scholarly journals Squeezing and stick–slip friction behaviors of lubricants in boundary lubrication

2018 ◽  
Vol 115 (26) ◽  
pp. 6560-6565 ◽  
Author(s):  
Rong-Guang Xu ◽  
Yongsheng Leng
Keyword(s):  
Phase Transition ◽  
Boundary Lubrication ◽  
Sliding Friction ◽  
Surface Force ◽  
Lubricant Film ◽  
Force Balance ◽  
Surface Boundary ◽  
Stick Slip ◽  
Squeeze Out ◽  
Dynamics Simulations

The fundamental questions of how lubricant molecules organize into a layered structure under nanometers confinement and what is the interplay between layering and friction are still not well answered in the field of nanotribology. While the phase transition of lubricants during a squeeze-out process under compression is a long-standing controversial debate (i.e., liquid-like to solid-like phase transition versus amorphous glass-like transition), recent different interpretations to the stick–slip friction of lubricants in boundary lubrication present new challenges in this field. We carry out molecular dynamics simulations of a model lubricant film (cyclohexane) confined between molecularly smooth surfaces (mica)––a prototypical model system studied in surface force apparatus or surface force balance experiments. Through fully atomistic simulations, we find that repulsive force between two solid surfaces starts at about seven lubricant layers (n= 7) and the lubricant film undergoes a sudden liquid-like to solid-like phase transition atn< 6 monolayers thickness. Shear of solidified lubricant films at three- or four-monolayer thickness results in stick–slip friction. The sliding friction simulation shows that instead of shear melting of the film during the slip of the surface, boundary slips at solid–lubricant interfaces happen, while the solidified structure of the lubricant film is well maintained during repeated stick–slip friction cycles. Moreover, no dilation of the lubricant film during the slip is observed, which is surprisingly consistent with recent surface force balance experimental measurements.

scholarly journals On the question of whether lubricants fluidize in stick–slip friction

2015 ◽  
Vol 112 (23) ◽  
pp. 7117-7122 ◽  
Author(s):  
Irit Rosenhek-Goldian ◽  
Nir Kampf ◽  
Arie Yeredor ◽  
Jacob Klein
Keyword(s):  
Surface Force ◽  
Lubricant Film ◽  
Force Balance ◽  
Smooth Surfaces ◽  
Stick Slip ◽  
Lubricant Films ◽  
Slip Event ◽  
Stick Slip Motion ◽  
Molecular Layers ◽  
Slip Motion

Intermittent sliding (stick–slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick–slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4–0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.

scholarly journals PHASE DYNAMICS AND KINETICS OF THIN LUBRICANT FILM DRIVEN BY CORRELATED TEMPERATURE FLUCTUATIONS

2007 ◽  
Vol 07 (02) ◽  
pp. L111-L133 ◽  
Author(s):  
ALEXEI V. KHOMENKO ◽  
IAKOV A. LYASHENKO
Keyword(s):  
Sliding Friction ◽  
Temperature Fluctuations ◽  
Lubricant Film ◽  
Stick Slip ◽  
Flat Surfaces ◽  
Phase Dynamics ◽  
Damped Oscillations ◽  
Atomically Flat ◽  
Kinetics Of ◽  
Stable Sliding

The melting of an ultrathin lubricant film is studied at friction between atomically flat surfaces. We take into account fluctuations of lubricant temperature, which are defined by the Ornstein-Uhlenbeck process. Phase diagrams and portraits are calculated for second- and first-order transitions (the melting of an amorphous and that of a crystalline lubricants, respectively). It is shown that, in the former case, a stick-slip friction domain, separating the regions of dry and sliding friction, appears. In the latter case, three domains of stick-slip friction arise, which are characterized by the transitions between dry, metastable and stable sliding friction. The increase in the correlation time of lubricant temperature fluctuations leads to increasing in the rubbing-surface temperature needed for realization of sliding friction. The stationary states, corresponding to dry, stable and metastable sliding friction, are reached as a result of damped oscillations.

Atomistic Simulations of Friction at Sliding Diamond Interfaces

MRS Bulletin ◽  
1993 ◽  
Vol 18 (5) ◽  
pp. 50-53 ◽  
Author(s):  
Judith A. Harrison ◽  
Carter T. White ◽  
Richard J. Colton ◽  
Donald W. Brenner
Keyword(s):  
Sliding Friction ◽  
Surface Force ◽  
Theoretical Models ◽  
Theoretical Work ◽  
Atomic Scale ◽  
Related Phenomenon ◽  
Scientific Instrumentation ◽  
Adsorbed Films ◽  
Dynamics Simulations ◽  
Analytic Models

Friction, or the resistance to motion of two bodies in contact, and the related phenomenon of wear are two of the more costly problems facing industry today. Despite their importance, a fundamental understanding of friction and wear, especially at the atomic scale, has remained elusive. This is rapidly changing, however, as new scientific instrumentation has been developed that allows, for the first time, the study of friction at the atomic scale. These pioneering efforts have led to the emergence of a rapidly growing field called nanotribology, the subject of this issue of the MRS Bulletin. Some of the contributing techniques include the surface force apparatus, which has been used to study the rheology of molecularly thin liquid layers, a quartz-crystal microbalance, which has been used to measure the sliding friction of molecularly thin adsorbed films, and the atomic force microscope (AFM), which has been used to measure the frictional force between a sharp tip (possibly a single asperity) and a flat surface during sliding. In addition to providing a vast amount of information related to friction on the atomic scale, these innovative experiments have provided the necessary data to test the validity of older theoretical models and have stimulated new theoretical work. For instance, atomic-scale friction has been investigated theoretically using analytic models, first principles calculations, and molecular dynamics simulations.

scholarly journals The friction of lubricated metals

1940 ◽  
Vol 239 (799) ◽  
pp. 1-27 ◽  
Keyword(s):  
Boundary Lubrication ◽  
Single Layer ◽  
Short Chain Fatty Acids ◽  
Lubricant Film ◽  
Surface Phenomenon ◽  
Surface Films ◽  
Metallic Surfaces ◽  
Stick Slip ◽  
Mineral Oils ◽  
Frictional Behaviour

An analysis has been made of the kinetic friction between metals sliding under conditions of boundary lubrication. With mineral oils and many other lubricants an intermittent clutching and breaking away of the surfaces still occurs through the oil film. The friction, the surface temperature and the area of contact all show violent fluctuations and the behaviour may be essentially the same as with unlubricated metals. Certain substances, however, are able to prevent this “ stick-slip” motion and allow continuous sliding to take place. With short-chain fatty acids, for example, the motion is “ stick-slip”, but when the chain reaches a certain length continuous sliding occurs. Alcohols and saturated hydrocarbons of the same, or longer, chain length do not cause continuous sliding. Even with the best lubricant the film breaks down to some extent during sliding and some wear of the surfaces takes place. The metal is torn to a depth which is large compared with the dimensions of a molecule. The frictional force between lubricated metals must therefore be greatly influenced by the bulk properties of the metals concerned. The frictional behaviour of metallic surfaces covered with successive monolayers of lubricant has also been investigated. A single layer can cause a large reduction in the friction but the film is soon worn away. With multilayers the rate at which the film wears off is markedly dependent on its molecular thickness and methods are described for measuring the rate of wear of lubricant films. A single film of long-chain fatty acid molecules is more effective than a single film of the flat leaf-shaped cholesterol molecule. It is clear that a primary film is not sufficient, but that for effective boundary lubrication it is necessary to have present a layer of lubricant several molecules thick. The experiments show that boundary lubrication cannot be regarded as a purely surface phenomenon. On the basis of these experiments a theory has been put forward to explain boundary lubrication. In general it appears that even with lubricated surfaces the local pressures in the region of contact are very high, so that the lubricant film between the surfaces is partly broken down. If the sliding speeds are appreciable this breakdown is aided by the local high temperatures. Metallic junctions, the size of which is large compared with the dimensions of a molecule, are formed between the surfaces. There will, of course, be some resistance due to the interaction of the surface films themselves, but under many conditions of sliding the resistance to motion is due mainly to the force necessary to break the junctions. The frictional behaviour of boundary lubricated surfaces is therefore largely governed by the extent to which the lubricant film breaks down during sliding.

scholarly journals The Role of Hyaluronic Acid in Cartilage Boundary Lubrication

Cells ◽  
2020 ◽  
Vol 9 (7) ◽  
pp. 1606 ◽  
Author(s):  
Weifeng Lin ◽  
Zhang Liu ◽  
Nir Kampf ◽  
Jacob Klein
Keyword(s):  
Hyaluronic Acid ◽  
Synovial Fluid ◽  
Surface Force ◽  
Force Balance ◽  
New Paradigm ◽  
Low Friction ◽  
Cartilage Surface ◽  
Boundary Lubricant ◽  
Lipid Layers

Hydration lubrication has emerged as a new paradigm for lubrication in aqueous and biological media, accounting especially for the extremely low friction (friction coefficients down to 0.001) of articular cartilage lubrication in joints. Among the ensemble of molecules acting in the joint, phosphatidylcholine (PC) lipids have been proposed as the key molecules forming, in a complex with other molecules including hyaluronic acid (HA), a robust layer on the outer surface of the cartilage. HA, ubiquitous in synovial joints, is not in itself a good boundary lubricant, but binds the PC lipids at the cartilage surface; these, in turn, massively reduce the friction via hydration lubrication at their exposed, highly hydrated phosphocholine headgroups. An important unresolved issue in this scenario is why the free HA molecules in the synovial fluid do not suppress the lubricity by adsorbing simultaneously to the opposing lipid layers, i.e., forming an adhesive, dissipative bridge between them, as they slide past each other during joint articulation. To address this question, we directly examined the friction between two hydrogenated soy PC (HSPC) lipid layers (in the form of liposomes) immersed in HA solution or two palmitoyl–oleoyl PC (POPC) lipid layers across HA–POPC solution using a surface force balance (SFB). The results show, clearly and surprisingly, that HA addition does not affect the outstanding lubrication provided by the PC lipid layers. A possible mechanism indicated by our data that may account for this is that multiple lipid layers form on each cartilage surface, so that the slip plane may move from the midplane between the opposing surfaces, which is bridged by the HA, to an HA-free interface within a multilayer, where hydration lubrication is freely active. Another possibility suggested by our model experiments is that lipids in synovial fluid may complex with HA, thereby inhibiting the HA molecules from adhering to the lipids on the cartilage surfaces.

GRANULAR FAILURE: THE ORIGIN OF EARTHQUAKES?

2009 ◽  
Vol 23 (28n29) ◽  
pp. 5374-5382 ◽  
Author(s):  
MASSIMO PICA CIAMARRA ◽  
LUCILLA DE ARCANGELIS ◽  
EUGENIO LIPPIELLO ◽  
CATALDO GODANO
Keyword(s):  
Confining Pressure ◽  
Stick Slip ◽  
Slip Regime ◽  
Constant Rate ◽  
Large Fluctuations ◽  
System Flow ◽  
System Phase Diagram ◽  
Dynamics Simulations ◽  
Stick Slip Motion ◽  
System Phase

Via Molecular Dynamics simulations, we investigate the stick-slip motion in a model of fault, where two surfaces subject to a constant confining pressure P, and enclosing granular particles, are subject a shear stress σ. When the system sticks, the stress increases with a constant rate [Formula: see text], while the stress decreases when the system flow. We dermine the system 'phase diagram' in the pressure P load velocity [Formula: see text] plane, locating the transition form the continuos flow regime to the stick-slip regimes, and show that the transition between these two regimes is characterized by the presence of large fluctuations. In the stick-slip regime, the system reproduces the behaviour of a segment of a fault of fixed lenght.

Modifying surface forces through control of surface potentials

2017 ◽  
Vol 199 ◽  
pp. 261-277 ◽  
Author(s):  
Ran Tivony ◽  
Jacob Klein
Keyword(s):  
Surface Potential ◽  
Electrolyte Solution ◽  
Strong Electric Field ◽  
Salt Water ◽  
Surface Force ◽  
Adhesion Energy ◽  
Force Balance ◽  
Electrostatic Energy ◽  
Added Salt ◽  
Vdw Interaction

Combining direct surface force measurements with in situ regulation of surface potential provides an exceptional opportunity for investigating and manipulating interfacial phenomena. Recently, we studied the interaction between gold and mica surfaces in water with no added salt, while controlling the metal potential, and found that the surface charge at the metal may vary, and possibly even change its sign, as it progressively approaches the (constant-charge) mica surface [Langmuir, 2015, 31(47), 12845–12849]. Such a variation was found to directly affect the nature of the contact and adhesion between them due to exclusion of all mobile counterions from the intersurface gap. In this work, we extend this to examine the potential-dependent response of the adhesion and interaction between gold and mica to externally applied voltages and in electrolyte solution. Using a surface force balance (SFB) combined with a three-electrode electrochemical cell, we measured the normal interaction between gold and mica under surface potential regulation, revealing three interaction regimes – pure attraction, non-monotonic interaction from electrostatic repulsion to attraction (owing to charge inversion) and pure repulsion. Accordingly, the adhesion energy between the surfaces was found to vary both in no added salt water and, more strongly, in electrolyte solution. We justify this potential-dependent variation of adhesion energy in terms of the interplay between electrostatic energy and van der Waals (vdW) interaction at contact, and attribute the difference between the two cases to the weaker vdW interaction in electrolyte solution. Finally, we showed that through abruptly altering the gold surface potential from negative to positive and vice versa, the adhesion between gold and mica can be reversibly switched on and off. We surmise that the process of bringing the surface into contact is associated with the formation of a strong electric field O (108 V m−1) in the intersurface gap.

Frictional Behavior of Textile Fabrics Part I: Sliding Phenomena of Fabrics on Metallic and Polymeric Solid Surfaces

1997 ◽  
Vol 67 (11) ◽  
pp. 793-802 ◽  
Author(s):  
Luis Virto ◽  
Arun Naik
Keyword(s):  
Sliding Friction ◽  
Solid Material ◽  
Solid Surfaces ◽  
Friction Behavior ◽  
Stick Slip ◽  
Compression Load ◽  
Physical Description ◽  
Frictional Behavior ◽  
Textile Fabrics ◽  
Polymeric Solid

This paper presents experimental results on the sliding of fabrics on metallic and polymeric solid surfaces, showing the influence of the compression load at the solid-fabric interface and the nature of the solid material, and the effect of sliding speed on the sliding friction coefficient. At the same time, a physical description of the sliding phenomenon is given. On the basis of these observations, a theoretical approach is developed to explain the sliding friction behavior of fabrics on solid surfaces. Part II will deal with the waving and stick-slip phenomena, which are evident in the sliding process under certain conditions.

scholarly journals Predicting the structures and associated phase transition mechanisms in disordered crystals via a combination of experimental and theoretical methods

2018 ◽  
Vol 211 ◽  
pp. 425-439 ◽  
Author(s):  
Michael T. Ruggiero ◽  
Johanna Kölbel ◽  
Qi Li ◽  
J. Axel Zeitler
Keyword(s):  
Phase Transition ◽  
Solid State ◽  
Time Domain ◽  
Density Functional ◽  
Molecular Crystals ◽  
Terahertz Time Domain Spectroscopy ◽  
Functional Theory ◽  
Theoretical Methods ◽  
Dynamics Simulations ◽  
Transformation Processes

Experimental terahertz time-domain spectroscopy and theoretical solid-state ab initio density functional theory and molecular dynamics simulations are used to elucidate the structures, dynamics, and phase transformation processes of molecular crystals undergoing a solid-state order–disorder transition.

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