Coupling of Normal and Transverse Motions during Frictional Sliding

Dynamic stiction without static friction: The role of friction vector rotation

In the textbook formulation of dry friction laws, static and dynamic friction (stick and slip) are qualitatively different and sharply separated phenomena. However, accurate measurements of stick-slip motion generally show that static friction is not truly static but characterized by a slow creep that, upon increasing tangential load, smoothly accelerates into bulk sliding. Microscopic, contact-mechanical, and phenomenological models have been previously developed to account for this behavior. In the present work, we show that it may instead be a systemic property of the measurement apparatus. Using a mechanical model that exhibits the characteristics of typical setups of measuring friction forces-which usually have very high transverse stiffness-and assuming a small but nonzero misalignment angle in the contact plane, we observe some fairly counterintuitive behavior: Under increasing longitudinal loading, the system almost immediately starts sliding perpendicularly to the pulling direction. Then the friction force vector begins to rotate in the plane, gradually approaching the pulling direction. When the angle between the two becomes small, bulk sliding sets in quickly. Although the system is sliding the entire time, macroscopic stick-slip behavior is reproduced very well, as is the accelerated creep during the "stick" phase. The misalignment angle is identified as a key parameter governing the stick-to-slip transition. Numerical results and theoretical considerations also reveal the presence of high-frequency transverse oscillations during the "static" phase, which are also transmitted into the longitudinal direction by nonlinear processes. Stability analysis is carried out and suggests dynamic probing methods for the approaching moment of bulk slip and the possibility of suppressing stick-slip instabilities by changing the misalignment angle and other system parameters.

Design of a Tribotester Based on Non-Contact Displacement Measurements

A tribotester with an integrated load sensor based on a strain gauge is typically used to measure the friction coefficient generated by the contact-related sliding motion of two objects. Since the friction coefficient is obtained by dividing the measured friction force by the applied normal force, the normal and friction forces must be measured for accurate analysis. In this study, a tribotester was used to measure the displacement of a cantilever tip using the fiberoptic sensor in a non-contact method. The friction coefficient measurement using the fiberoptic sensor was proven to be valid by calibrating the tip displacement due to normal/friction forces after designing a basic structural cantilever tip based on experiments and simulation analyses. The results obtained by using the fiberoptic sensor-cantilever tip-based tribotester were compared with those obtained using commercial and/or custom-built tribotesters under the same conditions. By designing various shapes of cantilever tips and using simulation analysis, the calibrations of the normal/friction forces and tip displacement could be verified and the coupling effect was evaluated. The performance and reliability of the fiberoptic sensor-cantilever tip-based tribotester, which can be used to determine the normal/friction forces by non-contact displacement measurements without a strain gauge, were verified.

Velocity dependence of sliding friction on a crystalline surface

We introduce and study a minimal 1D model for the simulation of dynamic friction and dissipation at the atomic scale. This model consists of a point mass (slider) that moves over and interacts weakly with a linear chain of particles interconnected by springs, representing a crystalline substrate. This interaction converts a part of the kinetic energy of the slider into phonon waves in the substrate. As a result, the slider experiences a friction force. As a function of the slider speed, we observe dissipation peaks at specific values of the slider speed, whose nature we understand by means of a Fourier analysis of the excited phonon modes. By relating the phonon phase velocities with the slider velocity, we obtain an equation whose solutions predict which phonons are being excited by the slider moving at a given speed.

Electro-responsive polyelectrolyte-coated surfaces

The anchoring of polymer chains at solid surfaces is an efficient way to modify interfacial properties like the stability and rheology of colloidal dispersions, lubrication and biocompatibility. Polyelectrolytes are good candidates for the building of smart materials, as the polyion chain conformation can often be tuned by manipulation of different physico-chemical variables. However, achieving efficient and reversible control of this process represents an important technological challenge. In this regard, the application of an external electrical stimulus on polyelectrolytes seems to be a convenient control strategy, for several reasons. First, it is relatively easy to apply an electric field to the material with adequate spatiotemporal control. In addition, in contrast to chemically induced changes, the molecular response to a changing electric field occurs relatively quickly. If the system is properly designed, this response can then be used to control the magnitude of surface properties. In this work we discuss the effect of an external electric field on the adhesion and lubrication properties of several polyelectrolyte-coated surfaces. The influence of the applied field is investigated at different pH and salt conditions, as the polyelectrolyte conformation is sensitive to these variables. We show that it is possible to fine tune friction and adhesion using relatively low applied fields.

Perturbation of the yield-stress rheology of polymer thin films by nonlinear shear ultrasound

We investigate the nonlinear response of macromolecular thin films subjected to high-amplitude ultrasonic shear oscillation using a sphere-plane contact geometry. At a film thickness comparable to the radius of gyration, we observe the rheological properties intermediate between bulk and boundary nonlinear regimes. As the driving amplitude is increased, these films progressively exhibit oscillatory linear, microslip, and full slip regimes, which can be explained by the modified Coulomb friction law. At highest oscillation amplitudes, the interfacial adhesive failure takes place, being accompanied by a dewettinglike pattern. Moreover, the steady state sliding is investigated in thicker films with imposed shear stresses beyond the yield point. We find that applying high-amplitude shear ultrasound affects not only the yielding threshold but also the sliding velocity at a given shear load. A possible mechanism for the latter effect is discussed.

Onset of sliding in amorphous films triggered by high-frequency oscillatory shear

We investigate the change of the static friction threshold of weakly adhesive amorphous interfaces in the presence of the shear ultrasonic oscillation. Prior to sliding, a softening of the shear interfacial stiffness is observed under either static or high-amplitude oscillatory shear. We find that the nonlinear shear ultrasound, regardless of its polarization, triggers the macroscopic sliding at these interfaces far below the static threshold. Such unjamming transition is due to the vibration-induced decrease of the apparent coefficient of static friction, which provides a mechanism for understanding the reduction of the yielding threshold of granular media by the acoustic fluidization.

Probing and tuning frictional aging at the nanoscale

Time-dependent increase of frictional strength, or frictional aging, is a widely observed phenomenon both at macro and nanoscales. The frictional aging at the nanoscale may result from nucleation of capillary bridges and strengthening of chemical bonding, and it imposes serious constraints and limitations on the performance and lifetime of micro- and nanomachines. Here, by analytical model and numerical simulations, we investigate the effect of inplane oscillations on friction in nanoscale contacts which exhibit aging. We demonstrate that adding a low amplitude oscillatory component to the pulling force, when applied at the right frequency, can significantly suppress aging processes and thereby reduce friction. The results obtained show that frictional measurements performed in this mode can provide significant information on the mechanism of frictional aging and stiffness of interfacial contacts.

Electric-field-induced friction reduction and control

Friction is always present when surfaces in contact are set in motion. In this work I describe how a precise, active control of the global friction is possible by adjusting the local molecular conformation of a polyelectrolyte coating via the application of an alternating electric field. The intensity of the applied field determines the degree of interpenetration between polymer brushes in contact, regulating chain stretching while sliding, which is the process at the origin of the global friction. The dynamics of the problem is controlled by the relaxation times of the polyelectrolyte.

Solid-on-solid single-block dynamics under mechanical vibration

The suppression of friction between sliding objects, modulated or enhanced by mechanical vibrations, is well established. However, the precise conditions of occurrence of these phenomena are not well understood. Here we address these questions focusing on a simple spring-block model, which is relevant to investigate friction both at the atomistic as well as the macroscopic scale. This allows us to investigate the influence on friction of the properties of the external drive, of the geometry of the surfaces over which the block moves, and of the confining force. Via numerical simulations and a theoretical study of the equations of motion, we identify the conditions under which friction is suppressed and/or recovered, and we evidence the critical role played by surface modulations and by the properties of the confining force.

Stabilizing stick-slip friction

Even the most regular stick-slip frictional sliding is always stochastic, with irregularity in both the intervals between slip events and the sizes of the associated stress drops. Applying small-amplitude oscillations to the shear force, we show, experimentally and theoretically, that the stick-slip periods synchronize. We further show that this phase locking is related to the inhibition of slow rupture modes which forces a transition to fast rupture, providing a possible mechanism for observed remote triggering of earthquakes. Such manipulation of collective modes may be generally relevant to extended nonlinear systems driven near to criticality.

Dynamic superlubricity on insulating and conductive surfaces in ultra-high vacuum and ambient environment

Atomic-scale friction between a sharp tip at the end of a micro-fabricated silicon cantilever and atomically flat surfaces (NaCl, KBr, HOPG and mica) can be significantly reduced by piezo-induced perpendicular mechanical oscillations at specific resonance frequencies of the cantilever in gentle contact with the sample. The reported measurements confirm and extend the applicability of the effect recently demonstrated using electro-capacitive actuation on alkali halide surfaces in ultra-high vacuum (Socoliuc et al 2006 Science 313 208). A controlled reduction of friction is now observed even on a conductive surface and under ambient conditions, which is quite promising for applications to micro-electromechanical devices. The theory previously used to interpret 'dynamic superlubricity' is supported by new measurements showing that the contact can be maintained in that regime and that the initial reduction of friction is linear versus oscillation amplitude. The calibration of the oscillating component of the normal force is also discussed.

Controlling microscopic friction through mechanical oscillations

We study in detail the recent suggestions by Tshiprut [Phys. Rev. Lett. 95, 016101 (2005)] to tune tribological properties at the nanoscale by subjecting a substrate to periodic mechanical oscillations. We show that both in stick-slip and sliding regimes of motion friction can be tuned and reduced by controlling the frequency and amplitude of the imposed substrate lateral excitations. We demonstrate that the mechanisms of oscillation-induced reduction of friction are different for stick-slip and sliding dynamics. In the first regime the effect results from a giant enhancement of surface diffusion, while in the second regime it is due to the interplay between washboard and oscillation frequencies that leads to the occurrence of parametric resonances. Moreover, we show that for a particular set of parameters it is possible to sustain the motion with only the oscillations.

Principles of atomic friction: from sticking atoms to superlubric sliding

Tribology-the science of friction, wear and lubrication-is of great importance for all technical applications where moving bodies are in contact. Nonetheless, little progress has been made in finding an exact atomistic description of friction since Amontons proposed his empirical macroscopic laws over three centuries ago. The advent of new experimental tools such as the friction force microscope, however, enabled the investigation of frictional forces occurring at well-defined contacts down to the atomic scale. This research field has been established as nanotribology. In the present article, we review our current understanding of the principles of atomic-scale friction based on recent experiments using friction force microscopy.

Atomic-Scale Control of Friction by Actuation of Nanometer-Sized Contacts

Stiction and wear are demanding problems in nanoelectromechanical devices, because of their large surface-to-volume ratios and the inapplicability of traditional liquid lubricants. An efficient way to switch friction on and off at the atomic scale is achieved by exciting the mechanical resonances of the sliding system perpendicular to the contact plane. The resulting variations of the interaction energy reduce friction below 10 piconewtons in a finite range of excitation and load, without any noticeable wear. Without actuation, atomic stick-slip motion, which leads to dissipation, is observed in the same range. Even if the normal oscillations require energy to actuate, our technique represents a valuable way to minimize energy dissipation in nanocontacts.

Dynamics and Kinetics of Heat Transfer at the Interface of Model Diamond {111} Nanosurfaces

A molecular dynamics simulation was performed to study the effect of an applied force on heat transfer at the interface of model diamond [111] nanosurfaces. The force was applied to a small, hot nanosurface at 800, 1000, or 1200 K brought into contact with a larger, colder nanosurface at 300 K. The relaxation of the initial nonequilibrium interfacial force occurs on a subpicosecond time scale, much shorter than that required for heat transfer. Heat transfer occurs with exponential kinetics and a rate constant that increases linearly with the interfacial force according to 7 x 10(-4) ps(-1)/nN. This rate constant only increases by at most 10% as the temperature of the hot surface is increased from 800 to 1200 K. Replacing the interfacial H-atoms on both surfaces by D atoms also has a very small effect on the heat transfer. However, if one nanosurface has H atoms on its interface and the other nanosurface's interface has D atoms, then there is a marked 25% decrease in the rate constant for heat transfer. Increasing the size of the hot surface, and, thus, the interfacial contact area, increases the rate of heat transfer but not the rate constant. For the same interfacial force, different anharmonic models for the nanosurfaces' potential energy function give the same heat transfer rate constant. The possibility of quantum effects for heat transfer across the diamond interface is considered.

Tuning diffusion and friction in microscopic contacts by mechanical excitations

We demonstrate that lateral vibrations of a substrate can dramatically increase surface diffusivity and mobility and reduce friction at the nanoscale. Dilatancy is shown to play an essential role in the dynamics of a nanometer-size tip which interacts with a vibrating surface. We find an abrupt dilatancy transition from the state with a small tip-surface separation to the state with a large separation as the vibration frequency increases. Atomic force microscopy experiments are suggested which can test the predicted effects.

Nonequilibrium energy dissipation at the interface of sliding model hydroxylated α-alumina surfaces

Nonequilibrium molecular dynamics simulations were performed to study the dynamics of energy transfer at the interface of a small nanoscale hydroxylated alpha-alumina surface sliding across a much larger surface of the same material. Sliding velocities of 0.05, 0.5, 5, and 50 ms and loads of 0, 0.0625, 5, 15, 25, and 100 nN were considered. Nonequilibrium energy distributions were found at the interface for each of these conditions. The velocity distribution P(v) for the atoms in a sublayer of the smaller surface oscillates during the sliding, reflecting the periodicity of the interfacial intermolecular potential. When averaged over the sliding, this P(v) for each of the sublayers is bimodal with Boltzmann and non-Boltzmann components. The non-Boltzmann component, with temperatures in excess of 1000 K and as high as 2500 K, is most important for the interfacial H-atom sublayer and becomes less important in moving to a sublayer further from the interface. Similarly, the temperature of the Boltzmann component decreases for sublayers further from the interface and approaches the 300 K temperature of the boundary. The temperature of the Boltzmann component decreases, but the importance of the non-Boltzmann component increases, as the sliding velocity is decreased. The temperature of the non-Boltzmann component is relatively insensitive to the sliding velocity. Friction forces are determined by calculating the energy dissipation during the sliding, and different regimes are found for variation in the friction force versus sliding velocity v(s) and applied load. For v(s) of 0.05, 0.5, and 5 ms, the friction force is inversely proportional to v(s) reflecting the increased time for energy dissipation as v(s) is decreased.

The nonlinear nature of friction

Tribology is the study of adhesion, friction, lubrication and wear of surfaces in relative motion. It remains as important today as it was in ancient times, arising in the fields of physics, chemistry, geology, biology and engineering. The more we learn about tribology the more complex it appears. Nevertheless, recent experiments coupled to theoretical modelling have made great advances in unifying apparently diverse phenomena and revealed many subtle and often non-intuitive aspects of matter in motion, which stem from the nonlinear nature of the problem.

Control of friction at the nanoscale

We propose a new algorithm to control frictional dynamics of a small array of particles towards preassigned values of the average sliding velocity. The control is based on the concepts of non-Lipschitzian dynamics and terminal attractor. Extensive numerical simulations illustrate the robustness, efficiency, and convenience of the algorithm.

Boundary lubrication properties of materials with expansive freezing

We have performed molecular dynamics simulations of solid-solid contacts lubricated by a model fluid displaying many of the properties of water, particularly its expansive freezing. Near the region where expansive freezing occurs, the lubricating film remains fluid, and the friction force decreases linearly as the shear velocity is reduced. No sign of stick-slip motion is observed, even at the lowest velocities. We give a simple interpretation of these results, and suggest that, in general, good boundary lubrication properties will be found in the family of materials with expansive freezing.

Transverse thermal depinning and nonlinear sliding friction of an adsorbed monolayer

We study the response of an adsorbed monolayer under a driving force as a model of sliding friction phenomena between two crystalline surfaces with a boundary lubrication layer. Using Langevin-dynamics simulation, we determine the nonlinear response in the direction transverse to a high symmetry direction along which the layer is already sliding. We find that below a finite transition temperature there exist a critical depinning force and hysteresis effects in the transverse response in the dynamical state when the adlayer is sliding smoothly along the longitudinal direction.

Tuning friction with noise and disorder

We present numerical and experimental evidence which demonstrates that under certain conditions friction can be reduced by spatial disorder and/or thermal noise. We discuss possible mechanisms for this behavior.