Stick-slip experiment

The factor that is least controllable in the deduction of static and kinetic friction values from stick-slip experiments is the condition of the contacting surfaces. This explains why Brockley and Davis [20] in their study of the influence of the time of contact on were unable to obtain repeatable results, as shown in Fig. 8-15a. For each series of determinations made with a single placement of the rider on the track, the plot of against the time of quiescent contact shows satisfactory self-consistency. But when the experiment is repeated with a fresh placement of the rider on the track, a different curve is obtained, self-consistent but not a duplicate of the previous experiment. This was traced to the variability of the surface with location on the rubbing track. Furthermore, it was demonstrated that a model of time-dependent junction growth could be made to yield the following expression ... 

Figure 12. Image (8 x 4 pm) of the polymeric surface after a stick-slip experiment realised at 25 nm s The grey scale represents the topogrs hy.

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]. 

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... 

The initial stages of the STM experiment require the positioning of the tip in proximity of the surface such that a tunnelling current can be detected this often means moving the tip by several micrometres or even millimetres. The piezoelectric materials used for scanning are not suitable for this initial approach and most instruments therefore contain a second coarse positioning driver frequently this is also a piezoelectric material in a stick-slip kind of design.27... 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

F. Heslot, T. Baumberger, B. Caroli, and C Caroli, Creep, Stick-slip and Dry Friction Dynamics Experiments and a Heuristic Model, Phys. Rev., E49, 4973 (1994). 

The only observation of a stick-slip transition in a simple shear geometry is the unique experimental study of Laun [39]. This controlled stress experiment not only observed a stick-slip transition but also explicitly recorded the time scale (a few milliseconds) on which the boundary condition (BC) evolved from... 

Thus Eq. (9) shows that the number density v of chains entangled with the loops in Fig. 5c would indeed have an Mw 11 5 dependence. When each of these adsorbed chains experiences a critical force of Fe they become disentangled and a stick-slip transition follows as observed in Fig. 12. The molecular weight dependence of the critical stress given by the combination of Eqs. (6) and (9) explains the experimental finding [27] for linear polyethylenes. ... 

In the SFA experiments there is no way to determine whether shear occurs primarily within the film or is localized at the interface. The assumption, made by experimentalists, of a no-slip flow boundary condition is invalid when shear localizes at the interface. It has also not been possible to examine structural changes in shearing films directly. MD simulations offer a way to study these properties. Simulations allow one to study viscosity profiles of fluids across the slab [21], local effective viscosity inside the solid-fluid interface and in the middle part of the film [28], and actual viscosity of confined fluids [29]. Manias et al. [28] found that nearly all the shear thinning takes place inside the adsorbed layer, whereas the response of the whole film is the weighted average of the viscosity in the middle and inside the interface. Furthermore, MD simulations also allow one to examine the structures of thin films during a shear process, resulting in an atomic-scale explanation [12] of the stick-slip phenomena observed in SFA experiments of boundary lubrication [7]. 

Sliding of the Steel Sphere on Flat PET Surfaces. Figure 2 shows typical friction traces in the sliding of a steel sphere at a speed 0,25 mm/s under a load 8 N. It is seen that the static friction is considerably higher than the kinetic friction and there is no stick-slip phenomenon. Comparing the friction of PET with that of other polymers obtained in the sliding experiment ( ) similar to that in the present work, it was found that PET exhibited relatively lower... 

There is another class of frictional interaction that we might call semi-deterministic. These actually behave like a hybrid between stick/slip oscillations and random peak/valley fiiction interactions. Most of these semi-deterministic interactions are caused by human-made objects, created through intentional engineering of the surface features of these objects. Essentially any surface that has a deterministic (especially periodic) structure has the potential to slide with a quasi-periodic motion. Some obvious examples of these come from our driving experience, like the ka-dunk, ka-dunk... sound produced by the seams between freeway concrete tiles, or the scored pavement patterns (or Botts dots) placed by the road edges to tell us we re straying from the allowed driving surface. 

The finite element model described in Section 11.2 was used here to model the friction experiments described above. However, to simulate the 16 Nm torque, a bolt pre-stress of 227 MPa was applied. This value was obtained experimentally from the axial gauges in the shank of a specially manufactured instmmented bolt, as discussed previously. For comparison, both the continuous and stick-slip [25] fiiction models (available in MSC Marc finite element code) was used to account for fiiction between the contacting interfaces. The fiiction coefficients were chosen to be 0.1, 0.3 and 0.45 between the bolt/laminate, washer/laminate and laminate/laminate interfaces, respectively. More details on fiiction coefficient selection can be found in [17]. 

The load-deflection response from both friction models (i.e. the continuous and stick-slip friction models) are shown in Figure 11.11 together with the experimental curve. Both friction models represent the behaviour in the experiment reasonably well. The sharp change from static fiiction Slope 1) to kinetic fiiction transition region) is slightly better represented by the stick-slip friction model. This is because... 

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