Friction kinetic

Reiter G, Demirel A L and Graniok S 1994 From static to kinetic friction in confined liquid films Science 263 1741-4... 

In Figure 2-15a, a 10-lb block slides down a ramp inclined at an angle of 30°. If the coefficient of kinetic friction between the block and the ramp is 0.1, what will be the acceleration of the block. ... 

Friction is the resistance against change in the relative positions of two bodies touching one another. If the area of contact is a plane, the relative motion will be a sliding one and the resistance will be called sliding or kinetic friction. If the material in the area of contact is loaded beyond its strength, abrasion or wear will take place. Both phenomena are affected by numerous factors such as the load, relative velocity, temperature, and type material. 

Kinetic friction It is the friction developed between two bodies in motion. 

Kinetic friction in the quasistatic limit of v 0 exhibits a similar dependence on a and b. Friction disappears if the strength b is less than a critical value bf, but above this threshold it is the instability similar to that discussed in the... 

Tomlinson model that results in a finite kinetic friction. 

At finite velocity kinetic friction behaves quite differently in the sense that the commensurability plays a less significant role. Besides, the system shows rich dynamic properties since Eq (16) may lead to periodic, quasi-periodic, or chaotic solutions, depending on damping coefficient y and interaction strength h. Based on numerical results of an incommensurate case [18,19], we outline a force curve of F in Fig. 23 asafunction ofv, in hopes of gaining a better understanding of dynamic behavior in the F-K model. 

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 friction and wear properties of fullerene LB fihns have been investigated. The coefficient of kinetic friction was measured using a steel ball-on-glass disk method, with the LB films deposited onto the glass disk [326,327]. The friction coefficient dropped from 0.8... 

Static friction is the force which must be overcome in order to start the motion of a body over a surface. Kinetic friction is the force which must be continuously overcome in order to maintain uniform motion of the body over the surface. The coefficient of friction (static or kinetic respectively) is the frictional force divided by the perpendicular or normal force between the surfaces in contact. 

Kinetically controlled epoxy curing reactions, 10 423 Kinetic barriers, 11 529 Kinetic friction, 15 224 Kinetic incompatibility, in acrylonitrile copolymerization, 11 203 Kinetic measurements, 14 607-629. See also Very fast kinetics combined methods for unstable reagents, 14 621... 

The dWi are Gaussian white noise processes, and their strength a is related to the kinetic friction y through the fluctuation-dissipation relation.72 When deriving integrators for these methods, one has to be careful to take into account the special character of the random forces employed in these simulations.73 A variant of the velocity Verlet method, including a stochastic dynamics treatment of constraints, can be found in Ref. 74. The stochastic... 

Once temperature comes into play, the jumps of atoms between minima may be invoked prematurely, i.e., before the formation of instabilities, via thermal fluctuations. These thermally activated jumps decrease the force that is required to pull the surface atom, which leads to a decrease in the kinetic friction. The probability that a jump will be thermally activated is exponentially related to the energetic barrier of the associated process, which can be understood in terms of Eyring theory. In general, the energetic barriers are lower when the system is not at its thermal equilibrium position, which is a scenario that is more prominent at higher sliding velocities. Overall, this renders Fk rate or velocity dependent, typically in the following form ... 

Figure 5 Typical velocity relationship of kinetic friction for a sliding contact in which friction is from adsorbed layers confined between two incommensurate walls. The kinetic friction F is normalized by the static friction Fs. At extremely small velocities v, the confined layer is close to thermal equilibrium and, consequently, F is linear in v, as to be expected from linear response theory. In an intermediate velocity regime, the velocity dependence of F is logarithmic. Instabilities or pops of the atoms can be thermally activated. At large velocities, the surface moves too quickly for thermal effects to play a role. Time-temperature superposition could be applied. All data were scaled to one reference temperature. Reprinted with permission from Ref. 25. 

The kinetic friction Fy is also affected by commensurability. If two crystalline surfaces are separated by one atomic layer only, Fy may actually be reduced because of commensurability, although static friction is increased.25 The strikingly different behavior for commensurate and incommensurate systems is demonstrated in Figure 6. 

Figure 18 Coverage dependence of the kinetic friction coefficient pk of a system containing 0.25-2.5 monolayers of a simple fluid. Commensurate systems (c) are denoted with open symbols, and incommensurate systems (ic) are designated with closed symbols. Reproduced with permission from Ref. 82.

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]. 

Monolayer Slider/Substrate Coefficient of kinetic friction, fik Ref. 

Early in the history of polytetrafluoroethylene, W. A. Zisman recognized its unusual surface properties, and the polymer now finds many uses because its low coefficient of friction eliminates the need for lubrication (Fitzsimmons and Zisman). Shooter and Thomas called attention to the remarkable resistance of polytetrafluoroethylene to seizure, and Bowden (1950) described use of composite structures. These and other investigations have been reviewed and summarized by Allan (1958) (also Allan and Chapman), who showed that the dependence of the coefficient of kinetic friction, fk, on the load in grams, W, is given by the following equation ... 

Koberstein and coworkers121 have examined the effects of a polydimethylsiloxane-polystyrene (PDMS-PS) block copolymer on the interfacial tensions of blends of PDMS and polystyrene. As little as 0.002 wt% of the copolymer, added to the siloxane phase, was sufficient to lower the interfacial tension by 82% in the case of a blend of polystyrene (Afn = 4,000) and PDMS (Mn = 4,500). No further reduction in interfacial tension was observed at higher copolymer levels due to micelle formation. Riess122 has polymerized styrene in the presence of a silicon oil and a polydimethylsiloxane-polystyrene block copolymer to obtain a polystyrene in which 0.1-1 pm droplets of silicone oil are dispersed. This material displayed a lowered coefficient of kinetic friction on steel compared to pure polystyrene. 

It is useful to differentiate between static and dynamic friction. Dynamic friction, also called kinetic friction, is the mechanical force between sliding or rolling surfaces that resists the movement. Static friction must be overcome to start the movement between two bodies which are initially at rest. 

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