Dynamic phase transitions in confined lubricant fluids under shear

Alternatives to Friction Coefficient: Role of Frictional Instabilities on Fine Touch Perception

Fine touch perception is often correlated to material properties and friction coefficients, but the inherent variability of human motion has led to low correlations and contradictory findings. Instead, we hypothesized that humans use frictional instabilities to discriminate between objects. We constructed a set of coated surfaces with minimal physical differences that by themselves, are not perceptible to people, but instead, due to modification in surface chemistry, the surfaces created different types of instabilities based on how quickly a finger is slid and how hard a human finger is pressed during sliding. We found that participant accuracy in tactile discrimination most strongly correlated with formations of steady sliding, and response times negatively correlated with stiction spikes. Conversely, traditional metrics like surface roughness or average friction coefficient did not predict tactile discriminability. In fact, the typical method of averaging friction coefficients led to a spurious correlation which erroneously suggests that distinct objects should feel identical and identical objects should feel distinct. Identifying the central role of frictional instabilities as an alternative to using friction coefficients should accelerate the design of tactile interfaces for psychophysics and haptics.

Dynamical force measurements for contacting soft surfaces upon steady sliding: Fixed-depth tribology

The tribology between surfaces can have a profound impact on the response of a mechanical system, such as how granular particles are driven to flow. In this work, we perform experiments that time resolve the tangential and normal components of the force between two semicylindrical polydimethylsiloxane samples immersed in fluid, as they slide against each other in a range of controlled speeds. The time-averaged friction force shows a nonmonotonic dependence on the sliding speed over four decades, which is consistent with the paradigmatic Stribeck diagram and three dynamical regimes associated with it. Our specially designed fixed-depth setup allows us to study the fluctuation of force that exhibits strong stick-slip patterns in one of the regimes. Data from repetitive experiments reveal that both the "onset speed" for the stick-slip patterns and its spatial location along the sample change gradually during the course of our experiments, indicating changes on the sample surfaces. In addition, we conduct counterpart experiments by using spherical samples rubbing against each other, to make a direct connection of the interparticle tribology to the granular flow reported in our previous work [J.-C. Tsai et al., Phys. Rev. Lett. 126, 128001 (2021)10.1103/PhysRevLett.126.128001].

Reexamination of Damping in Sliding Friction

Friction is responsible for about one-third of the primary energy consumption in the world. So far, a thorough atomistic understanding of the frictional energy dissipation mechanisms is still lacking. The Amontons' law states that kinetic friction is independent of the sliding velocity while the Prandtl-Tomlinson model suggests that damping is proportional to the relative sliding velocity between two contacting objects. Through careful analysis of the energy dissipation process in atomic force microscopy measurements, here we propose that damping force is proportional to the tip oscillation speed induced by friction. It is shown that a physically well-founded damping term can better reproduce the multiple peaks in the velocity-dependent friction force observed in both experiments and molecular dynamics simulations. Importantly, the analysis gives a clear physical picture of the dynamics of energy dissipation in different friction phases, which provides insight into long-standing puzzles in sliding friction, such as velocity weakening and spring-stiffness-dependent friction.

Structures of Nanoconfined Liquids Determined by Synchrotron X-ray Diffraction

We have successfully performed X-ray diffraction measurements of the liquids octamethylcyclotetrasiloxane (OMCTS, a quasi-spherical-shaped molecule) and n-hexadecane (a normal alkane) confined between mica surfaces at surface separation distances (D's) from 500 nm to the hard-wall thickness (1.9 nm for OMCTS and 1.0 nm for hexadecane). At all of the studied D's, we observed diffraction peaks corresponding to their mean intermolecular spacing at q = 8.6 nm^-1 (d = 0.73 nm) for OMCTS and q = 13.6 nm^-1 (d = 0.45 nm) for n-hexadecane. The peak intensity increased at D < ca. 50 nm for OMCTS even with the decreasing distance and exhibited a local maximum at D = 17-13 nm, indicating the sharp increase in the molecular order in this distance range. The peak intensities normalized by the D and I_normalized values of OMCTS and n-hexadecane were nearly constant at D's greater than 100 nm, though they appeared to increase slightly. The increase then became more significant with decreasing D below 100 nm, and finally the I_normalized values became 120 (for OMCTS) and 160 (for n-hexadecane) at the hard wall. These results clearly demonstrated the significant increase in the structural order of OMCTS and n-hexadecane under nanoconfinement, especially below 100 nm. The fwhm values of the peaks of OMCTS and n-hexadecane showed no significant change until small distances when the confinement effect was significant. These results indicated that the increase in the structural order should be mainly ascribed to the ordering of the molecules in the parallel plane in the enhanced layered structure formed under the confinement. The viscous parameters (b₂) of OMCTS and n-hexadecane obtained from the resonance shear measurement showed no increase at D's down to ca. 7 nm. This indicated that a certain ordering of the confined molecules was required for the observable increase in the viscosity.

Thermal Friction Enhancement in Zwitterionic Monolayers

We introduce a model for zwitterionic monolayers and investigate its tribological response to changes in applied load, sliding velocity, and temperature by means of molecular-dynamics simulations. The proposed model exhibits different regimes of motion depending on temperature and sliding velocity. We find a remarkable increase of friction with temperature, which we attribute to the formation and rupture of transient bonds between individual molecules of opposite sliding layers, triggered by the out-of-plane thermal fluctuations of the molecules' orientations. To highlight the effect of the molecular charges, we compare these results with analogous simulations for the charge-free system. These findings are expected to be relevant to nanoscale rheology and tribology experiments of locally-charged lubricated systems such as, e.g., experiments performed on zwitterionic monolayers, phospholipid micelles, or confined polymeric brushes in a surface force apparatus.

Lubricated friction around nanodefects

The lubrication properties of nanoconfined liquids underpin countless natural and industrial processes. However, our current understanding of lubricated friction is still limited, especially for nonideal interfaces exhibiting nanoscale chemical and topographical defects. Here, we use atomic force microscopy to explore the equilibrium and dynamical behavior of a model lubricant, squalane, confined between a diamond tip and graphite in the vicinity of an atomic step. We combine high-resolution imaging of the interface with highly localized shear measurements at different velocities and temperatures to derive a quantitative picture of the lubricated friction around surface defects. We show that defects tend to promote local molecular order and increase friction forces by reducing the number of stable molecular configurations in their immediate vicinity. The effect is general, can propagate over hundreds of nanometers, and can be quantitatively described by a semiempirical model that bridges the molecular details and mesoscale observations.

Solidification and superlubricity with molecular alkane films

Hydrocarbon films confined between smooth mica surfaces have long provided an experimental playground for model studies of structure and dynamics of confined liquids. However, fundamental questions regarding the phase behavior and shear properties in this simple system remain unsolved. With ultrasensitive resolution in film thickness and shear stress, and control over the crystallographic alignment of the confining surfaces, we here investigate the shear forces transmitted across nanoscale films of dodecane down to a single molecular layer. We resolve the conditions under which liquid-solid phase transitions occur and explain friction coefficients spanning several orders of magnitude. We find that commensurate surface alignment and presence of water at the interfaces each lead to moderate or high friction, whereas friction coefficients down to [Formula: see text] are observed for a single molecular layer of dodecane trapped between crystallographically misaligned dry surfaces. This ultralow friction is attributed to sliding at the incommensurate interface between one of the mica surfaces and the laterally ordered solid molecular film, reconciling previous interpretations.

Probing the Self-Assembly and Nonlinear Friction Behavior of Confined Gold Nano-Particles

For the wide application of nanoparticles (NPs) (e.g., in nanotribology), it is of fundamental and practical importance to understand the self-assembly and lubrication behavior of confined NPs. In this work, a systematic study was conducted to probe the assembly and associated surface forces of spherical gold nanoparticles (Au NPs, diameter ∼5 nm) confined between pairs of mica (negatively charged) and (3-aminopropyl)triethoxysilane modified mica (APTES-mica, positively charged) surfaces using a surface forces apparatus (SFA) under aqueous conditions. It is observed that Au NPs were squeezed out of the confined gap between two mica surfaces during the loading process, resulting from the repulsive electric-double layer force. In contrast, multilayers of Au NPs were confined between two APTES-mica surfaces because of the attractive double-layer force between oppositely charged Au NPs and APTES-mica. Interestingly, the interaction between Au NPs and APTES-mica is stronger than the interactions between Au NPs, resulting in the rearrangement of the confined Au NPs under shearing. Importantly, a large friction coefficient (μ > 0.7) with unexpected nonlinear stick-slip friction was observed when sliding two APTES-mica surfaces with thin layers of Au NPs (∼20 nm) confined in between. The observed stick-slip motion could be explained by the velocity-dependent friction model where a critical shear velocity was required for transiting from stick-slip to smooth sliding. Our study provides useful information on the assembly and interaction forces of confined nanoparticles on charged surfaces, with implications for predicting the behaviors of NPs under confinement in various engineering applications.

Effect of temperature on the viscoelastic properties of nano-confined liquid mixtures

The behaviour of fluids confined in nanoscale gaps plays a central role in molecular science and nanofluidics, with applications ranging from biological function to multiscale printing, osmosis and filtration, lab-on-chip technology and friction reduction. Here atomic force microscopy is used to shear five different mixtures of hexadecane and squalane confined between the tip apex and atomically flat graphite. The shearing amplitudes are typically <2 nm, hence reflecting highly localised information at the interface. The evolution of each mixture's viscoelastic properties is studied as a function of temperature, between 20 °C and 100 °C. The results, complemented by sub-nanometre resolution images of the interface, show that spatial organisation of the liquid molecules at the surface of graphite largely dominates the measurements. Squalane presents a higher effective affinity for the surface by forming a robust self-assembled layer in all mixtures. This results in a step-like change of the viscous and elastic response of the confined liquid as the confining pressure increases. In contrast, measurements in pure hexadecane show a continuous and linear increase in the apparent viscosity with pressure at all temperatures. This is interpreted as a more fragile interfacial layer and images show that it can be completely removed at high temperatures. Depending on the mixture composition, measurements can be strongly location-dependent which suggests molecular clustering and nanoscale phase separation at the interface.

Friction and nonlinear dynamics

The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the essentially nonlinear phenomena involved in friction.

Modeling the response of a quartz crystal microbalance under nanoscale confinement and slip boundary conditions

Nanorheology and boundary slip play an important role in the micro/nanofluidics, and micro/nano-electromechanical systems, especially for research on DNA, proteins and polymers. Herein, a nanoscale confinement structure, called a nanocell, is established by assembling a parallel plate on the quartz crystal microbalance (QCM) chip to study the nanorheology of liquids and the boundary slip on the interface. The corresponding analytical models are established and verified experimentally with high consistency. We reveal that the responses of QCM with the nanocell assembled are dependent on the nanocell confinement thickness, the acoustic impedance of the nanocell lid (parallel plate), as well as the boundary slip on the interface. A critical influence thickness of the assembled nanocell d = 2δ is indicated, above which the assembly of a nanocell has no influence on the QCM response. And the interfacial boundary slip results in obvious decreases of relative frequency shift and relative half-bandwidth variation. We find that adopting a nanocell lid with the same acoustic impedance as the tested liquids will evidently simplify the experimental analysis. In the paper, the nanocell provides an effective method to investigate the nanorheology of confined liquids and the interfacial boundary slip, and the established models offer a theoretical basis for the analysis of the nanocell-assembled QCM response.

Effective temperature dynamics of shear bands in metallic glasses

We study the plastic deformation of bulk metallic glasses with shear transformation zone (STZ) theory, a physical model for plasticity in amorphous systems, and compare it with experimental data. In STZ theory, plastic deformation occurs when localized regions rearrange due to applied stress and the density of these regions is determined by a dynamically evolving effective disorder temperature. We compare the predictions of STZ theory to experiments that explore the low-temperature deformation of Zr-based bulk metallic glasses via shear bands at various thermal temperatures and strain rates. By following the evolution of effective temperature with time, strain rate, and temperature through a series of approximate and numerical solutions to the STZ equations, we successfully model a suite of experimentally observed phenomena, including shear-band aging as apparent from slide-hold-slide tests, a temperature-dependent steady-state flow stress, and a strain-rate- and temperature-dependent transition from stick-slip (serrated flow) to steady-sliding (nonserrated flow). We find that STZ theory quantitatively matches the observed experimental data and provides a framework for relating the experimentally measured energy scales to different types of atomic rearrangements.

Boundary lubrication with a liquid crystal monolayer

We study boundary lubrication characteristics of a liquid crystal (LC) monolayer sheared between two crystalline surfaces by nonequilibrium molecular dynamics simulations, using a simplified rigid bead-necklace model of the LC molecules. We consider LC monolayers confined by surfaces with three different atomic structures, subject to different shearing velocities, thus approximating a wide variety of materials and driving conditions. The time dependence of the friction force is studied and correlated with that of the orientational order exhibited by the LC molecules, arising from the competition between the effect of the structure of the confining surfaces and that of the imposed sliding direction. We show that the observed stick-slip events for low shear rates involve order-disorder transitions, and that the LC monolayer no longer has enough time to reorder at high shear rates, resulting in a smooth sliding regime. An irregular stick-slip phase between the regular stick-slip and smooth sliding is observed for intermediate shear rates regardless of the surface structure.

Microslips to "Avalanches" in Confined, Molecular Layers of Ionic Liquids

We have measured forces between mica surfaces across two hydrophobic ionic liquids with a surface forces apparatus. Both surface-adsorbed water and alkyl-chain length on the imidazolium cation influence the structure of the nanoconfined film and the dynamics of film-thickness transitions. Friction shows accumulative microslips as precursors to collective "avalanches" that abruptly reduce friction momentarily. This behavior is interpreted as a consequence of interlayer ion correlations within the 1 to 2 nm thick film; we identify this to be analogous to the friction response of crackling noise systems over a broad range of sizes.

Liquid-liquid-solid transition in viscoelastic liquids

Liquid-liquid-solid transitions (LLST) are known to occur in confined liquids, exist in supercooled liquids and emerge in liquids driven from equilibrium. Molecular dynamics (MD) simulations claim many successes in forecasting the phenomena. The transitions are also studied in the framework of thermodynamics based methods and minimalistic models. In here, the proposed approach is derived in the framework of continuum and includes spatial and temporal dynamic heterogeneities; the approach is meant to capture the material behavior at small scales. We conjecture that the liquid-like and solid-like behaviors are dissimilar enough for the two to be governed by different constitutive relations. In this way, we gain additional degree of freedom, which is found essential when predicting the transitional phenomena. As a result, we derive the LLST criteria for liquids in equilibrium, during steady flow and at transient conditions. Lastly, we forecast short-lived LLSTs in human blood during cardiac cycle.

Traction and nonequilibrium phase behavior of confined sheared liquids at high pressure

Nonequilibrium molecular dynamics simulations of confined model liquids under pressure and sheared by the relative sliding of the boundary walls have been carried out. The relationship between the time-dependent traction coefficient, μ(t), and the state of internal structure of the film is followed from commencement of shear for various control parameters, such as applied load, global shear rate, and solid-liquid atom interaction parameters. Phase diagrams, velocity and temperature profiles, and traction coefficient diagrams are analyzed for pure Lennard-Jones (LJ) liquids and a binary LJ mixture. A single component LJ liquid is found to form semicrystalline arrangements with high-traction coefficients, and stick-slip behavior is observed for high pressures and low-shear velocities, which is shown to involve periodic deformation and stress release of the wall atoms and slip in the solid-liquid boundary region. A binary mixture, which discourages crystallization, gives a more classical tribological response with the larger atoms preferentially adsorbing commensurate with the wall. The results obtained are analyzed in the context of tribology: the binary mixture behaves like a typical lubricant, whereas the monatomic system behaves like a traction fluid. It is discussed how this type of simulation can give insights on the tribological behavior of realistic systems.

Origin of intermittent plastic flow and instability of shear band sliding in bulk metallic glasses

Intermittent or serrated plastic flow is widely observed in the deformation of bulk metallic glasses (BMGs) or other disordered solids at low temperatures. However, the underlying physical process responsible for the phenomena is still poorly understood. Here, we give an interpretation of the serrated flow behavior in BMGs by relating the atomic-scale deformation with the macroscopic shear band behavior. Our theoretical analysis shows that serrated flow in fact arises from an intrinsic dynamic instability of the shear band sliding, which is determined by a critical stiffness parameter in stick-slip dynamics. Based on this, the transition from serrated to nonserrated flow with the strain rate or the temperature is well predicted and the effects of various extrinsic and intrinsic factors on shear band stability can be quantitatively analyzed in BMGs. Our results, which are verified by a series of compression tests on various BMGs, provide key ingredients to fundamentally understand serrated flow and may bridge the gap between the atomic-scale physics and the larger-scale shear band dynamics governing the deformation of BMGs.

Earthquake-like dynamics in Myxococcus xanthus social motility

Myxococcus xanthus is a bacterium capable of complex social organization. Its characteristic social ("S")-motility mechanism is mediated by type IV pili (TFP), linear actuator appendages that propel the bacterium along a surface. TFP are known to bind to secreted exopolysaccharides (EPS), but it is unclear how M. xanthus manages to use the TFP-EPS technology common to many bacteria to achieve its unique coordinated multicellular movements. We examine M. xanthus S-motility, using high-resolution particle-tracking algorithms, and observe aperiodic stick-slip movements. We show that they are not due to chemotaxis, but are instead consistent with a constant TFP-generated force interacting with EPS, which functions both as a glue and as a lubricant. These movements are quantitatively homologous to the dynamics of earthquakes and other crackling noise systems. These systems exhibit critical behavior, which is characterized by a statistical hierarchy of discrete "avalanche" motions described by a power law distribution. The measured critical exponents from M. xanthus are consistent with mean field theoretical models and with other crackling noise systems, and the measured Lyapunov exponent suggests the existence of highly branched EPS. Such molecular architectures, which are common for efficient lubricants but rare in bacterial EPS, may be necessary for S-motility: We show that the TFP of leading "locomotive" cells initiate the collective motion of follower cells, indicating that lubricating EPS may alleviate the force generation requirements on the lead cell and thus make S-motility possible.

Aging and stiction dynamics in confined films of a star polymer melt

The stiction properties of a star polyisoprene (PIP) melt (having 22 arms and an arm molecular weight of around 5000, M(w) ≈ 110,000) confined between mica surfaces were investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P; the time constants of the phase transitions in the stiction dynamics (freezing on stopping and melting on starting) were obtained from the force relaxation behaviors. The results were compared with those of a confined linear-PIP melt (M(w) ≈ 48,000) and other confined fluid systems; the effect of star architecture on the phase transitions in confinement during aging is discussed. Estimation of the molecular size gives that the confined star-PIP films consist of three molecular layers; a non-adsorbed layer sandwiched between two layers adsorbed on opposed mica surfaces. There are (at least) four time constants in the freezing transition of the confined star-PIP melt; fast (τ(1)) and slow (τ(2)) time constants for lateral force relaxation on stopping, critical aging time for freezing (τ(f)), and the logarithmic increase of the spike height against t. The three time constants on stopping, τ(1), τ(2), and τ(f), increase with the increase of P (decrease of the thickness D). As regards the melting transition on starting, spike force decay was fitted by a single exponential function and one time constant was obtained, which is insensitive to P (D). Comparison of the time constants between freezing and melting, and also with the results of linear-PIP reveals that the stiction dynamics of the star-PIP system involves the relaxation and rearrangement of segmental-level and whole molecular motions. Lateral force relaxation on stopping is governed by the individual and cooperative rearrangements of local PIP segments and chain ends of the star, which do not directly lead to the freezing of the system. Instead, geometrical rearrangements of the soft star-PIP spheres into dense packing between surfaces (analogous to the concept of a colloidal glass transition) are the major mechanism of the freezing transition (stiction) after aging. Interdigitation of PIP segments/chain ends between neighboring star molecules also contributes to the spike growth along with aging, and the melting transition on starting.

Static and dynamic friction in sliding colloidal monolayers

In a pioneer experiment, Bohlein et al. realized the controlled sliding of two-dimensional colloidal crystals over laser-generated periodic or quasi-periodic potentials. Here we present realistic simulations and arguments that besides reproducing the main experimentally observed features give a first theoretical demonstration of the potential impact of colloid sliding in nanotribology. The free motion of solitons and antisolitons in the sliding of hard incommensurate crystals is contrasted with the soliton-antisoliton pair nucleation at the large static friction threshold F(s) when the two lattices are commensurate and pinned. The frictional work directly extracted from particles' velocities can be analyzed as a function of classic tribological parameters, including speed, spacing, and amplitude of the periodic potential (representing, respectively, the mismatch of the sliding interface and the corrugation, or "load"). These and other features suggestive of further experiments and insights promote colloid sliding to a unique friction study instrument.

"Freezing" of nanoconfined fluids under an electric field

The problem of the solidlike transition of fluids in a nanogap has drawn much fundamental and practical attention. Here, we directly observed the disappearance of the fluidity of liquids confined within a gap with a surface separation of >10 nm under an EF in a ball-plate system, which is called the "freezing" of liquids. The flow of the nanoconfined liquid became very weak as the EF intensity was increased to a critical value and was correlated with the liquid polarity and the film thickness. It is deduced that the EF can induce more liquid molecules to be aligned to form more ordered layers in the nanogap.

Stick-slip instabilities and shear strain localization in amorphous materials

We study the impact of strain localization on the stability of frictional slipping in dense amorphous materials. We model the material using shear transformation zone (STZ) theory, a continuum approximation for plastic deformation in amorphous solids. In the STZ model, the internal state is quantified by an effective disorder temperature, and the effective temperature dynamics capture the spontaneous localization of strain. We study the effect of strain localization on stick-slip instabilities by coupling the STZ model to a noninertial spring slider system. We perform a linear stability analysis to generate a phase diagram that connects the small scale physics of strain localization to the macroscopic stability of sliding. Our calculations determine the values of spring stiffness and driving velocity where steady sliding becomes unstable and we confirm our results through numerical integration. We investigate both homogeneous deformation, where no shear band forms, and localized deformation, where a narrow shear band spontaneously forms and accommodates all of the deformation. Our results show that at a given velocity, strain localization leads to unstable frictional sliding at a much larger spring stiffness compared to homogeneous deformation, and that localized deformation cannot be approximated by a homogeneous model with a narrower material. We also find that strain localization provides a physical mechanism for irregular stick-slip cycles in certain parameter ranges. Our results quantitatively connect the internal physics of deformation in amorphous materials to the larger scale frictional dynamics of stick-slip.

Structural aging and stiction dynamics in confined liquid films

The static friction (stiction) of the molecularly thin films of an irregularly shaped molecule 1,3-dimethylbutyl octyl ether (DBOE) confined between mica surfaces was investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P. The results show two relaxation processes, one on stopping and one on starting, where each process has a fast and a slow time constant. For stopping mode, there is no stiction spike when t is shorter than a characteristic nucleation time, tau(n) (fast time constant). When t exceeds tau(n), stiction spike appears whose height increases logarithmically with t. With regard to starting, the relaxation behavior was evaluated by a double exponential fit of the slipping regime (force decay) of the spike and two time constants (tau(1) and tau(2)) were obtained. The fast time constant on starting tau(1) is almost equal to that on stopping tau(n). To the best of our knowledge, this is the first direct observation of the agreement of the time constant on stopping and that on starting, indicative of a reversible structural transition (solid-liquid transition) in the stop-start stiction dynamics. The two fast time constants exhibit exponential dependence on P, which implies a glasslike nature of the transition. Comparison with the stick-slip friction reveals that the solid-liquid transition involved in stiction and that in stick-slip dynamics is different for DBOE; first-order-like discontinuous transition is suggested for stick-slip friction. Origins of the different solid-liquid transition dynamics in stiction and in stick-slip friction are discussed by comparing with the dynamics of other confined liquid systems.

Molecular dynamics simulation of complex particles in three dimensions and the study of friction due to nonconvexity

We investigate the macroscopic properties of frictionless nonconvex particles using molecular dynamics. The calculations are based on a simple and efficient method to simulate complex-shaped interacting bodies. The particle shape is represented by Minkowski operators. A multicontact time-continuous interaction between bodies is derived using simple concepts of computational geometry. Three-dimensional simulations of hopper flow show that the nonconvexity of the particles strongly affects the jamming on granular flow. Also the model allows the representation of complex bodies with rough surfaces as in friction studies and the reproduction of a wide range of friction and dilatancy angles as in true triaxial tests.

Properties of confined and sheared rhodamine B films studied by SFA-FECO spectroscopy

We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area. We also observed crystallization (solidification) parallel to the shearing direction.

Combined atomic force microscopy and fluorescence correlation spectroscopy measurements to study the dynamical structure of interfacial fluids

We have studied the dynamic structure of thin (approximately a few nanometers) liquid films of a nearly spherical, nonpolar molecule tetrakis(2-ethylhexoxy)silane (TEHOS) by using a combination of atomic force microscopy (AFM) and fluorescence correlation spectroscopy (FCS). Ultra-sensitive interferometer-based AFM was used to determine the stiffness (force gradient) and the damping coefficient of the liquid film. The experiments show oscillations in the damping coefficient with a period of approximately 1 nm, which is consistent with the molecular dimension of TEHOS as well as previous X-ray reflectivity measurements. Additionally, we performed FCS experiments for direct determination of the molecular dynamics within the liquid film. From the fluctuation autocorrelation curve, we measured the translational diffusion of the probe molecule embedded within the fluid film formed on a solid substrate. The autocorrelation function was best fitted with two components, which indicate that the dynamics are heterogeneous in nature. However, the heterogeneity is not as pronounced as had been previously observed for molecularly thin liquid films sandwiched between two solid substrates.

Low friction lubrication between amorphous walls: Unraveling the contributions of surface roughness and in-plane disorder

Using molecular dynamics simulations, we show that dodecane films confined between amorphous surfaces at 300 K retain liquid-like behavior down to film thicknesses of at least 1.8 nm and possibly smaller. This is in stark contrast to the behavior of films confined between crystalline surfaces which show an abrupt transition to a very high viscosity state at a film thickness of 4 nm. We show that it is the small increase in surface roughness in going from crystalline to amorphous walls, rather than the in-plane disorder, that is responsible for disrupting the crystalline bridges found in the crystal-confined films. The main consequences of the in-plane disorder are the removal of the orientational pinning of the local domain alignment and the reduction of the critical thickness at which the transition to film rigidity appears.

A Modified Surface Forces Apparatus for Single Molecule Tracking

To unravel molecular motion within confined liquids, we have combined a surface forces apparatus (SFA) with a highly sensitive fluorescence microscope. Details of our setup including important modifactions to enable the tracking of single dye molecules within nanometer thin confined liquid films are presented. The mechanical and optical performance of our setup is discussed in detail. For a load of 20 mN we observed a circular-shaped contact region (d approximately 300 microm), which results in a confining pressure of about 280 kPa. First experiments on liquid films of tetrakis(2-ethylhexoxy)silane (TEHOS) doped with rhodamine B demonstrated the ability to track single dye molecules within the confining gap of a SFA. The mean diffusion constant was independent of the liquid film thickness of approximately 3x10(-8) cm2/s and thus 10 times smaller than the diffusion constant of rhodamine B in bulk TEHOS. This points to the existence of a thin interface layer with slower molecular dynamics and an attractive potential parallel to the solid surface trapping molecules in this interface region.

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.

Nanotribology of ethers: effects of molecular asymmetry and fluoroalkyl chains

The tribological properties of the molecularly thin films of asymmetric ether (1,3-dimethylbutyl octyl ether, AE) and fluorinated asymmetric ether (1H,1H,2H,2H-perfluorooctyl-1,3-dimethylbutyl ether, FAE) were investigated. Friction forces and dynamic thicknesses (thicknesses during sliding) were simultaneously measured using the surface forces apparatus, and the effects of molecular asymmetry and fluoroalkyl chains on the friction properties are analyzed. The friction forces (both kinetic and static) and dynamic thicknesses are larger for the AE film than for the FAE film. The two ethers exhibit stick-slip friction at low sliding velocity, but the stick-slip patterns are different. For the AE film, one stick-slip cycle consists of two or more spikes; a large spike is followed by one or more small spike(s) in the cycle. On the other hand, regular stick-slip spikes are observed for the FAE film. The results suggest that the responsible friction mechanisms are completely different between the two ether films. The asymmetric shape of the AE molecule results in a variety of shear-ordered liquid structures in confinement, and the friction (stick-slip) behavior follows the "phase-transition model". In contrast, the FAE molecule is rigid, and the shape of the molecule is rather close to a symmetric cylinder, which leads to a well-ordered two-layer film in confinement. The each molecular layer is strongly adsorbed on adjacent mica substrate and behaves as a fluorinated coating. The friction is governed by the molecular scale "bumpiness" of the fluoroalkyl chains lying on mica surfaces and basically follows the "cobblestone model". The advantage of the thin FAE film as a practical lubricant is also discussed.

Superlubricity: a paradox about confined fluids resolved

Using the method of Frantz and Salmeron to cleave mica [Tribol. Lett. 5, 151 (1998)]] we investigate alkane fluids in a surface forces apparatus and confirm several predictions of molecular dynamics (MD) simulation. An oscillatory force-distance profile is observed for the methyl-branched alkane, squalane. Boundary slip is inferred from the frictional sliding of molecularly thin fluids and also from the hydrodynamic flow of thicker films. These findings resolve the paradox that prior experiments disagreed with these aspects of MD predictions, and demonstrate that exceptionally low energy dissipation is possible when fluids move past solid surfaces that are sufficiently smooth.

Effect of inertia and elasticity on stick-slip motion

A hybrid simulation method is used to study the transition from stick-slip motion to steady sliding as the sliding velocity increases above a critical value v(c). The effects of the geometry, elasticity, and mass M of the sliding object are varied to test competing theories. When the slider has a tapered geometry, v(c) scales as M(-1/2), and the elasticity of the slider is irrelevant. When the slider has a constant columnar cross section, elasticity dominates, and v(c) is independent of mass as M--> infinity. The tapered geometry is more typical of existing measurements, but the columnar geometry could be realized using a nanotube.

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.

Boundary lubrication with a glassy interface

Recently introduced constitutive equations for the rheology of dense, disordered materials are investigated in the context of stick-slip experiments in boundary lubrication. The model is based on a generalization of the shear transformation zone (STZ) theory, in which plastic deformation is represented by a population of mesoscopic regions which may undergo nonaffine deformations in response to stress. The generalization we study phenomenologically incorporates the effects of aging and glassy relaxation. Under experimental conditions associated with typical transitions from stick-slip to steady sliding and stop-start tests, these effects can be dominant, although the full STZ description is necessary to account for more complex, chaotic transitions.

Nanoscopic liquid bridges exposed to a torsional strain

In this paper we investigate the response to a torsional strain of a molecularly thin film of spherically symmetric molecules confined to a chemically heterogeneous slit pore by means of Monte Carlo simulations in the grand canonical ensemble. The slit pore comprises two identical plane-parallel solid substrates, the fluid-substrate interaction is purely repulsive except for elliptic regions attracting fluid molecules. Under favorable thermodynamic conditions the confined film consists of fluid bridges where the molecules are preferentially adsorbed by the attractive elliptic regions, and span the gap between the opposite substrate surfaces. By rotating the upper substrate while holding the lower one in position, bridge phases can be exposed to a torsional strain 0< or =theta< or =pi/2 and the associated torsional stress T(theta) of the (fluidic) bridge phases can be calculated from molecular expressions. The obtained stress curve T(theta)(theta) is qualitatively similar to the one characteristic of sheared confined films: as the torsion strain increases, T(theta) rises to a maximum (yield point) and then decays monotonically to zero. By changing the ellipses' aspect ratio while keeping their area constant, we also investigate the influence of the attractive elliptic patterns' shape on T(theta)(theta).

Transitions between smooth and complex stick-slip sliding of surfaces

Shear measurements were performed on mica surfaces with molecularly thin films of squalane (C30H62) confined between them. Squalane is a branched hydrocarbon liquid that can be in the liquid, glassy, or liquid-crystalline state under confinement. The friction forces, especially the transitions between smooth and intermittent (e.g., stick-slip) sliding, were measured over a wider range of applied loads (pressures), sliding velocities (shear rates), and temperatures than in previous studies. The results reveal that, depending on the conditions, qualitatively different behavior can arise in the same system. These include both abrupt and continuous transitions, both upper and lower critical transition temperatures, short and very long transient effects, and chaotic, sawtooth, or sinusoidal stick-slip that can slowly decay with time or distance sheared. The differences between these branched and simpler, e.g., spherical, unbranched molecules are compared, as well as with unlubricated (dry) surfaces and macroscopic (geological) systems.

Rearrangements and dilatancy for sheared dense materials

Constitutive equations are proposed for dense materials, based on the identification of two types of free-volume activated rearrangements associated with shear and compaction. Two situations are studied: the case of an amorphous solid in a stress-strain test, and the case of a lubricant in tribology test. Varying parameters, strain softening, shear thinning, and stick-slip motion can be observed.

Confined molecules under shear: from a microscopic description to phenomenology

A coarse grained two-state model is derived starting from a molecular dynamics description of a molecular system under shear. This model captures the main features of the response of a confined system under shear, and generalizes the phenomenological Tomlinson model for the response of a driven system. The derivation is based on the solution of coupled microscopic equations using a mean field approximation. The two-state model agrees well with the direct numerical solution and offers a practical approach to investigate the response of sheared systems under a broad range of parameters.