Stick-slip friction

Figure 11.3 Example of a system that exhibits stick-slip friction. Stick-slip is also illustrated as a schematic plot of spring elongation versus time at constant pulling speed v.

Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57]. 

Discuss why stick-slip friction is favored if fi decreases with sliding speed. 

Foam rheology has been a challenging area of research of interest for the yield behavior and stick-slip flow behavior (see the review by Kraynik [229]). Recent studies by Durian and co-workers combine simulations [230] and a dynamic light scattering technique suited to turbid systems [231], diffusing wave spectroscopy (DWS), to characterize coarsening and shear-induced rearrangements in foams. The dynamics follow stick-slip behavior similar to that found in earthquake faults and friction (see Section XU-2D). 

Yoshizawa FI and Israelaohvili J N 1993 Fundamental mechanisms of interfacial friction. 2. Stick-slip friction of spherical and chain molecules J. Phys. Chem. 97 11 300-13... 

The use of hydrostatic slideways - in which pressurized oil or air is employed - completely eliminates stick-slip and reduces friction to very low values but there are disadvantages in the form of higher costs and greater complication. 

The solidihed layer yields and returns to the liquid phase if the shear stress excesses the critical value, which initiates the sliding. When the stress is relaxed as a result of slip, the solid phase resumes again. The periodic transition between the solid and liquid states has been interpreted in the literature as a major cause of the stick-slip motion in lubricated sliding. Understanding the stick-slip and static friction in terms of solid-liquid transitions in thin films makes a re-... 

The dependence of friction on sliding velocity is more complicated. Apparent stick-slip motions between SAM covered mica surfaces were observed at the low velocity region, which would disappear when the sliding velocity excesses a certain threshold [35]. In AFM experiments when the tip scanned over the monolayers at low speeds, friction force was reported to increase with the logarithm of the velocity, which is similar to that observed when the tip scans on smooth substrates. This is interpreted in terms of thermal activation that results in depinning of interfacial atoms in case that the potential barrier becomes small [36]. 

Fig. 19—Shear stress and chain angle as a function of sliding distance, from simulations of alkanethiolates on Au(111) at temperature 1 K (a) results from commensurate sliding show a stick-slip motion with a period of 2.5 A, (b) in incommensurate case both shear stress and chain angle exhibit random fluctuations with a much smaller average friction [45],...

In the studies that attribute the boundary friction to confined liquid, on the other hand, the interests are mostly in understanding the role of the spatial arrangement of lubricant molecules, e.g., the molecular ordering and transitions among solid, liquid, and amorphous states. It has been proposed in the models of confined liquid, for example, that a periodic phase transition of lubricant between frozen and melting states, which can be detected in the process of sliding, is responsible for the occurrence of the stick-slip motions, but this model is unable to explain how the chemical natures of lubricant molecules would change the performance of boundary lubrication. 

Berman, A. D., Ducker, W. A., and IsraelachivUi, J. N., "Experimental and Theoretical Investigations of Stick-Slip Friction Mechnisms," Physics of Friction, B. N. J. Persson and E. Tosatti, Eds., Kluwer Academic Publishers, 1996, pp. 51-67. 

It has been recognized that the behavior of atomic friction, such as stick-slip, creep, and velocity dependence, can be understood in terms of the energy structure of multistable states and noise activated motion. Noises like thermal activities may cause the atom to jump even before AUq becomes zero, but the time when the atom is activated depends on sliding velocity in such a way that for a given energy barrier, AI/q the probability of activation increases with decreasing velocity. It has been demonstrated [14] that the mechanism of noise activation leads to "the velocity... 

In summary, sliding can be regarded as a process during which interfacial atoms would experience a series of stick-slip motions, similar to the jump in and out in the adhesion case, and it is the energy loss in this approach/separation cycle that determines the level of friction. 

From the point of view of system d5mamics, the transition from rest to sliding observed in static friction originates from the same mechanism as the stick-slip transition in kinetic friction, which is schematically shown in Fig. 31. The surfaces at rest are in stable equilibrium where interfacial atoms sit in energy minima. As lateral force on one of the surfaces increases (loading), the system experiences a similar process as to what happens in the stick phase that the surface... 

So far we have compared the static friction with the stick-slip transition. In both cases the system has to choose between the states of rest and motion, depending on which one is more favorable to the energy minimization. On the other hand, the differences between the two processes deserve a discussion, too. In stick-slip, when the moving surface slides in an average velocity V, there is a characteristic time, t =d.Ql V, that defines how long the two surfaces can... 

The mechanisms of static friction and stick-slip motion, as discussed in the last section, are supposed to be a good description of dry friction. Another case, perhaps more general in engineering practices, to be addressed in this section is lubricated sliding where liquid lubricant, consisting of a few molecule layers, is confined between two solid walls. Both experimental and theoretical studies indicate, as we have discussed in Chapter 5, that there are substantial changes in rheology of the confined lubricant, and the liquid may transit practically to a solid-like state when film thickness becomes molecularly thin [32,33]. 

In static friction, the change of state from rest to motion is caused by the same mechanism as the stick-slip transition. The creation of static friction is in fact a matter of choice of system state for a more stable and favorable energy condition, and thus does not have to be interpreted in terms of plastic deformation and shear of materials at adhesive junctions. 

Zhang, T., Wang, H., and Hu, Y. Z., Atomic Stick-Slip Friction Between Commensurate Self-Assembled Monolayers," Tribology Letters, Vol. 14, No. 2,2003, pp. 69-76. 

McGuiggan, P. M. (2008) Stick slip contact mechanics between dissimilar materials Effect of charging and large friction. Langmuir, 24, 3970-3976. 

In their study, Park et al.100 investigated the frictional properties of fluorine-terminated alkanethiol SAMs grafted to gold surfaces. The frictional properties of the system were investigated by sliding two SAMs past one another at velocities in the stick-slip regime under various external loads. The simulations yield the shear stress as and the kinetic friction coefficient pk can be estimated from the slope of a plot of as versus load, using the relationships contained in Eqs. [4] and [5]. 

Friction and Atomic Level Stick-Slip Motion. 

Transition from Stick-Slip to Continuous Sliding in Atomic Friction Entering a New Regime of Ultralow Friction. 

The frictional properties of PTFE arc unique Its unusually low static coefficient of friction decreases with increasing load and is lower than the dynamic coefficient of friction. This precludes stick-slip behavior. The low surface energy also prevents wetting by liquids other than low-surface-tension fluids like fluorocarbons. 

Differences in the frictional properties of most plastics can be explained in terms of the ratio of shear strenghth to hardness. Shooter and Tabor observed that the coefficients of friction for polytetrafluoroethylene are 2—3 times lower than anticipated by this calculation. It is believed that this discrepancy is caused by the inherently low cohesive forces between adjacent polymer chains and is responsible for the absence of stick-slip. The large fluorine atoms effectively screen the large carbon-fluorine dipole, reducing molecular cohesion so that the shear force at the interface is low. The shear strength of the bulk material is higher because of interlocking molecular chains. 

In technical applications, stick-slip friction is detrimental in terms of wear, vibrations, and precision of movement. Stick-slip ... 

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