a bleaching solution leads to the removal of the adhesion and solely a double-layer interaction. Two crossed hair fibers obey Amontons’ classic law of friction, with a linear relation between applied load and frictional force, allowing the determination of a friction coefficient; positively charged surfactant adsorption is shown to reduce the friction coefficient between the fibers in a manner consistent with boundary lubrication by a palisade layer.
Introduction The surface of hair is a complex barrier which provides a range of functions, not the least of which is a waterproofing film in the natural state. This hydrophobic coating layer on the outermost cuticle surfaces is still far from being completely understood.1,2 It is prone to weathering, modification, and indeed elimination by external agents such as commercial chemical treatments. Recent experiments3 show the existence of polar, charged regions at the cuticle edges which are also the target area for chemical modification. It has been less clear how these various regions contribute to the overall interactions (forces and friction) between the fibers, which determine the behavior of the hair in both water and the dry state. Interfiber interaction is the basis of an individual’s everyday perception of the state of their hair, and it is therefore not unexpected that its comprehension is of interest to the hair-care and cosmetic industry.4,5 Still, the means by which a complex multifiber system may adopt different three-dimensional shapes, or how a system of hundreds of thousands of fibers interact among each other while in motion, are questions for the physics community and go beyond a purely cosmetic interest and extend to other fields, ranging from textiles6,7 to character animation in the cinema and entertainment industries.8 Economics have historically meant that cosmetics interests have dominated research, but increasingly the difficulties of representing hair realistically are receiving attention. In this area the major stumbling block is *To whom correspondence should be addressed. Tel: þ46-8-790-9914. Fax: þ46-8-20-8998. E-mail:
[email protected].
(1) Huson, M.; Evans, D.; Church, J.; Hutchinson, S.; Maxwell, J.; Corino, G. J. Struct. Biol. 2008, 163, 127–136. (2) Breakspear, S.; Smith, J. R.; Luengo, G. J. Struct. Biol. 2005, 149, 235–242. (3) Dupres, V.; Langevin, D.; Guenoun, P.; Checco, A.; Luengo, G.; Leroy, F. J. Colloid Interface Sci. 2007, 306, 34–40. (4) Bhushan, B. Handbook of Nanotechnology, 2nd revised and extended ed.; Springer: New York, 2007; Chapter 36, p 40. (5) Robbins, C. R. Chemical and Physical Behavior of Human Hair, 4th ed.; Springer: New York, 2001; Chapters 1, 2, 5, and 8. (6) Mizuno, H.; Kjellin, M.; Nordgren, N.; Pettersson, T.; Wallqvist, V.; Fielden, M.; Rutland, M. W. Aust. J. Chem. 2006, 59, 390–393. (7) Huang, F.; Li, K. C.; Kulachenko, A. J. Mater. Sci. 2009, 44, 3770–3776. (8) Ward, K.; Bertails, F.; Kim, T. Y.; Marschner, S. R.; Cani, M. P.; Lin, M. C. IEEE Trans. Vis. Comput. Graph. 2007, 13, 213–234.
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the absence of a method to provide a quantitative description of hair interactions. Groundbreaking studies have been performed on probing the hair surface with an AFM tip,2,9 and these studies have significantly advanced understanding of the hair surface and its behavior; however, the crucial and most relevant measurement; that of the symmetric interaction;remains difficult. This paper describes experiments which effectively overcome this problem, using the crossed fiber probe configuration, first employed for polyester fibers,6 a derivative of the now institutionalized colloid probe.10 At the heart of this force measuring technique is the atomic force microscope (AFM)11 first developed for imaging surfaces and which underpins the whole of modern nanoscience. The AFM can also be used to measure surface forces between particles or fibers6,10 and is particularly useful for systems that do not lend themselves to measurement with the surface force apparatus (SFA).12,13 Briefly, the fiber-probe AFM technique employs the same crossed cylinder geometry as the SFA but achieves this through gluing the fiber along the main axis of the AFM cantilever, which interacts with a second fiber mounted on a moveable substrate at right angles to the cantilever direction. A schematic of this setup can be found in Figure 1. Here we use the AFM in crossed fiber mode to measure both the forces in contact (friction and adhesion, which are of most interest to the applications mentioned earlier1,2,9,14) and the precontact forces in both air and aqueous medium. These latter forces consist of the van der Waals interaction and charge interactions and in addition to providing fundamental information about the nature of the hair surface are also interactions that should be taken into account to understand and predict the behavior of any multifiber system especially in a humid or aqueous environment. (9) LaTorre, C.; Bhushan, B. Ultramicroscopy 2005, 105, 155–175. (10) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239–241. (11) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930–933. (12) Briscoe, W. H.; Titmuss, S.; Tiberg, F.; Thomas, R. K.; McGillivray, D. J.; Klein, J. Nature 2006, 444, 191–194. (13) Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J. N. Science 2002, 297, 379–382. (14) Sadaie, M.; Nishikawa, N.; Ohnishi, S.; Tamada, K.; Yase, K.; Hara, M. Colloids Surf., B 2006, 51, 120–129.
Published on Web 11/30/2010
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Figure 1. A representation of the crossed human hair fiber system used in the AFM. The nominal lengths of the upper and lower hair segments are typically 60 and 500 μm, respectively. The hair diameter ranged from 53 to 94 μm.
Materials and Methods Native (variously otherwise denoted as natural, virgin, or untreated) female Caucasian hair samples were pretreated before use according to two protocols. A standardized oxidative treatment with hydrogen peroxide (H2O2) for 10 min was performed on some of the hairs, henceforth referred to as “bleached” hair which mimics weathering and damaged to hair. Both native and bleached human hair samples were sonicated in 1% aqueous sodium dodecyl sulfate (SDS, g99%, Sigma-Aldrich, Germany) solution for 2 min, rinsed with a copious amount of filtered, deionized water, and then dried with nitrogen which has earlier been identified as a reproducible cleaning protocol.2 Initial cuts of the fibers into lengths of ∼200 μm were performed with ordinary household clippers. The AFM cantilevers were silicon and tipless (NSC12, MikroMasch, Estonia) with a specified normal spring constant of 0.65 N/m. The actual normal and torsional spring constants are calibrated using a technique based on the hydrodynamic damping of the intrinsic thermal noise.15 The values for the various cantilevers used ranged from 0.3 to 0.56 N/m and 6.8 10-9 to 1.4 10-8 N m/rad, respectively. Etched tungsten wires attached to a micromanipulator (Eppendorf) were used to position the glue and the fiber pieces respectively on an AFM cantilever under a stereo microscope. A challenge in all force measuring techniques, particularly at the nanoscale, is quantification of the forces measured. In the original fiber probe article 6, such quantification was rendered difficult by the length of the fibers employed. Figure 2 shows a scanning electron micrograph of a hair fiber that has been trimmed using a focused ion beam (FIB)16 to alleviate this issue. If the hair fiber on the AFM cantilever is long, then it will also bend and deform under normal and lateral forces, and there is no way of reliably obtaining the effective spring constant. Second, the AFM cantilever actually sits at about 12° to the surface. Therefore, if the hair protrudes too far beyond the cantilever, it may potentially strike the substrate before the hair-hair contact is made and preclude force measurement. The outer (left-hand) cut is that performed with the FIB, and it is apparent that this edge is finer than the inner cut which was performed with ordinary clippers. The experimental technique allows the separation between the two fibers to be controlled with nm resolution and both the normal force and friction between hair fibers to be determined (15) Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Rev. Sci. Instrum. 2004, 75, 1988–1996. (16) Volkert, C. A.; Minor, A. M. MRS Bull. 2007, 32, 389–399.
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Figure 2. SEM image of the hair probe attached to a cantilever. The fiber has been truncated with a FIB to reduce overhang and minimize the effect of the fiber on the mechanical response of the FM cantilever. The FIB cut is considerably smoother than the conventional cut. In this example the “root” direction of the fiber is to the right. with a resolution of nN. The crossed cylindrical geometry is equivalent to that of a sphere interacting with a flat surface, and the truncated fiber is still long enough for this assumption to be valid. The height of the cuticle edges is variable but of the order of 500 nm, and the surface roughness on the cuticle surfaces is around 12 ( 8 nm.9 The force and friction experiments were performed using an atomic force microscope (Nanoscope IIIa, Picoforce, Veeco) according to protocols described in an IUPAC report.17 Briefly, each AFM experiment started by acquiring two normal force curves at a constant scan rate of 400 nm/s. Afterward, friction measurements were run at sliding velocities varying between 4 and 20 μm/s with scan size of 10 μm. These rates are relatively slow compared to typical movements of hair but are experimentally convenient. Speeds of mm/s are impossible in AFM due to hydrodynamic and inertial effects. The friction forces are nonetheless only weakly dependent on the rate over the range accessed here. An initially low applied load was gradually increased to the maximum value (typically 100-120 nN) and then reduced again until the surfaces spontaneously separated. These latter experimental parameters are relevant for the interactions experienced by hair in daily life. (Such motion and forces are determining in the cosmetics industry for example in “style retention” and “volume” of hair.5) The relatively large scan size was chosen to ensure that the probe would cross a cuticle edge at least once while sliding (the distance between cuticle edges is around 5 μm as can be seen in Figure 2). For the measurements performed in liquid the hair fibers were exposed to each surfactant solution for ∼15 min before force and friction measurements. The surfactant was tetradecyltrimethylammonium bromide (TTAB) (g99%, Sigma-Aldrich, Germany). A small amount of background electrolyte (1 mM NaCl, 99.99%, Merck, Germany) was used for the aqueous systems, which is standard practice in surface force measurement since it limits the thickness of any electrical double layer to a convenient experimental range and renders the fitting of the force curves much simpler.10,18 To simplify, we refer to the solutions not containing surfactant as “water”.
Results and Discussion Native Hair in Solution. Figure 3 shows the forces on approach and the effect of surfactant addition to the solution (17) Ralston, J.; Larson, I.; Rutland, M. W.; Feiler, A. A.; Kleijn, M. Pure Appl. Chem. 2005, 77, 2149–2170. (18) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975–1001.
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Figure 3. Normal force curves for native hair measured in 1 mM NaCl in the presence of trimethyltetradecylammonium bromide solutions (TTAB): (0) no surfactant, (2) 1 cmc, and (O) 10 cmc. The cmc of TTAB at 1 mM NaCl is 3.2 mM. κ-1 is 9.6 nm for the water, 4.7 nm for the 1 cmc solution, and 2.6 nm for the 10 cmc solution. The fitted surface potentials are -42 mV for water and 33 mV for 10 cmc. The surface potential for 1 cmc is constrained to a value of 50 mV, and the ensuing effective radius is 1.0 μm (see text). The Debye length has been constrained to the calculated value from the solution conditions, except for the 10 cmc case where the calculation is dependent on knowledge of the degree of dissociation of the micelles.
for the case of native hair. It is possible to observe a repulsive force at longer range, and analysis of the distance dependence shows clearly that this force arises from overlap of the electrical double layers19 associated with an electrical charge at the surface, arising primarily from dissociation of polar proteinaceous moieties.5 While it is strictly impossible to assign the sign of the charge from such measurements, the change in force on addition of surfactant as well as a range of documented studies of the charging of hair5,20,21 allows us to conclude that the sign is negative. At shorter range, below about 15 nm, an attractive force component is observed before a final elastic/steric compression of the surfaces into a rigid compliance. This attractive component appears to be slightly too long ranged to be solely due to dispersion forces (van der Waals or Lifshitz interaction) and may also reflect the inherent hydrophobicity of the native hair caused by the palisade layer of fatty acids (see the inset in Figure 5). A continuous, tightly packed lipid layer with the alkyl chains presented to the aqueous phase is expected to display hydrophobic behavior and forces between such surfaces are known to be attractive,22-24 though the range of this force is highly variable depending on the specific surface involved and various mechanisms have been proposed.25-27 There is however broad consensus28 that at short range (of the order (19) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Dover Publications: New York, 1999; Chapter IV. (20) Jachowicz, J.; Berthiaume, M. D. J. Colloid Interface Sci. 1989, 133, 118–134. (21) Michael, A. S. Surf. Interface Anal. 1996, 24, 522–528. (22) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357–5360. (23) Tyrrell, J. W. G.; Attard, P. Phys. Rev. Lett. 2001, 87, 176104. (24) Stevens, H.; Considine, R. F.; Drummond, C. J.; Hayes, R. A.; Attard, P. Langmuir 2005, 21, 6399–6405. (25) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci. 2001, 91, 391–436. (26) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15739–15746. (27) Eriksson, J. C.; Henriksson, U. Langmuir 2007, 23, 10026–10033. (28) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. Soc., Faraday Trans. 2 1989, 85, 163–176. (29) Tyrode, E.; Johnson, C. M.; Kumpulainen, A.; Rutland, M. W.; Claesson, P. M. J. Am. Chem. Soc. 2005, 127, 16848–16859.
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Figure 4. Variation of friction coefficient (9) and adhesion (O) as a function of TTAB concentration. The inset shows a typical friction-load relationship from which the friction coefficient is obtained as the gradient (2 cmc, (open diamond) loading and (filled diamond) unloading). The friction coefficient is the average of the values of three such measurements using a different pair of fibers each time. The trend of the variation in friction coefficient with concentration was equally clear in the individual experiments.
of that observed here) this force is related to the unfavorable water structure associated with the hydrocarbon-water interface.29,30 The precision of the force measurement is very good, but despite the calibration of the force constants, accurate quantification of the forces is rendered more difficult by the inherent surface structure of the fibers (Figure 2). The conventional approach would be to use the measured radii of the two hair fibers to normalize the measured force such that a free energy of interaction of two equivalent flat surfaces can be calculated.31 From this data fitting of DLVO theory would allow determination of both a surface potential and effective surface charge. For the case of the native hair, the values are -7 mV and -0.04 μC/cm2. These values are very much lower bounds since normalization with the fiber radius presupposes a smooth surface, and the cuticle structure of hair seen in Figure 2 necessarily negates this assumption;the local effective radius of interaction is almost certainly considerably smaller than the mean of the fiber radii. In some cases the effective local curvature can be directly estimated from surface features;32 however, in this case that is not possible. Fortunately, we have an internal reference inherent in the experiments which allows an estimate of the effective interaction radius to be obtained. The nature of this internal reference is implicit in the surfactant adsorption and will be discussed after the surface force discussion below. (One should also consider that the hair surface is not molecularly smooth9 so that there is a further degree of uncertainty induced in the numbers generated by fitting theories to results.) Surfactant adsorbs to the hydrophobic layer via a hydrophobic adsorption mechanism, and surfactant aggregates of opposite orientation to the lipid film are formed;the polar headgroups point toward the aqueous phase. This is extracted from the force curves via the fact that the hydrophobic attractive component gradually switches off, and the force becomes steeply repulsive. The fact that the force becomes shorter ranged with increasing surfactant concentration reflects the fact that the double layer (30) Hore, D. K.; Walker, D. S.; Richmond, G. L. J. Am. Chem. Soc. 2008, 130, 1800–1801. (31) Derjaguin, B. V. Kolloid Z. 1934, 69, 155–164. (32) Cardenas, M.; Valle-Delgado, J. J.; Hamit, J.; Rutland, M. W.; Arnebrant, T. Langmuir 2008, 24, 7262–7268.
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Figure 5. Normal force curves of bleached hair measured in 1 mM NaCl aqueous solution with corresponding DLVO fits (solid and dashed line). The curve is the average of 10 individual measurements and normalized by the effective radius, Reff. The Debye length, κ-1 was 9.6 nm and was not a fitting parameter. The solid line is the constant charge limit, and the dashed line is the constant potential limit. A somewhat arbitrary Hamaker constant of 5 10-21 J was employed based on the Hamaker constant in air (∼4.4 10-20 J), calculated from the Lifshitz-van der Waals surface energy component of H2O2 treated hair fiber.52 No attractive component is visible in the interaction, and no adhesion was observed. A surface potential of -55 mV was estimated from the DLVO fits. The inset is a schematic of the native hair surfaces facing each other in crossed fiber configuration. The outer surface consists predominantly of lipids, the majority of which are 18-methyleicosanoic acid (18-MEA) covalently bonded to the underlying protein.
is becoming compressed due to the ionic nature of the surfactant. At the cmc (critical micelle concentration) the surfactant forms micelles, which do not contribute to the double-layer compression; however, since the micelles are charged (not every surfactant in the micelle is balanced by a counterion), there are significant numbers of counterions capable of screening the surface charge.33,34 The changes in the repulsive force with surfactant adsorption are also reflected in the adhesive force which is measured on separation and shown in Figure 4 (open circles). The adhesion decreases with increasing concentration until it disappears at about the cmc where the adsorbed film has already reached its maximum packing density. This result is expected and uncontroversial35-38 and reflects the fact that a layer of surfactant remains between the surfaces in contact. Surface aggregation of cationic surfactants, like bulk aggregation, is determined by the balance of hydrophobic attraction and Coulombic repulsion between the molecules. Thus, the surface potential of the aggregates is expected to be relatively constant, independent of the nature of the surface to which they adsorb. Thus, since the surface potential of TTAB is known between surfaces of well-characterized radius, the radius can be estimated by scaling the measured force to fit the expected double-layer interaction. In fact, this idea is not new, and the expected double-layer interactions (33) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569–578. (34) Pashley, R. M.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2902–2904. (35) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219–304. (36) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110–1121. (37) Stiernstedt, J.; Fr€oberg, J. C.; Tiberg, F.; Rutland, M. W. Langmuir 2005, 21, 1875–1883. (38) Koopal, L. K.; Leermakers, F. A. M.; Lokar, W. J.; Ducker, W. A. Langmuir 2005, 21, 10089–10095. (39) Senden, T. J.; Drummond, C. J.; Kekicheff, P. Langmuir 1994, 10, 358–362.
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of mica-silicon nitride39 and of adsorbed cationic surfactants40 have previously been used to obtain interaction radii in exactly the same manner as performed here (or alternatively spring constants in systems where the radius is well-known). In a similar vein the expected hydrodynamic force has also been used to scale the measured forces to obtain either the spring constant or the local radius.41 The potential of a TTAB surface has previously been obtained,37 and with a small correction reflecting the slightly different salt conditions (Graham equation) the surface potential is expected to be of the order of 50 mV. In each experiment employing a different pair of hairs the double layer force in 1 cmc solution was scaled to fit this value and resulted in effective radii of curvature between 1.0 and 1.5 μm (as opposed to the mean curvatures of the fibers of around 40 μm). Thus, the radii employed in the force curves reflect the effective radius of interaction obtained in this manner rather than the macroscopic radii of the fibers. The surface potentials obtained from fitting the curves with DLVO theory42 are thus -42 mV (native hair in solution), 50 mV at 1 cmc (constrained), and 33 mV at 10 cmc. The fits are shown in Figure 3, and note that the Debye length is calculated from the solution concentration and therefore not used as a fitting parameter. This constraint was relaxed for the case of 10 cmc where calculation of the effective Debye length requires knowledge of the degree of dissociation of the micelles.33,34 Native hair surface is well-known to be negatively charged, and the zeta potential of hair has been reported to be around -12 mV at pH 5.5.20 While zeta potential and surface potential are related, the zeta potential is generally lower because it is measured further from the charged surface.43 In addition, the zeta potential value was averaged over the hair surface, which would result in a smaller value than a local measurement at the cuticle edge. At 10 cmc the lower potential reflects the higher electrolyte concentration associated with the counterions and may well also reflect heterogeneity associated with a loss of hydrophobicity due to lipid removal (see discussion of friction behavior below). The zero of separation in the force curves is actually determined by the force17 rather than being an absolute zero of separation as in the SFA.18 Thus, the zero of separation shown in for the case of 1 cmc and 10 cmc almost certainly corresponds to the contact of the palisade layer of surfactants.36 This is confirmed by the disappearance of the adhesion (Figure 4) described below at these concentrations.35-37 The relationship between surface forces and friction is the subject of much study,12,44-48 and it is now well-known that adsorbed surfactant films reduce friction by providing a boundary lubricating layer and that short-ranged, highly repulsive forces can dramatically reduce friction.44,45 The presence of hydrated cations, such as those present in the polar part of the surfactant used here, can also provide a “molecular ball bearinglike” effect.12 It is thus interesting to see how the difference in surface properties indicated by the forces are reflected in the (40) Senden, T. J.; Drummond, C. J. Colloids Surf., A 1995, 94, 29–51. (41) Senden, T. J.; Ducker, W. A. Langmuir 1994, 10, 1003–1004. (42) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283–285. (43) Giesbers, M.; Kleijn, J. M.; Cohen Stuart, M. A. J. Colloid Interface Sci. 2002, 248, 88–95. (44) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J. F.; Jerome, R.; Klein, J. Nature 2003, 425, 163–165. (45) Feiler, A. A.; Bergstroem, L.; Rutland, M. W. Langmuir 2008, 24, 2274– 2276. (46) Yoshizawa, H.; Chen, Y. L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128– 4140. (47) Plunkett, M. A.; Feiler, A.; Rutland, M. W. Langmuir 2003, 19, 4180–4187. (48) Drobek, T.; Spencer, N. D. Langmuir 2007, 24, 1484–1488.
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frictional, or nanotribological, interaction as the hair fiber single contact is loaded and sheared. Figure 4 shows the frictional coefficient as a function of the surfactant concentration. The friction coefficient is extracted from data such as that in the inset to the figure, where the linear relationship between load and friction is clearly seen; the behavior is the same irrespective of whether the load is stepwise increased (loading) or decreased (unloading). A finite friction is observed at zero applied load in the inset due to the presence of a small adhesion in that particular case. It is clear that the presence of surfactant significantly reduces the friction compared to the interaction in water. Above the cmc a plateau is observed in the friction since the chemical potential of adsorbing monomers does not change above this value and the density of the adsorbing surfactant boundary film does not change. What is interesting and unexpected compared to observations in other systems6,49 is that a minimum in the friction occurs below the cmc, at roughly the concentration where the secondary surfactant palisade would be expected to reach its saturation value.35 We speculate that this is probably due to the formation of protein-surfactant aggregates at the lower concentration and possibly depletion of lipid material by surfactant solubilization at higher concentrations, though this remains to be investigated. The directionality of the friction is treated in a separate section below. Effect of Bleaching. Figure 5 shows the forces in aqueous solution between two bleached hair fibers. Bleaching of hair is commonly performed to lighten hair color but also to allow penetration of dyes into the hair fiber, which is otherwise rather resistant to such chemical treatment. A side effect of this treatment is a reduction in the hydrophobicity of the protective outer layer which is provided by a palisade layer of lipid molecules, schematically shown in the inset.1,2,50,51 In this case no force measurements were performed in surfactant solutions so the internal standard for effective radius calculation is lacking. However, the three pairs of hairs used to obtain the results in e.g. Figure 3 returned extremely similar results, so the same value for Reff was used as in Figure 5 (1.0 μm). Assuming that this value is reliable then the double-layer potential obtained from fitting DLVO theory to the data returns a surface potential of -55 mV and surface charge 0.49 μC/cm2 which is larger than for the case of native hair shown in Figure 3. The larger value in the case of the bleached hair is consistent with the exposure of a larger amount of charge bearing protein, ordinarily protected by the lipid layer. The difference between the two cases is also clearly perceived at short range. An attractive force component is manifested in the native hair case, leading to an adhesive force as the fibers are separated of 3.1 ( 1.1 mN/m (Figure 4) which we ascribe to the more hydrophobic nature of the surface. No adhesion is observed in the case of the bleached hairs. Any dispersion force component to this interaction would be expected to be comparable for the two hair types, but since this should also be present in the case of the bleached hair, it is likely that this force component is overwhelmed by the charge effects. (This is also good evidence that the effective radius is of the correct order, since DLVO theory predicts that lower surface potentials than those observed here would be insufficient to mask the dispersion interaction.) Interactions in Air. It is also possible to extract a friction coefficient in ambient air, the value of which is 0.16 ( 0.04 (at a (49) Theander, K.; Pugh, R. J.; Rutland, M. W. J. Colloid Interface Sci. 2005, 291, 361–368. (50) Jones, L. N.; Rivett, D. E. Micron 1997, 28, 469–485. (51) Swift, J. A. J. Cosmet. Sci. 1999, 50, 23–47.
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Figure 6. Long-ranged attractive interaction in air. The effective radius is 1.0 μm obtained as described in text. The solid line fits the functional form of the Coulomb attraction to the data, and the apparent zero separation of the Coulomb interaction is offset by 250 nm to the apparent zero of separation for the force measurement. (This corresponds to about 50% of the cuticle height.) Inset: the short-ranged part of the interaction where the van der Waals interaction dominates the interaction below ∼7 nm, and the surfaces jump into contact due to a spring instability. The solid line in the inset corresponds to the theoretical van der Waals interaction assuming a Hamaker constant of 2 10-20 J.
relative humidity (RH) of ∼30% at the time of the measurements). While the adhesive interactions (which act as an extra applied load) render the absolute value of the friction force quite high in this case, the friction coefficient is much lower than any of the cases in solution. This is because the hairs swell and soften in the water, becoming more deformable and conformable. Force measurement, on the other hand, is rendered more difficult by the formation of static charges, and the curves reflect a long-range attraction characteristic of a Coulombic interaction which is illustrated in Figure 6. This implies that after the initial contact charge transfer occurs from one hair fiber to the other. In this case the force is negative, indicating a uniformly attractive force. As the solid line demonstrates, the form of the force is consistent with a Coulombic interaction. (It is difficult to extract reliable parameters because of the uncertainty in the zero of separation for this force as well as the relevant effective radius.) The force is so long ranged that the cuticle height starts to influence the appearance of the force. The zero of separation in the figure is where physical contact is made between the surfaces; this does not necessarily coincide with the effective zero of the Coulombic interaction. The offset in Figure 6 to the zero of separation of the Coulombic interaction is 250 nm;about half the average cuticle height. The inset to the figure nonetheless shows that at very short separations a steeply attractive van der Waals force takes over from the electrostatic force. While both forces diverge at short separations, the very different “zeroes of separation” mean that they manifest independently of one another. The last part of the force profile is inaccessible due to spring instability (known as a “jump” in the surface force community), and the range of this jump is highly reproducible. Once again the absolute magnitude of the force is dependent on the effective radius but, using that value obtained earlier, leads to a Hamaker constant for the system of 2 10-20 J. This value is slightly lower than that predicted from contact angles52 (52) Molina, R.; Comelles, F.; Julia, M. R.; Erra, P. J. Colloid Interface Sci. 2001, 237, 40–46.
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Figure 7. One of the friction “loops” taken in air at a load of 9 nN. The upper and lower lines display the photodiode response to the friction force measured as the hair probe slides along the cuticle (root to tip) and against cuticle (tip to root), respectively. The prominent peaks reflect mechanical contact of hair probe with cuticle edges and the distance between the peaks is 4.5 μm, which corresponds to a typical distance between cuticle edges. As a result of the crossed cylindrical geometry, no directional effects are observed.
but is of the right order of magnitude. Taking into account that to preserve simplicity, we apply the nonretarded interaction for fitting (which overestimates the interaction for a given Hamaker constant) a slight underestimation would be expected. This indicates once again that the effective radii used are appropriate, at least to describe the classical surface interactions. When two hair fibers interact in vivo, they do so in a much more humid environment which would discharge the static electricity. A study of the effect of humidity is outside the scope of this work but is planned. We note that in extremely dry conditions, where the humidity in the hair itself is also reduced, static electricity is commonly observed, for example during brushing. (In this case of course the interaction is repulsive between the hairs as the charge transfer occurs between the hair and the brush and the hairs have the same charge sign.) Opposite charging in a symmetric system is somewhat unexpected, but in fact this phenomenon is rather well documented for hair, where the heterogeneity of the surfaces renders this possible if the cuticle edges (which tend to have a higher density of polar, ionizable material) are implicated in the contact.5 Directional Effects. Directional effects are an important consideration in hair friction;5,53-55 usually the friction coefficient is larger when a hair is sheared against the cuticle (root to tip) due mechanical interlock with the cuticles. In the results shown here no such cuticle/directional effects are observed, which is a direct result of the experimental geometry and measurement protocols as discussed below. A single friction “loop” is depicted in Figure 7. Such loops are the raw data upon which for example the inset to Figure 4 are based. Each point in that figure is the average of 10 such loops, where the friction force is obtained from the hysteresis in the scanning direction, the calibration of the photodiode, and the torsional spring constant (see for example refs 56 and 17). In air, the measured friction was often larger at the edge of the cuticles due to the “roll” effect caused by the height change as the (53) Bhushan, B.; Wei, G.; Haddad, P. Wear 2005, 259, 1012–1021. (54) Adams, M. J.; Briscoe, B. J.; Wee, T. K. J. Phys. D: Appl. Phys. 1990, 23, 406–414. (55) LaTorre, C.; Bhushan, B. Ultramicroscopy 2006, 106, 720–734. (56) Pettersson, T.; Nordgren, N.; Rutland, M. W.; Feiler, A. Rev. Sci. Instrum. 2007, 78, 093702.
18914 DOI: 10.1021/la103001s
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probe slides over it and which manifested as a prominent peak or spike in the friction loop. In our measurements this peak height and width appeared to be independent of the sliding direction, indicating that, as expected with the crossed fiber configuration, there was no possibility of mechanical interlocking;which would only be experienced by a probe of comparable size or roughness in the sliding direction, such as a finger or an anti/parallel fiber.53,54 (The radius of the hair probe is large relative to the groove between cuticle edges.) The different peak heights reflect the fact that the cuticles are not uniform in thickness. The Supporting Information consists of a sketch which shows how the directional effects are related to the geometry of the probe. When adhesion (or static charging) was large, the peak was not clearly seen since the intrinsic friction became large in comparison to the cuticle effects. Interestingly, the prominent peak was much less observed in water presumably due to the smaller elastic modulus of wet hair than that of dry hair.4 This implies that the surface chemistry of hair, or the adsorbed film, is a more important influence on the friction in water than physical contact of the cuticle edges. To quantify the effect of sliding direction on friction coefficients, it is necessary to vary the angle between probe and substrate and development of such an experimental protocol is currently underway.
Conclusion The results presented above show that it is entirely feasible to measure both the contact and noncontact forces between hair fibers at nanoscale contacts, with high resolution of both the force and separation. The action of well-characterized cationic surfactants, which nonetheless are rather good models for species found in commercial cosmetic products, clearly alter the range and magnitude of these forces in a manner consistent with earlier studies of adsorption on smooth model surfaces. A “correct” balance of such forces clearly must determine the look and feel of hair, or indeed any multifiber system, and a quantitative understanding is essential for modeling. Such quantitative information is now shown to be accessible. Despite the relatively complex chemical structure of hair and the nonideal nature of such natural biofiber surfaces for surface force measurements (usually molecularly smooth surfaces such as mica are preferred), the novel fiber-fiber probe technique shows the interactions to depend on the following classical forces: van der Waals and PoissonBoltzmann in liquid and Coulombic and van der Waals attraction in air. The forces can be quantitatively understood from internal referencing against interactions of known potential. Somewhat surprisingly, the fibers demonstrate a classical Amontons behavior in their nanotribological behavior with linear dependence between force and load, and surfactant adsorption mediates their frictional response via a classical boundary lubrication mechanism. The crossed cylinder geometry precludes observation of any directional effects on the frictional response, and our current research encompasses experiments in which the fiber angle is changed. However, the much smaller mechanical effects associated with sliding over the cuticle edges permits the conclusion to be drawn that directional effects should be of much lower importance in the wet state. Acknowledgment. M.W.R. is a fellow of the Swedish Research Council (VR). The AFM was funded by a grant from the Knut and Alice Wallenberg Foundation. VR is acknowledged for partial financial support of H.M. David Haviland and Anders Liljeborg are thanked for access to and assistance with the FIB. F. Leroy (L’Oreal) is thanked for his support. M.W.R. is a Langmuir 2010, 26(24), 18909–18915
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cofounder of the SSF financed Swedish Centre for Biomimetic Fibre Engineering, Biomime. Supporting Information Available: Effect of cuticle edges on friction for a smooth surface (probe reduced in scale
Langmuir 2010, 26(24), 18909–18915
Article
compared to surface), cuticle causes roll which is directionally independent (Figure A); effect of cuticle edges on friction for a rough surface;directional dependence due to mechanical interlock (Figure B). This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la103001s
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