Accepted Abstracts


Force-Free Measurements of Friction: Dielectric, X-ray Scattering, and Optical Studies of Molecular Tribology

Steve Granick

In attempts to obtain direct molecular information boundary layer films by other methods mentioned in the title of this abstract. In findings obtained at the time this abstract was written, dielectric measurements show that the glass transition temperature is enhanced relative to the bulk and that molecular mobility above the glass transition is retarded relative to the bulk. Furthermore, step-scan infrared spectroscopy shows the shear-induced orientation of various functional groups of confined molecules as they are sheared at various shear rates.



What does a cricket tell us about tribology?

M. Scherge

Microtribology Group, Institut fuer Physik, TU Ilmenau, PF 100565, D-98684 Ilmenau, Germany


S. N. Gorb

Biological Microtribology Group, Max-Planck-Institut fuer Entwicklungsbiologie,

P.O. Box 21 09, D-72011 Tuebingen, Germany

We present a study on friction, adhesion and indentation of attachment pads of the great green bush cricket, using a system of bio-substrate versus a flat surface. The pads are evolutionary optimized frictional devices which are adapted to attach the insect to a variety of natural surfaces. The attachment pads of both living and dead insects were tested and evaluated using several analysis techniques. These techniques included friction and adhesion analysis by microtribometry, microindentation, scanning electron microscopy (SEM), histologic sectioning, and shock freezing. Microtribological analysis showed that, the friction between pad and surface is anisotropic. The anisotropy increases with increasing normal force. Microindentation revealed that the pads respond very flexible to applied loads leading to an optimized morphology with respect to contact area. The adhesion tests provided useful information on monolayer lubrication phenomena. The pads are covered by a hydro-carbon layer, rendering the surface hydrophobic. Thus capillary action can be ruled out as origin of adhesion. It is assumed that van der Waals (dispersion) forces and the perfect ability to achieve an intimate contact between pad and surface are responsible for the optimized adhesion of the insect. All results make clear that the microtribological properties are a perfect combination of surface structure, lubrication and active interface control.



Comparison between modulated lateral force microscopy (MLFM) and magnetic force modulation microscopy (MFMM) for elastic property characterization of surfaces: advantages, drawbacks and limits.

O. Piétrement, J.L. Beaudoin and M. Troyon

Université de Reims, Unité de Thermique et Analyse Physique, EA 2061. 21, rue Clément Ader, 51685 Reims Cédex 2, France.

SFM has proved to be an ideal tool for nanomechanical investigations. Different kinds of modulation techniques in the direction perpendicular to the sample have been suggested in order to image the surface elastic properties. However, it has been underlined [1] that, in most cases, the contrast of the elasticity images is mainly due to the friction in the direction parallel to the cantilever beam and not the result of the indentation, because the normal contact stiffness is generally two orders of magnitude higher than the flexion cantilever stiffness. Two methods, MFMM [2] and MLFM [3], should allow the quantitative determination of the elastic properties without being hindered by the above problem. The first one under the condition that the magnetic force is well applied at the tip apex, that is, by fixing and centering a small magnetic particle just above the tip, and the second one because lateral contact stiffness and torsion cantilever stiffness are of the same order of magnitude.

We have used and compared these two methods on three different samples presenting a large range of elastic moduli: carbon fibers embedded in an epoxy matrix, a polyethylene film deposited on silicon and mica substrates. The measurements are given in the table below and compared to the bulk sample values. Concerning the MLFM, we have used a new calibration procedure based on the direct estimation of the lateral contact stiffness versus applied load and on the adjustment of the elasticity contact theories to the experimental data [4].

Samples Carbon fibers Epoxy Mica Polyethylene Silicon

Bulk value (GPa) 200-240 5 176 0.75-1 160

MLFM values(GPa) 140 ± 45 17 ± 6 117 ± 35 4 ± 2 106 ± 33

MFMM values(GPa) Not Sensitive (NS) under study NS 1.1 ± 0.4 NS

Advantages: the main advantage of MLFM comes from the use of the lock-in technique and from the fact that our calibration procedure does not need to know the lateral displacement for estimating the elastic modulus, unlike the other methods. Concerning the MFMM, the indentation being normal to the surface, the elastic modulus is directly deduced from the measurement of the indentation depth and the elasticity contact theories.

Drawbacks: It is very difficult in MFMM to fix very precisely the small magnet just above the tip apex, what is conditioning the quality of the measurements.

Limits: results (see table) seem to indicate that MLFM gives enough good measurements when the samples are stiff, whereas it is not the case for compliant ones, and it is the opposite for MFMM. In fact, in MLFM a precise characterization of the tip-sample contact area is essential for a correct estimation of the elastic modulus. We have experimentally noticed it is easier to characterize the contact area on a stiff surface (silicon, mica, carbon fibers) than on a compliant surface (polyethylene, epoxy). MFMM is more appropriate for soft sample because this technique is only sensitive to the indentation depth of the tip in the material but, for stiff samples, the indentation depth is so small that measurements become impossible; the contrast in the images is then almost entirely disappearing.

[1] P.E. Mazeran and J.L. Loubet, Tribology Lett. 3 (1997) 125.

[2] E.L. Florin, M. Radmacher, B. Fleck and H.E. Gaub, Rev. Sci. Instrum. 65 (1994) 639.

[3] T. Göddenheinrich, S. MŸller and C. Heiden, Rev. Sci. Instrum. 65 (1994) 2870.

[4] O. Piétrement, J.L. Beaudoin and M. Troyon, to be published in Tribology Lett.



Aspects of molecular structure and dynamics on polymer surfaces near Tg .

M. Heuberger1), G. Luengo2), J. Israelachvili3)

1) Department of Materials, ETH, CH-8092 ZŸrich, Switzerland.

2) Dept. Applied Physics, L'OREAL Research, 92583 Clichy Cedex, France.

3) Dept. Chem. Eng. / MRL, UCSB, Santa Barbara, CA 93106, USA.

We have investigated the tribological properties of an asymmetric system using a Surface Forces Apparatus modified for reciprocating-friction testing. The sliding partners were mica and a 300nm thick film of Poly-n-buthylmetacrylate (PBMA). We compare results obtained in the two sister systems where one of the materials is sliding on a track of the other material and viceversa.

In both complementary systems we observe shear-induced effects during the first 3-6 sliding cycles, which in turn have a strong effect on the temporal development of the kinetic friction as well as the transition to steady-state sliding and intermittent friction.

The dynamic nature of the system is discussed in terms of the known molecular structure of PBMA. Different relaxation mechanisms are evidenced and attributed to side-chain and backbone relaxations.

An important conclusion of these measurements is that the number of cycles and the temperature rather than other quantities like load, velocity, time, sliding distance or confinement govern the transition to steady-state sliding.

Our results indicate possible molecular mechanisms that explain why polymer surfaces tend to wear faster in a multidirectional sliding environment.



Molecular tribology of highly ordered monolayers

D. Gourdon

Institut de Genie Atomique-Departement de Physique,


CH-1015 Lausanne Swotzerland

In order to investigate friction at a fundamental level, atomic force microscopy (AFM) in the wearless regime was performed on a model system Ñ a highly ordered thiolipid monolayer on mica. In the monolayer, condensed domains with long-range orientational order were present. These domains revealed strong friction anisotropies. The directionality of this effect appeared to correlate well with the tilt direction of the molecules in the monolayer, as determined by two independent techniques (electron diffraction and Brewster angle microscopy). The molecular tilt responsible for the strong frictional effects was surprisingly small, less than 15¡ demonstrating that even small tilts can make a major contribution to friction at the molecular scale. In order to better elucidate the mechanical and chemical contributions to friction, on one hand AFM tips of various size, shape and chemical nature were used; on the other hand, monolayers formed by different molecules were probed. These experiments generally indicated that the observed friction anisotropies mostly persisted, whichever tip was used. In order to understand the molecular events responsible for the dissipation, friction was measured on our model system as a function of applied load and sliding velocity. The measurements versus load revealed two or three different frictional regimes (depending on the load range), that correlated well with a systematic stepwise behavior of the height of the domain as measured simultaneously. These discrete effects, in addition to the measured directionality of friction (related to tilt direction) were attributed to the creation of molecular gauche defects induced by the scanning tip. The measurements versus velocity also showed different frictional regimes. At high velocity, friction was found to be independent of velocity (similar to dry friction). However, at low velocity, friction essentially showed a viscous behavior. It was finally demonstrated that the viscosity of the monolayer exhibits an anisotropy, which is closely related to the relative orientation of the molecules with respect to the directionality of the stress applied by the tip in the monolayer.



Friction dynamics of mesoscopic single asperities

W.P.Vellinga and C.P.Hendriks

Materials Technology, Mechanical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB, Eindhoven, the Netherlands

Experiment and theory analysing dynamic friction behaviour have made significant contributions to the understanding of friction. It is now widely realised that drastically different types of dynamic behaviour are intrinsic to sliding systems. In fact this complicated behaviour may offer clues for better characterisation and eventually better understanding of sliding friction systems. Experimental studies of dynamic behaviour, eg. of time dependent ÔstictionÕ and transitions from steady sliding to Ôstick-slipÕ, have uncovered remarkable parallels between widely different materials at widely different time- and lengthscales. On the other hand so called "rate-and-state" have been quite succesfull in reproducing observed behaviour. For the type of sliding studied in this paper, often called ÔdryÕ friction, the state variable is usually loosely associated with the contact area between slider and substrate, evolving by some creep process. Although adequate, rate-and-state models usually contain some parameters that have no definite physical interpretation. Providing these descriptions with explicit physical background is of fundamental interest. It is also of practical importance, because it is clearly linked to possible predictive use. Most of the experiments in "dry" dynamic friction have been performed on multiasperity contacts. The load per asperity in many of these cases is estimated to be around 10mN with typical contact areas of the asperities in the order of 10 to 100 mm2. Quantitative experimental results on the sliding of single asperities at this scale, often referred to as the Ômesoscopic scaleÕ, may be expected to lead to new insights.

We have studied the dynamic behaviour of such single mesoscopic asperities sliding on, or ploughing through, several viscoelastic and viscoplastic polyester surfaces. The measurements were performed with a versatile home-built device, the Lateral Force Apparatus, LFA, that will be briefly introduced.

In short we are able to measure independently normal and lateral forces between 400 nN and 150 mN, for tips with radii from 100 nm to 100 µm. Driving speeds are between 1 and 40 µm/s.

We have observed dynamic behaviour that is in many aspects analogous to that of systems described by rate-and-state models reported in literature. For example we find transitions from steady sliding to stick-slip for decreasing driving speed, decreasing driving spring stifness, and increasing normal force. Peculiar to the system are similar transitions, for decreasing cross-link density, and decreasing asperity radius. Moreover we find that stick-slip, and changes in asperity speed in general are always accompanied by normal displacements. In a simple picture we assume that some of the material displaced by the tip passes underneath it, leading to a coupling of asperity speed and strain rate normal to the interface. We are investigating systems of nonlinear differential equations that incorporate this coupling and that allow critical comparison to experiment. Preliminary results indicate that the assumption that the material behaves in a simple viscoplastic manner reproduces qualitatively, and to some extent quantitatively the experimental record.



Recent Progress of Microtribology in Japan

Reizo Kaneko

A recearch committee for microtribology was organized with the Japan Society of Tribologists in 1990. In 1996, a national project for microtrbology was started in Japan. Today, microtribology is advancing in a wide range of fields, such as analysis and simulation with moleculer dynamics, evaluations and enhancements of solid surfaces, tribochemical processes, tribological properties of bio-mechanisms, and applications. Recent works on microtribology in Japan will be introduced as a review.



Deducing Energy Dissipation From Rheological Response.

M. Urbakh, M. Porto, V. Zaloj, and J. Klafter

School of chemistry, Tel Aviv University, Tel Aviv 69978, Israel

We investigate the response of confined molecular systems to a harmonic drive. A model is introduced which mimics recent measurements on friction using surface force apparatus. The model predicts a critical driving amplitude below which the response is linear. For higher amplitudes the system exhibits a nonlinear behavior and shear thinning. We propose a method for analyzing the nonlinear response regime which is base on calculating the energy dissipation in the system. This approach provides a way to deduce the microscopic parameters which are responsible for the dissipation. A novel origin of thinning is proposed which stems from energy dissipation due to stick-slip motion and the transition to smooth sliding. We establish relationships between the microscopic parameters of the system and phenomena observed in rheology and tribology.

Special attention is paid to a response in the normal direction induced by a lateral drive. Results to date show already that the transition, from stick to slip, is accompanied by dilation of the film thickness. The excitation of normal oscillations by the lateral drive becomes efficient only when the amplitude of the lateral oscillations are large enough to feel a nonlinearity of the liquid-plate potential. As a result, the dilatancy and additional energy dissipation due to normal motion occurs for driving velocities which exceed some critical value which depends on the load. The existence of an additional channel of dissipation should manifest itself as shear thickening, where an effective viscosity increases with increasing velocity.

Presenting Author:

Prof. Michael Urbakh,

School of Chemistry, Tel-Aviv University,

Ramat-Aviv, 69978 Tel-Aviv, Israel;

Tel: 972-3-640-8324, Fax: 972-3-640-9293;




Atomic Scale Friction: From Basic Characteristics to Control.

J. Klafter, M.G. Rozman, V. Zaloj and M. Urbakh

School of chemistry, Tel Aviv University, Tel Aviv 69978, Israel

The basic characteristics of molecular systems under shear are briefly reviewed and methods to control friction are proposed. Controlling frictional forces has been traditionally approached by chemical means, namely by using lubricating liquids. A different approach, discussed here, is by controlling the system mechanically. Our goal is twofold: (a) to achieve smooth sliding at low driving velocities, which otherwise correspond to the stick-slip regime, (b) to modify the frictional forces. Two different methods of control are proposed here using mechanical means: the first one uses a feedback control method, and the second one relies on a "brute-force'' modification of the system dynamics mechanically, but without a feedback. The possibilities to control friction are demonstrated using a model system which displays the main experimentally observed behaviors, obtained in measurements on nanoscale confined liquids and granular layers. The methods should be applicable to real systems for which time series of dynamical variables are experimentally available. The only necessary condition is the existence of (possibly unstable) sliding regimes of motion in the experimental systems.

Presenting Author:

Prof. Joseph Klafter,

School of Chemistry, Tel-Aviv University,

Ramat-Aviv, 69978 Tel-Aviv, Israel;

Tel: 972-3-640-8254, Fax: 972-3-640-9293;




Length scale effects in the scratching of poly(methyl methacrylate); transitions from nano to macroscopic deformations

M.J. Adams1, B.J. Briscoe2, D.M. Gorman1 and S.A. Johnson1

1 Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K.

2 Department of Chemical Engineering, Imperial College, London, SW7 2BY, U.K.

The paper describes a series of experimental studies that have examined the scratch deformation processes occurring in poly(methyl methacrylate) which is a typical amorphous polymer. The emphasis will be upon the rather peculiar influences on the data that were produced by varying the length scale of the deformation. Various data are described using nano-, micro- and macro-probes in order to obtain length scales ranging from the nanometer into the millimeter regime. The data are provided as deformation maps, scratch profiles and frictional responses. The indenters used to induce the deformation were chosen to be geometrically self-similar; essentially cones and triangular based pyramids. A range of loads and sliding velocities were investigated for the different indenter geometries. Selected data are also included for lubricated systems.

Comparisons are made with normal indentation data obtained for a similar range of length scales and loading rates. However, the focus of the paper and the analysis is upon the marked influences on the deformation processes that are induced as the sizes of the deformations are changed. This is exemplified in a variety of ways but primarily by the large elastic recovery compared with normal indentation and the apparent decrease in the scratch hardness with increasing depth of the penetration. First order analyses would suggest that the hardness should not be a marked function of the scale of penetration and the paper seeks to provide a rationale for this effect. It is argued that the nature of the interface friction, or stress boundary condition, between the indenter and the deforming substrate is significantly influenced by the absolute scale of the deformation. In particular, a key factor is the relative extent of stick and slip at this interface. There are precedents that may be identified in related areas such as fretting and metal working to justify this proposition.

An apparent decrease in the normal hardness of materials with increasing depth is well established. However, this is limited generally to depths of less than 100 nm rather than the larger range of length scales that have been observed in the current work for the scratch hardness of poly(methyl methacrylate). The decrease in the normal hardness is attributed to uncertainties in the tip defect and also a real difference in the mechanical properties at the surface probably due to the manufacturing process. A transition from plastic deformation to brittle fracture with increasing depth is also a common feature of the normal indentation characteristics of glassy polymers. The interpretation of such length-scale transitions in the failure of brittle materials is a well-established phenomenon. Friction at the material-indenter interface plays an important role in the transitions observed for more complex mechanical responses observed in scratching.



Molecular Dynamics Study on Possible Designing Mesoscopic Materials of Desired Wearless-Frictional Characteristics by Phonon-Band Engineering

Keiji HAYASHI a, Akifumi MAEDA a, Tomohisa TERAYAMA a, Kazuo HARAOKA b, Toshio KAWAI c, and Noriyuki SAKUDO a

a Advanced Materials Science R&D Center, Kanazawa Institute of Technology,

3-1, Yatsukaho, Matto, Ishikawa 924-0838, JAPAN

b Study Group for Friction,

1-24-8-201, Nishigahara, Kita-ku, Tokyo 114-0024, JAPAN

c Chitose Institute of Science and Technology,

758-65, Bibi, Chitose, Hokkaido 066-8655, JAPAN

In improving frictional characteristics of artificial materials for sub-micrometer size mechanisms and actuators, a better atomistic understanding of wearless sliding friction is indispensable. The goal of our investigation is to clarify law of wearless friction in sub-micrometer size systems based on study from the atomistic point of view, and to apply the law to material design as a guiding principle. We have been studying wearless-frictional resistance accompanying steady sliding motion between mesoscopic lattices by means of molecular dynamics (MD) simulations using atomistic simplified two- and one- dimensional models [1-3]. Systematic exploration of universal features lying in the friction-like phenomena revealed unique dependence of the frictional force on sliding velocity, applied load, and local temperature distribution.

In the present study how the dependence reflects phonon spectra of the lattices was examined using modified two- and one- dimensional MD models where particles belonging to some layers parallel to the interface were replaced with their isotopes. This paper reports relationship between change in the dependence due to the isotope substitution and the distance from the interface to the isotope layer. Furthermore, a possible atomistic origin of the dependence is discussed in terms of "size effect" on the basis of a hypothesis that the frictional power corresponds to rate of energy dissipation via forced vibration and subsequent energy transfer between normal phonon modes due to anharmonicity of inter-particle potential.

[1] K. Hayashi, et al., Computational Materials Science, to be published.

[2] K. Hayashi, et al., Surface and Coatings Technology 83 (1996) 313.

[3] K. Hayashi, et al., Progress of Theoretical Physics, to be published.



Numerical and Theoretical Study of friction between atomically clean

surfaces and the effects of impurities

H. Matsukawa and T.Kawaguchi^

Dep. of Phys., Osaka University, Toyonaka, Osaka, Japan

^Dep. of Technology, Fac. of Education, Shimane University, Matsue, Japan

We study sliding friction between atomically clean surfaces with and without impurities numerically and theoretically. The vanishing static-frictional-force state and its stability against impurities are investigated for the case of incommensurate surfaces. The maximum static frictional force shows fractal behavior as a function of the elastic constant of the lattice. The velocity dependences of the kinetic frictional force for both cases of the absence and presence of impurities are calculated. The results are discussed in relation with the change of the lattice structure. The transverse pinning, which resist the transverse motion in the sliding state, is also discussed.


[1] Matsukawa H. and Fukuyama H., Physical Review B49 (1994) 17286.

[2] Matsukawa H. and Fukuyama H., Physics of Sliding Friction,

B.N.J.Persson and E.Tosatti(eds.) , 139-148, Kluwer.

[3] Kawaguchi T. and Matsukawa H., Physical Review B56 (1997) 4261.

[4] Kawaguchi T.and Matsukawa H., Physical Review B56 (1997)13932.

[5] Kawaguchi T. and Matsukawa H., Physical Review B58 (1998)15877.

[6] Kawaguchi T. and Matsukawa H., submitted to Physical Review B.



Noncontact Dissipation Force Microscopy

M. Gauthier and M. Tsukuda

Dept. of Physics, Graduate School of Science, University of Tokyo

True atomic resolution of surfaces can now be routinely achieved using dynamic force microscopy by measuring the cantilever resonant frequency shift. More recently, it has also been realized that the damping of the cantilever motion could also be useful for analyzing surfaces. Most of such experiments have been conducted in the so-called tapping mode where the tip interacts relatively strongly with the surface. In this mode, adhesion hysteresis between the tip and the surface have been generally believed to be responsible for dissipating energy. However in the noncontact mode, such hysteresis are likely to become negligible since the local surface relaxation time is considerably shorter than the time spent by the tip in the vicinity of the surface. On the other hand, to infer that elastic surface deformations in the noncontact mode may reveal atomic information also seems questionable. Surface deformations would probably involve a number of surface atoms and in this case, local dissipation measurements may reflect some local average surface property. In the present work, we have demonstrated the existence of an intrinsically local dissipation mechanism between the tip of the noncontact dynamic force microscope and a surface by using the Langevin equation approach. This mechanism is fundamentally different from that related to adhesion hysteresis. A useful expression for the friction caused by the tip-surface interaction is obtained, which can be seen to be a particular case of the fluctuation-dissipation theorem. In the first place, a scheme referred to as noncontact dissipation force microscopy is suggested for producing atomically resolved images reflecting local surface nanoscale mechanics. Moreover, the results have raised new prospects, for example, the possibility of detecting nanoscale structures embedded beneath the surface, or studying nanomechanical properties of larger molecules adsorbed on the surface.



Applications of the Nano-Scratch technique for characterisation of ultra-thin films

N. X. Randall and R. Consiglio

CSEM Instruments, Jaquet-Droz 1, CH-2007 Neuch‰tel, Switzerland

As the thickness of functional coatings continually decreases to satisfy the structural and protective needs of modern day applications, quantitative instrumentation has become a necessity for adequate evaluation of material properties, particularly scratch resistance and adhesion at the film-substrate interface. The Nano Scratch Tester (NST) is a new instrument overcoming the limitations of both the classical stylus scratch test (normal force range) and the scanning force microscope (SFM) technique (short sliding distances), allowing scratch lengths of up to 10 mm. Tangential force and penetration depth are simultaneously measured during the scratching process, both in a multipass contact fatigue mode. For high resolution inspection of the deformed or damaged area, a scanning force microscope (SFM) is integrated into the system. Experimental results are presented for a range of applications concerning ultra-thin films, as well as curved sample geometries and the use of scratch mapping. The results indicate very good reproducibility and confirm the application of this new instrument for the accurate characterisation of elasticity, hardness, adhesion, friction and mechanical integrity in coated systems where the film thickness is less than 1 µm.



Molecular Dynamics Study of Wearless Friction using Simplified One-Dimensional Model

Kazuo Haraoka(Study Group for Frcition,Japan) Keiji Hayashi(Kanazawa Institute of Technology,Japan) Koichiro Shida(Musashi Institute of Technology,Japan) Kiyoshi Tsutsui(Keio University,Japan) Toshio Kawai(Chitose Inst. of Sci. and Tech.,Japan)

Contact person: Kazuo Haraoka

Rn201, 1-24-8-201,Nishi-ga-Hara,Kitaku Tokyo,114-0024,Japan

In improving frictional characteristics of artificial materials for nano-meter size mechanisms and actuators, a better atomistic understanding of wearless sliding friction is indispensable. Although there have been a number of studies on dynamic friction from the atomistic point of view in terms of theoretical or computational physics,the obtained knowledge is still fragmentary and thus is quite insufficient to apply to design of artificial materials for nano-scale machines. In particular, in spite of pioneering works which pointed out that frictional characteristic of a mesoscopic lattice depends on its thickness manner of the dependence and its underlying atomistic mechanismshave not been elucidated yet. We have been systematically studying law of wearless friction accompanying steady sliding motion between mesoscopic lattices by means of molecular dynamics (MD) simulations using atomistic simplified two- [1] and one- [2,3] dimensional models. Exploration of universal features lying in the friction-like phenomena revealed unique dependence of the frictional force on sliding velocity, applied load, and local temperature distribution [4]. Furthermore, how the dependence reflects thickness and phonon spectra of the lattices was examined using a modified two-dimensional MD model where particles belonging to some layers parallel to the interface were replaced with their isotopes [5]. This paper presents a complementary study, where we discuss an atomistic origin of the law of friction based on a simplified one-dimensional lattice model perpendicular to the interface.

[1] K. Hayashi, et al., Surface and Coatings Technology 83 (1996) 313.

[2] K. Haraoka, et al., Proc. the 31st Symposium on Celestial Mechanics, Japan, pp. 93-98 (1999).

[3] K.Haraoka et al., Proc. the 5th Int. Conf. on Computational Physics, submitted.

[4] K. Hayashi, et al., Proc. the 9th Int. Conf. on Production Engineering, pp1001-1006

[5] K.Haraoka et al., Proc. the 5th Int. Conf. on Computational Physics, submitted.



A general equation for indentation and normal contact stiffness versus normal load: application to the estimation of elastic modulus by Magnetic Force Modulation Microscopy (MFMM).

O. Piétrement, J.L. Beaudoin and M. Troyon

Université de Reims, Unité de Thermique et Analyse Physique, EA 2061. 21, rue Clément Ader, 51685 Reims Cedex 2, France.

Several continuum mechanics models have been developed which are able to predict the contact area between two elastic bodies in the presence of adhesion. JKR [1] and DMT [2] models give relatively simple analytical expressions between contact area and applied load or between indentation depth and applied load. These two models constitute in fact two limiting cases. JKR model describes well the case of compliant materials with strong adhesion, whereas DMT model applies well in the case of stiff materials and weak adhesion. But, the general intermediate case requires a more complex analysis. An analysis of this regime providing also an analytic solution has been proposed by Maugis [3], but the resulting equations are cumbersome if one wishes to compare with experimental data such as AFM measurements. Very recently, Carpick et al. [4] proposed a simpler general equation for fitting contact area versus load measurements that approximates Maugis' solution very closely. At the present time, direct relationships between indentation depth and normal contact stiffness versus load have never been presented in the case of intermediate regime. The aim of our work is thus to propose, in a similar way as Carpick et al. for the contact radius, two general equations, one between indentation depth d and normal applied load Fn and the other for the normal contact stiffness versus load that approximate Maugis' solution very closely [5]. The equation giving the normalized indentation depth is:

, with ,

where , , S(a) and b(a) are four nondimensional parameters depending on a fifth parameter a, which are determined by fit on the Maugis' solution. Since the normal contact stiffness kN is defined as the tip-surface force gradient, comes also a generalized equation between kN and applied load given by:


MFMM [6] is well adapted for using this general equation because the variations of indentation depth can be measured and then kN deduced versus load. By conventional curve fitting software routines, it is easy to determine the values of a, a02/R, Fad, S, b and then to deduce the elastic modulus of the material if the tip radius is known. Details of this work, empirical equations we propose, as well as the experimental protocol for the estimation of the elastic modulus will be presented.

[1] K.L. Jonhson, K. Kendall and A.D. Roberts, Proc. R. Soc. London A 324, 301 (1971).

[2] B.V. Derjaguin, V.M. Muller and Y.P. Toporov, J. Coll. Interface. Sci. 53, 314 (1975).

[3] D. Maugis, J. Coll. Interface Sci. 150, 243 (1992).

[4] R.W. Carpick, D.F. Ogletree and M. Salmeron, J. Coll. Interface Sci. 211, 395 (1999).

[5] O. Piétrement and M. Troyon, submitted to J. Coll. Interface Sci.

[6] E.L. Florin, M. Radmacher, B. Fleck and H.E. Gaub, Rev. Sci. Instrum. 65 (1994) 639.



Theoretical Simulations of Force Microscopy Based on First Principles DF theory in Nanotribology -- Application toSi(111) Ã3xÃ3 Ag Surface

Naruo Sasaki and Masaru Tsukada

{Department of Physics, Graduate School of Science, University of Tokyo,

7--3--1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

{Phone. +81-3-5814-4223}

{Fax. +81-3-5814-7587}


Scanning probe microscopy (SPM) such as Noncontact Atomic-Force Microscopy (NC-AFM) and Frictional-Force Microscopy (FFM) are powerful tools in nanotribology. Physics of SPM is unique, because macroscopic cantilever detects the microscopic tip-surface interaction force. This means SPM indicates a crossover between micro- and macroscopic physics, in other words, nano- and conventional tribological fields.

In this talk, we propose an efficient method to describe the SPM system for combining the macrocopic cantilever dynamics and the microscopic tip-surface interaction force~[1--3]. The tip-surface interaction force is calculated quantitatively based upon first-principles density functional (DFT-LDA) calculations. As an example interaction force between Si tip and Si(111) Ã3xÃ3--Ag substrate surface is discussed~[3]. Based on the three-dimensional distribution of the force, frequency shifts and AFM images are calculated and compared with experiments~[4,5]. Adhesive feature of the tip-surface interaction with energy dissipation is discussed. We would like to mention the first step of study about energy dissipation problem based on first principle DF calculations. Method of theoretical interpretations of FFM of graphite surface based on pure classical calculations, and whether the classical theory is enough or not is also discussed~[6--8].

[1] N. Sasaki and M. Tsukada, Jpn. J. Appl. Phys. {\bf 37}, L533 (1998).

[2] N. Sasaki and M. Tsukada, Appl. Surf. Sci.~{\bf 140}, 339 (1999).

[3] N. Sasaki, H. Aizawa and M. Tsukada, Appl. Surf. Sci., in press.

[4] Y. Sugawara, T. Minobe, S. Orisaka, T. Uchihashi, T. Tsukamoto, and S. Morita, Surf. Interface Anal.~{\bf 27}, 456 (1999).

[5] K. Yokoyama, T. Ochi, Y. Sugawara, and S. Morita, Phys. Rev. Lett., in press.

[6] N. Sasaki, K. Kobayashi and M. Tsukada, Phys. Rev. B~{\bf 54}, 2138 (1996).

[7] N. Sasaki, M. Tsukada, S. Fujisawa, Y. Sugawara, and S. Morita, Tribol. Lett.~{\bf 4}, 125 (1998).

[8] N. Sasaki, M. Tsukada, S. Fujisawa, Y. Sugawara, S. Morita and K. Kobayashi, Phys. Rev. B~{\bf 57}, 3785 (1998).



Mechanisms of nano particles detachment by stick-slip within a macrotribocontact

Marcel Brendle, P. Diss, P. Stempflé

Institut de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, BP 2488

F68057 Mulhouse Cedex

This paper discusses the mechanisms by which nano particles (20 to 300 nm) are detached within a macrocontact and which are the parameters controlling the size of the elemental particles and the rate of their detachment. The nanoscale considered here is apparently related to the size of the areas of actual contact but probably also to the microstructure of the material itself. The acting mechanisms is that of microstick-slip, where the stiffness of the contact is rather imposed by the material than by the tribometer. The model of contact is therefore of the type Burridge Knopoff. However, while this model is generally used for pure stick-slip phenomena without transfer, in our case we have also to consider the occurence of particle detachment.

This work is based on experimental data acquired with some particular systems (graphitic or polymeric material sliding against thoroughly polished steel discs), where the elemental debris reagglomerate to form a 3rd body distributed as isolated particles (displaying typically a size from 0.1 to 100 µm). On initially smooth flat surfaces, they form asperities easily quantifiable either by image analysis for their 2D characteristics or by 3D optical profilometry for their 3D characteristics. Owing to this peculiar volume distribution into a few number of isolated islands, it becomes possible to assess rates of particle detachment dhe/dn as small as 0.1 nanometer pro unit area and pro cycle.

These systems display another interesting feature : the rate of particle detachment decreases with increasing sliding speed like :

dhe/dn = a log v + b

in a similar way as the coefficient of friction :

µ = aÕ log v + bÕ

While a logarithmic decrease of µ is generally ascribed to the existence of stick-slip phenomena, a logarithmic decrease of the rate of particle detachment was never described before.

Therefore, a new model of stick-slip accounting for a cohesive rupture will be proposed and its validity experimentally checked, at least for the qualitative aspects related to the existence of a critical sliding speed above which no particle may be detached. Referring to this model, the rate of elemental particle detachment is normally related to the frequency of stick-slip (i.e. the duration of a stick), the size of elemental particles and the number of microsticks, all entities which vary with environment and are not necessarily independant.

An independant determination of the the size of the elemental particles using AFM was therefore achieved and the influence of main experimental parameters (i.e. rubbing time, relative humidity, temperature, sliding speed etc...) systematically investigated.

The results will be discussed.



Atomic-scale friction of self-assembled monolayers

in scanning force microscopy

Y. S. Leng and S. Jiang

Department of Chemical Engineering

University of Washington

Seattle, WA 98195-1750, USA

When modeling atomic-scale friction in the atomic force microscope/friction force microscope (AFM/FFM), it is often assumed that the AFM tip, modeled by a single atom connected to a dynamic cantilever, interacts with a substrate having regular corrugation potential. Usually, the magnitude of this corrugated surface potential is strongly influenced by normal force. If the spring constants of the cantilever are less than some critical values, stick-slip will occur, resulting in net friction or energy dissipation (Tomanek et al. 1991). Two algorithms are commonly used to simulate friction processes at the atomic scale: (1) energy minimization approach (Sasaki et al. 1997) based on the conjugate gradient energy minimization, usually the Polak-Ribiere algorithm to find the nearest local minimum; (2) Ôdynamic elementÕ method based on the tip equations of motion (Holsher et al. 1997; Johnson et al. 1998). Both can effectively interpret stick-slip phenomena and AFM force images. However, the former approach can not yield any information about the dynamic evolution of the system as time proceeds. Furthermore, the thermal effect (temperature) can not be involved. On the other hand, the second method assumes that the surface potential is fixed. Such an assumption is valid only to the first-order approximation for solid surface potentials with regular corrugation. For self-assembled monolayers (SAMs) of our interests, where energy dissipation is in ÔpluckingÕ or continuous ÔviscousÕ modes (Glosli et al. 1993), the surface potential will evolve. The gauche defects near the contact region will dramatically disturb the corrugation of the surface potential. As the former approach, the thermal effects can not be involved.

Molecular dynamics (MD) simulations with parameterized atomic potentials open a new way to investigate atomic-scale friction. However, conventional MD encountered time-scale and length-scale problems: the simulation time is only in the order of nanoseconds, while the number of atoms involved in the simulation is far less than that in AFM experiments. To catch up with the high frequency motion of atoms, the scanning velocity of the AFM tip has to be high in MD simulations, usually many orders of magnitude larger than that in AFM experiments (Glosli et al. 1993; Bonner et al. 1997). The friction behavior in MD simulations is quite different from that in AFM experiments.

We propose a new simulation approach combining MD simulation method and dynamic AFM tip equations of motion based on continuum mechanics algorithm, making simulation time scale overlap with that in AFM experiments. In our tip-based simulations, an AFM tip scans over thin films of alkanethiol monolayers on Au (111) surfaces. The tip is connected to a rigid holder through three orthogonal springs, which represent the elastic stiffness of the AFM/FFM cantilever. Our results reveal that (1) elastic deformation of SAM contributes largely to energy dissipation, making the forces exerted on the AFM tip non-conservative; (2) conventional MD simulation depicts a high friction loop due to the high scanning velocity, resulting in a high viscous damping friction; and (3) friction decreases as chain length of SAMs increases, consistent with our recent quantitative AFM/FFM measurements of frictional properties of SAMs (Li et al. 1999).



Relation between Structure of Boundary Layers and their Microtribological Properties
Shigeyuki Mori, Takashi Igari and Hidetaka Nanao
Faculty of Engineering, Iwate University
4-3-5 Ueda, Morioka 020-8551 Japan

Langmuir-Brodgett(LB) films of a dye compound and fatty acids were used as a model of a boundary layer for microtribological tests. Ultra-violet spectra of the compound were obtained during ƒÎ-A curve measurements. Orientation of the dye molecule vertical to water surface was observed from the UV spectra. Friction coefficient and molecular orientation of the LB film of dye on a hard substrate(NiP coated Al alloy) were measured simultaneously at a load of 250 mN. Molecular orientation of dye molecules was decreased with increasing number of sliding and friction coefficient was increased. This result directly shows that tribological performance of the boundary film is closely dependent on the film structure, that is, molecular orientation. Mixed LB films composed of dye and partially fluorinated fatty acids were used as a boundary layer on a hard substrate. Two types of fatty acids were used for the sample, that is, CF3(CF2)5(CH2)10COOH (abbreviated as 6-10) and CF3(CF2)9(CH2)10COOH(10-10). Tribological properties of the mixed LB films were dependent on the chain structure of the fatty acids. Friction coefficient of the mixed film of dye and 5-10 was lower and its life was longer than the mixed film with 10-10. UV spectra of the mixed films revealed that molecular orientation of dye was affected by mixed components. Dye molecule mixed with 5-10 shows better orientation than that with 10-10. This means that 5-10 molecules support the orientation of dye molecules and the mixed film shows a good tribological property. Two-dimensional structures of mixed films were analyzed by TOF-SIMS. Although the mixed film with dye and 5-10 shows a uniform surface structure, that with dye and 10-10 has small islands of dye molecules in a sea of fatty acid, 10-10. The longer perfluorinated chains of 10-10 molecules may interact each other more strongly and they may form the island structure. It was found that two dimensional structure of mixed boundary films also plays an important role for a mechanical stability of the films which affects their tribological performances.



Interfacial Friction of Helium Films Physisorbe on Two-dimensional Mesoporous Hectorite

M. Hieda, T. Nishino, M Suzuki*

H. Yano$^{a}$, N.Wada$^{a}$ and K. Torii$^{b}$

Univ. of Electro-Communications, Chofu, Tokyo 182-8585, Japan\\

$^{a}$Univ. of Tokyo, Meguro-ku Tokyo 153-8902, Japan\\

$^{b}$TNIRI, Miyagi-ku, Sendai 983-8551, Japan\\

We report the sliding friction of $^{3}$He and $^{4}$He physisorbed films on a substrate of a two-dimensional mesoporous hectorite. The films, which move together with the vibrating substrate above $\sim$10~K, slipped and underwent decoupling from this substrate at low temperatures. The temperature dependence of the sliding friction of the films was well described by the thermally activated process. From comparison of the activation energy to the specific heat, it is suggested that the thermal excitations of the films play an important role in the friction of this system. ________________________________________________________________________


"Simulations of Friction Anisotropy in Tilted Ordered Organic Monolayers"

T. Ohzono and F. Fujihira, Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan

The friction anisotropy in the atomistic sliding interface has been reported for recent years by friction force microscopy (FFM). This anisotropy has some potentials for modifications and characterizations of the surfaces. However, the mechanisms for frictional phenomena including the anisotopy are more difficult to be understood than that for friction itself. The computational approach will give some helpful implications for the understanding of such atomistic systems. In this study, the molecular dynamics (MD) method was used to model the friction between a probe and ordered monolayers of alkyl chains, which tilt with a specific angle to the surface normal, bound at their ends to rigid substrate. The friction anisotropy was examined changing the scan direction. The results showed that the combination of sliding direction and azimuthal angle of the monolayer strongly affected on magnitude of friction, and suggested that this has correlation with the anisotropy in the deformational potential surface of the film with respect to the scan direction under the shear stress in the elastic regime. Although the results were very complex, some suggestions for the mechanisms of friction anisotropy observed on such highly ordered monolayers of alkyl chains will be given.



AFM manipulation investigations of carbon nanotubes on HOPG:

Friction of sliding, in-plane rotation, and rolling

M. R. Falvo(1,4), J. Steele(1,4), A. Buldum(1,4), J. D. Schall(2,4), R. M.

Taylor II(3,4), J.P. Lu(1,4), D. W. Brenner(2,4), R. Superfine(1,4)

1 Dept. of Physics and Astronomy, The University of North Carolina, Chapel


2 Dept. of Materials Science and Engineering, North Carolina State

University, Raleigh

3 Dept. of Computer Science, The University of North Carolina, Chapel Hill

4 North Carolina Center for Nanoscale Materials, The University of North

Carolina, Chapel Hill

In contrast to the majority of AFM/LFM friction studies that focus on the AFM tip/substrate friction, our approach is investigating the friction between a nanometer sized object and substrate through AFM manipulation. We believe manipulation studies offer some advantages over tip/substrate studies. With nanometer sized objects of well defined geometry, the properties of the contact zone such as contact area or relative crystalline orientation between the contacting surfaces, can be well characterized. Also, manipulation offers the opportunity to study a variety of modes of motion such as rolling or in-plane rotation that may provide insights that compliment those gleaned from sliding studies. Disadvantages of this method are the possibility of additional tip/substrate friction in the manipulation lateral force signal, and intermittent loss of contact between tip and object during stick-slip motion. Our AFM is equipped with a home-built interface, called the nanoManipulator, designed for advanced manipulation work. AFM manipulation studies require a large dynamic range in operating forces so that loosely bound particles can be both imaged stably and reliably manipulated while recording lateral force. Our interface answers this problem by providing the ability to switch transparently between non-contact mode (low-force) for imaging and contact-mode for manipulating. The particular system we are studying is carbon nanotubes (CNT) on graphite (HOPG). The contact zone between particle and substrate in this case consists of two graphene lattices and we find the friction is highly anisotropic depending on the degree of commensurability. We find there are certain orientations where the CNT "locks-in" to atomic registry with the substrate. The lateral force required to move the tube in this "locked-in" (commensurate) state is up to 10 times larger than the out-of-registry state. We also observe that rolling motion occurs only in this lock-in state. We have never observed rolling on any other substrate. We have also performed atomistic computer simulations identifying the energy barriers for sliding and rolling, elucidating atomic-scale features of slip-roll motion and explaining the details of the lateral force data in terms of the intrinsic faceting of multiwall carbon nanotubes. Our experiments and simulations show that interlocking of the atomic lattices in the contact region of two bodies can determine whether the body slides or rolls. In essence, the atomic lattice can act like a gear mechanism.



Mechanism of two-dimensional atomic-scale friction

Satoru Fujisawa

Mechanical Engineering Laboratory

Namiki 1-2, Tsukuba, Ibaraki 305, Japan

By using a two-dimensional frictional force microscope, weinvestigated experimentally and theoretically a mechanism of an atomic scale friction between a single asperity tip and an atomically flat surface of graphite and NaF. While the tip shows the two-dimensional discrete jump between the sticking-domains with the lattice periodicity at higher load region, at the lower load region the tip shows rather smooth motion. The extent of the sticking-domain decreases by increasing the load. Further, the appearance frequency of the zigzag motion of the discrete jump decreases by increasing the load. This load dependence causes a transition in the frictional force image. These results are explained by the load dependence of the effective adhesive region.



``Millipede'' -- An AFM Data Storage System at the Frontier of Nanotribology

U. Duerig, G.K. Binnig, G. Cross, M. Despont, U. Drechsler, W. Häberle, W. King,* M. Lutwyche, H. Rothuizen, R. Stutz, P. Vettiger, and R. Widmer}

IBM Research,

Zurich Research Laboratory,

CH--8803 Rueschlikon, ¬


* Also at: Thermosciences Division, Mechanical Engineering Department, Stanford University, Stanford, CA 94305--3030, USA

The ``Millipede'' data storage concept is based on the parallel operation of a large number of micromechanical levers which function as AFM sensors. The technique holds promise to evolve into a novel ultrahigh density, terabit capacity, and high data rate storage technology. Thermomechanical writing and reading in very thin polymer (PMMA) films is used to store and sense 30 to 40-nm-sized bits with similar pitch size, resulting in 400 to 500-Gbit/in2 storage densities. High data rates are achieved by operating very large arrays of 32x32 AFM sensors in parallel. Batch-fabrication of 32x32 AFM cantilever array chips has been successfully achieved and array reading and writing has been demonstrated. One of the most important considerations for the Millipede storage project is the polymer dynamics on the scale of the size of a bit. Scaling of rheological parameters measured for macroscopic polymer samples is likely to be incorrect due to the finite length of the underlying molecular polymer chain, a size which is comparable to the bit itself. In order to shed light on these issues we perfomed lifetime studies of regular arrays of nanometer size patterns using light scattering techniques and bit writing was studied using a variable temperature AFM with thermally active single levers similar to those in the actual ``Millipede'' chip.



Chemical Interaction at Lubricant/Solid Interface

Functional Devices Research Laboratories, NEC Corporation

Masahiro Yanagisawa

There are many interfacial issues at the interface between magnetic head sliders and magnetic disks, namely, (1) the slider-air interface, (2) the air-lubricant interface, (3) the slider-lubricant interface, (4) the lubricant-overcoat interface, (5) the overcoat-underlayer interface, as shown in Fig.1. We have studied the molecular conformation and the molecular mobility of perfluorinated polyethers lubricant on carbon surfaces by a variety of analytical methods, namely, FT-IR, ellipsometry, ESCA, micro-calorimetry. Raman spectroscopy, Electron Spin Resonance (ESR), and Nuclear Magnetic Resonance (NMR). According to our results, lubricant molecules seem to be conformed on carbon surfaces, as shown in Fig.2. Hydroxyl groups in lubricant molecules are bonded to dangling bonds on carbon surfaces. When the interface is exposed in a moist environment, lubricant molecules are desorbed from carbon surfaces by water molecules. Molecular mobility of lubricant, which is estimated by the relaxation time of NMR, clearly suggests the conformation.



Chemical Force Microscopy by Friction Force Microscopy M. Fujihira,* Y. Tani, M. Furugori, A. Nakasa, Y. Okabe, and K. Yagi
Tokyo Institute of Technology, Department of Biomolecular Engineering,
4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
Atomic scale frictional phenomena of ordered organic thin films, such as Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs), have been studied by friction force microscopy (FFM) for chemical force microscopy (CFM). In this presentation,
1) Effect of chemical modification of gold coated AFM tips with SAMs with various terminal functional groups
2) Effect of terminal functional groups of thiol compounds for SAMs on Au(111)
3) Effect of composition of phase-separated domains in mixed hydrocarbon and fluorocarbon LB films
4) Effect of chain length of thiols in SAMs prepared by micro-contact (m-contact) printing will be presented.
The information obtained by these experiments is indispensable when FFM is applied for chemical recognition (CFM) of surface chemical species.



FTIR Spectroscopy and Nanotribological comparative studies :
Influence of the adsorbed water layers on the tribological behaviour
T. Bouhacina*, J.P. Aimé, B.Desbat**.
· C.P.M.O.H., Univ. Bordeaux I
** L.P.C.M., Univ. Bordeaux I

Due to adsorption, a thin water film is present on silica surface. The nanobribological properties of silica strongly depend on the thickness of the water layer covering the substrate. In order to study the influence of the water at the interface between the tip and the surface, pull-off force (adhesion) and lateral force (friction) were systematically recorded at different temperatures. The roughness of the silica surface is about 0.3 nm on a area of 1µm2. A Molecular Imaging head was set in a glove box in order to keep dry the ambiant atmosphere. Several cycles of temperatures were performed between 20¡C and 160¡C. Infrared Spectroscopy (FTIR) experiments were achieved in the same conditions.
Figure 1: Both pull-off force (open circle) and lateral force (full square) show a drastic dependence on the temperature variation. When increasing temperature, these forces decrease. For example, at 160¡C, the respective values of these forces are about 20% of the ambiant ones. At room temperature and ambiant conditions, we assume that the water layer is thick enough to generate additionnal capillary forces between the tip and the surface.
FTIR experiments results show that the thickness of water film reduces when increasing the sample temperature.
Figure 2 : Pull-off forces as a fonction of temperature under dry conditions. The silica sample was first exposed to ambiant conditions, then gradually heated (full circle). A marked decrease of the pull-off is observed. Then several temperature cycles were done under controlled conditions (open circle and full square) and show a stationnary value of the pull-off force.
To prove the influence of water layer thickness, additive FTIR experiments are peformed in a vacuum chamber.



"Generation of surface defects during sliding and their contribution to energy dissipation in friction and wear"
Miquel Salmeron
Materials Science Division of the Lawrence Berkeley National Laboratory, University of California. Berkeley, CA, USA

Abstract: It is conceptually useful to discuss energy dissipation in friction in terms of elementary excitations. These include electronic excitations and phonon generation, which do not modify the atomic structure of the rubbing materials and which decay in time scales measured in picoseconds or less. Then there are viscoelastic processes, where defects are produced in the form of atomic or molecular displacements, tilts, or as internal defects (gauche distortions in lubricant molecules). In this type of processes, the materials return also to their original unmodified state, but in a much larger time scale, in the second or minute range, comparable to typical experimental times. Finally, there are processes, like the production of point defects, where the healing time is longer than the experimental time. In this case point defects can accumulate and eventually coalesce into new structures that represent a permanent modification of the interface or wear.
Understanding how these different processes are excited, whether there is a threshold energy for each of them, and their relative importance in the overall energy dissipation budget, is the goal of our research projects. We thus strive to design experiments such that one or only a few of these processes are contributing and such that their effects can be identified and measured. I will illustrate these ideas with several examples from my laboratory.


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