Dynamic transitions in molecularly thin liquid films under frictional sliding

The friction properties of the molecularly thin films of an asymmetric ether, 1,3-dimethylbutyl hexadecyl ether (DBHE), confined between mica surfaces were investigated using the surface forces apparatus. Kinetic friction was measured as a function of normal load and sliding velocity, and the static friction (stiction) was measured as a function of normal load and surface stopping time. Kinetic friction measurements exhibited unstable sliding dynamics: the friction force exhibited cyclic bumps and valleys in the sliding velocity range from about 10(-2) to 1 microm/s, but above and below the velocity range, smooth sliding was observed. Stop-start experiments showed a stiction spike when surface stopping time exceeded a characteristic nucleation time, indicative of the static friction state at very low sliding velocity. These results imply that the friction of the confined DBHE film has at least three responsible friction states--static friction and two different kinetic friction states--depending on the sliding velocity. The unstable sliding (bumps and valleys of the friction force) reflects the dynamic transition between two different kinetic states. The different friction states and their transitions are discussed on the basis of the recent experiments and theories of "inverted" stick-slip friction.     

SURFACE DYNAMIC FRICTION OF POLYMER GELS

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors ofthe hydrogels do not conform to Amonton's law F=μW which well describes the friction of solids. The frictional force andits dependence on the load are quite different depending on the chemical structures of the gels, surface properties of theopposing substrates, and the measurement condition. The gel friction is explained in terms of interracial interaction, eitherattractive or repulsive, between the polymer chain and the solid surface. According to this model, the friction is ascribed tothe viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chainfrom the substrate appears as friction. The surface adhesion between glass particles and gels measured by AFM showed agood correlation with the friction, which supported the repulsion-adsorption model proposed by the authors.     

Atomic-scale friction on diamond: a comparison of different sliding directions on (001) and (111) surfaces using MD and AFM

Atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations were conducted to examine single-asperity friction as a function of load, surface orientation, and sliding direction on individual crystalline grains of diamond in the wearless regime. Experimental and simulation conditions were designed to correspond as closely as state-of-the-art techniques allow. Both hydrogen-terminated diamond (111)(1 x 1)-H and the dimer row-reconstructed diamond (001)(2 x 1)-H surfaces were examined. The MD simulations used H-terminated diamond tips with both flat- and curved-end geometries, and the AFM experiments used two spherical, hydrogenated amorphous carbon tips. The AFM measurements showed higher adhesion and friction forces for (001) vs (111) surfaces. However, the increased friction forces can be entirely attributed to increased contact area induced by higher adhesion. Thus, no difference in the intrinsic resistance to friction (i.e., in the interfacial shear strength) is observed. Similarly, the MD results show no significant difference in friction between the two diamond surfaces, except for the specific case of sliding at high pressures along the dimer row direction on the (001) surface. The origin of this effect is discussed. The experimentally observed dependence of friction on load fits closely with the continuum Maugis-Dugdale model for contact area, consistent with the occurrence of single-asperity interfacial friction (friction proportional to contact area with a constant shear strength). In contrast, the simulations showed a nearly linear dependence of the friction on load. This difference may arise from the limits of applicability of continuum mechanics at small scales, because the contact areas in the MD simulations are significantly smaller than the AFM experiments. Regardless of scale, both the AFM and MD results show that nanoscale tribological behavior deviates dramatically from the established macroscopic behavior of diamond, which is highly dependent on orientation.     

Negative velocity dependence of friction for poly(2‐Acrylamido‐2‐methyl propanesulfonic acid) hydrogel sliding against a glass surface in the low‐velocity region

The frictional behavior of poly(2‐acrylamido‐2‐methylpropanesulfonic acid) (PAMPS) hydrogel sliding against a glass substrate in water over a wide sliding velocity (v) region has been investigated. The results showed that the frictional behavior of PAMPS gel conformed to a hydrodynamic lubrication mechanism only at relatively high sliding velocities. At low sliding velocities, a “negative” velocity dependence of friction was observed, which we believe not to be attributable to the experimental friction‐measuring mode used. This wider and weak mixed region at low sliding velocities is in contrast to the extremely narrow mixed region in the case of solid friction with a lubricant. The area of the PAMPS hydrogel surface subject to shearing decreased with increasing sliding velocity, and this would seem to be responsible for the weakly negative dependence of friction on velocity. In addition, the friction was found to increase with increasing the compressive modulus (E) of the gels attributing to the shearing exerted on the gel surface, in which the shear stress increased with E. The hydration layer between the sliding surfaces also contributes to the friction and weakens the dependence of friction on E to some extent. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 765–772     

Evaluation of nanotribological behavior of amorphous polystyrene: The macromolecular weight effect

Tribological testing of polymers is of prime importance in many industrial applications. Silicon nitride AFM tips have been used to mimic the contact between amorphous polystyrene surfaces and a hard asperity, which is useful in understanding of how a multitude of asperities behave in a macroscopic contact. In this study, the adhesion force and the friction force of four PS molecular weights were measured and the average contact pressure was calculated by using the JKR contact theory. The nanotribological behavior of polystyrene showed a dependence on macromolecular weight with varying applied normal force and sliding velocity. The study indicates that the length of polymer chains noticeably influences the tribological behavior of amorphous polystyrenes. Mechanisms governing such behavior differences were ascribed to energy dissipating modes.     

Intelligent gel – surface properties and functions of gels –

The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F = μW, which well describes the friction of solids. The frictional force and its dependencies of on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. According to this model, the frictional is ascribed to the viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chain from the substrate appears as friction. Surface adhesion between glass particles and gels measured by AFM showed a good correlation with the friction, which support the repulsion‐adsorption model proposed by authors.     

Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions

Adsorption of hydrated cations on hydrophilic surfaces has been related to a variety of phenomena associated with the short-range interaction forces and mechanisms of the adhesive contact between the surfaces. Here we have investigated the effect of the adsorption of cations on the lateral interaction. Using lateral force microscopy (LFM), we have measured the friction force between a silica particle and silica wafer in pure water and in electrolyte solutions of LiCl, NaCl, and CsCl salts. A significant lubrication effect was demonstrated for solutions of high electrolyte concentrations. It was found that the adsorbed layers of smaller and more hydrated cations have a higher lubrication capacity than the layers of larger and less hydrated cations. Additionally, we have demonstrated a characteristic dependence of the friction force on the sliding velocity of surfaces. A mechanism for the observed phenomena based on the microstructures of the adsorbed layers is proposed.     

Nanotribology of octadecyltrichlorosilane monolayers and silicon: self-mated versus unmated interfaces and local packing density effects

We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single-asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements in the wearless regime are performed using both uncoated and OTS-coated silicon AFM tips in contact with both uncoated and OTS-coated silicon surfaces, providing four pairs of either self-mated or unmated interfaces. Striking differences in the frictional responses of the four pairs of interfaces are found. First, lower friction occurs with OTS present on either the tip or substrate, and friction is yet lower when OTS is present on both. Second, the shape of the friction versus load plot strongly depends on whether the substrate is coated with OTS, regardless of whether the tip is coated. Uncoated substrates exhibit the common sublinear dependence, consistent with friction being directly proportional to the area of contact. However, coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution to the frictional behavior of OTS. Direct in situ comparison of two intrinsic OTS structural phases on the substrate is also performed. We observe frictional contrast for different local molecular packing densities of the otherwise identical molecules. The phase with lower packing density exhibits higher friction, in agreement with related previous work, but decisively observed here in single, continuous images involving the same molecules. Lateral stiffness measurements show no distinction between the two OTS structural phases, demonstrating that the difference in friction is not due to divergent stiffnesses of the two phases. Therefore, the packing density directly affects the interface's intrinsic resistance to friction, that is, the interfacial shear strength.     

Ab initio calculation of the adhesion and ideal shear strength of planar diamond interfaces with different atomic structure and hydrogen coverage

We propose a method to calculate the ideal shear strength τ of two surfaces in contact by ab initio calculations. This quantity and the work of adhesion γ are the interfacial parameters usually derived from tip-based friction force measurements. We consider diamond interfaces and quantitatively evaluate the effects of surface orientation and passivation. We find that in the case of fully passivated interfaces, γ is not affected by the orientation and the alignment of the surfaces in contact. On the contrary, τ does show a dependence on the atomic-scale roughness of the interface. The surface termination has a major impact on the tribological properties of diamond. The presence of dangling bonds, even at concentrations low enough to prevent the formation of interfacial C-C bonds, causes an increase in the resistance to sliding by 2 orders of magnitude with respect to the fully hydrogenated case. We discuss our findings in relation to experimental observations.     

Quantitative measurement of friction between single microspheres by friction force microscopy

The sliding friction between single silica microspheres was examined by applying friction force microscopy to probe the interaction between spherical silica particles glued to a tipless atomic force microscopy (AFM) cantilever and another particle glued to a glass slide. A three-dimensional model handling the complex contact geometry between spherical particles was established to compute friction and normal forces at the sliding interface from measured deflections of the AFM cantilever. Results obtained at different loads show a linear relationship between friction and normal force, with a friction coefficient of 0.4 between silica spheres. Friction in this system occurs at multi-asperity contacts. The results show that the macroscopic friction law of Amontons can be used to model the friction behavior of micrometer-sized granular matter. For plasma-treated silica particles, increased friction as well as wear could be observed during sliding.     

pH dependence of friction forces between silica surfaces in solutions

The pH dependence of the friction between a silica particle and a silica wafer was investigated using lateral force microscopy. Measurements were done in the range of 3.6 < or = pH < or = 10.6 and the effect of high loading force was also examined. It is found that the friction is independent of the pH of solutions and increases linearly with the applied load, when the pH is between 3.6 and 8.6. On the other hand, once the pH is above 9.0, the friction becomes extremely small and the dependence on the applied load becomes nonlinear. It is postulated that this transition is due to the development of a gel layer composed of polymer-like segments of silicilic acid anchored on the surface; at the lower applied load, this layer acts as a boundary lubricant between the surfaces, but, at the higher applied load, the entanglements of these segments and more direct contact between two solid surfaces leads to the increase of the friction. The effects found here are expected to play an important role in elucidating the basic mechanism of the planarization process of silica wafers.     

Influence of the work of adhesion on the dynamic wetting of chemically heterogeneous surfaces

The velocity dependence of the dynamic contact angle for a glycerol-water mixture wetting two different chemically heterogeneous surfaces (mixed thiols on gold and partially methylated titania, 16 samples in all) was studied. The molecular kinetic theory (MKT) of wetting was used to interpret the dynamic contact angle data. The equilibrium displacement frequency ( K 0) was predominantly determined by the viscous contribution from the bulk liquid, with a minor contribution from the surface. The mean distance between surface sites (lambda) decreased with increasing work of adhesion. The contact line friction coefficient zeta 0 was found to vary exponentially with the work of adhesion, enabling the unit flow volume of the liquid to be obtained.     

Surface friction of hydrogels with well-defined polyelectrolyte brushes

Hydrogels of poly(2-hydroxyethyl methacrylate) (PHEMA) with well-defined polyelectrolyte brushes of poly(sodium 4-styrenesulfonate) (PNaSS) of various molecular weights were synthesized, keeping the distance between the polymer brushes constant at ca. 20 nm. The effect of polyelectrolyte brush length on the sliding friction against a glass plate, an electrorepulsive solid substrate, was investigated in water in a velocity range of 7.5 x 10(-5) to 7.5 x 10(-2) m/s. It is found that the presence of polymer brush can dramatically reduce the friction when the polymer brushes are short. With an increase in the length of the polymer brush, this drag reduction effect only works at a low sliding velocity, and the gel with long polymer brushes even shows a higher friction than that of a normal network gel at a high sliding velocity. The strong polymer length and sliding velocity dependence indicate a dynamic mechanism of the polymer brush effect.     

Influence of solvent environment and tip chemistry on the contact mechanics of tip-sample interactions in friction force microscopy of self-assembled monolayers of mercaptoundecanoic Acid and dodecanethiol

Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.     

An empirically validated analytical model of droplet dynamics in electrowetting on dielectric devices

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.     

Adhesion and stable low friction provided by a subnanometer-thick monolayer of a natural polysaccharide

Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick "polymer brush" layers.     

On scale dependence in friction: transition from intimate to monolayer-lubricated contact

Scale dependence in friction is studied in the present paper using the newly developed mesoscale friction tester (MFT). A transition in frictional shear strength from several hundreds of MPa to several tens of MPa was observed over a very limited range of contact radii (20-30 nm) in both ambient and dry environments. Thus, a single apparatus has been able to establish these two limits which are consistent with the values previously obtained from friction experiments using atomic force microscopy (AFM) and the surface force apparatus (SFA), respectively. Consequently, it is hypothesized here that a shear strength in the hundreds of MPa results from intimate contact (solid-solid) and a shear strength in the tens of MPa results from a monolayer-lubricated contact. Furthermore, both the probe size and the normal pressure govern the interfacial conditions in the contact zone and it is these conditions, rather than the nominal environment, which in turn determine the resulting shear strengths. A continuum analysis based on the Lifshitz theory for van der Waals interactions is used to explain the quantized shear strengths which were obtained from our experiments and previous AFM and SFA friction experiments. This quantized friction behavior [J.N. Israelachvili, P.M. McGuiggan, A.M. Homola, Science 240 (1988) 189] results from the discrete separation due to the different interfacial conditions that can arise between two sliding surfaces. The consistency between the analysis and the experimental results shows that this analysis is applicable for nonwear friction with single asperity contact.     

Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers

We present a quantitative study of the nanoscale frictional properties of one-component (pure) and two-component (mixed) alkylsilane self-assembled monolayers (SAMs). The load and velocity dependence of the friction force was measured in air and ethanol using lateral force microscopy (LFM). It was observed that for SAMs with well-ordered structure (pure SAMs and mixed SAMs composed of two long chain molecules) friction depends nonlinearly on load, at low loads, both in air and in ethanol. These observations are consistent with the low-load contact area predictions of the Johnson-Kendall-Roberts (JKR) theory, indicating that for well-ordered SAMs friction force is proportional to contact area and that the true contact area is determined by elastic deformation of the SAM by the LFM probe. In ambient air, the magnitude of the friction force measured using mixed SAMs is found to be similar to that obtained using pure SAMs at the same external load. Changing the medium to ethanol, however, leads to dramatically lower friction in the mixed SAMs. An analysis of the friction data using a thermally activated Eyring model that takes into account the monolayer viscoelasticity suggests that the better friction properties of the mixed SAMs are a consequence of greater disorder and higher molecular mobility in the outer layer/canopy. These findings indicate that multi-tiered SAM coatings comprising a highly ordered underlayer and a disordered, mobile canopy can provide the basis for low-friction coatings for small mechanical systems.     

Studying friction and shear fracture in thin confined films using a rotational shear apparatus

This paper describes the effects of the elastic modulus and sliding velocity on the friction and shear fracture of smooth silanized rigid disks rotating against thin confined films of poly(dimethylsiloxane) (PDMS) elastomers. A rigid glass disk is rotated against thin PDMS films of different thicknesses and moduli bonded to a glass plate at various speeds. While the disk rotates on the PDMS coated glass plate, a load cell measures the resulting force with a cantilever beam. One end of the cantilever beam is glued to the glass plate, while its other end presses against a load cell. From the balance of forces and torques, the friction force at a given slip velocity is determined. The friction force increases with the slip velocity sublinearly, which is consistent with the results reported previously by Vorvolakos and Chaudhury (Langmuir 2003, 19, 6778). During rotation, however, the glass disk comes off the PDMS film when the shear stress reaches a critical value. This critical shear stress increases with the modulus of the film, but it decreases with its thickness, following a square root relationship, which is similar to the adhesive fracture behavior in thin films under pull-off conditions. A simple model is presented that captures the essential physics of the fracture behavior under shear mode.