Frictional response

Studying the friction of skin supplements other mechanical tests. Friction studies can be conducted with noninvasive methods and give a measure of the skin s health — skin hydration, for example Naylor1 showed that moistened skin has an elevated friction response and El-Shimi2 demonstrated that drier skin has a lowered friction response. Friction provides a quantitative measurement to assess skin. 

The frictional behaviour of polymers differs somewhat from that of perhaps more familiar materials. The frictional force tends to be proportional not to load (as in the classical case) but to speed. The coefficient of friction is very dependent on the nature of the two surfaces in contact, but is generally low, when suitable pairs are selected. This means that plastics gears can usually be run without external lubricants. Often the static friction coefficient is lower than the dynamic, which helps to explain the absence of slip-stick phenomenon exhibited by some plastics systems in motion this is especially marked with PTFE, which has an exceptionally low coefficient (around 0.02). The non-classical response of plastics materials results from their much lower modulus. Their frictional response is characterized by adhesion and deformation. 

In the surface-forces experiments reported here, PLL-g-PEG brushes showed a remarkably low frictional response to shear. Two opposing brushes of PLL(20)-g[2.9]-PEG(5) were able to glide against each other without measurable friction forces, even at considerable compressions. The increase in friction observed at compressions to less than 10% of the equilibrium film thickness can be attributed to an increase of viscosity when the solvent is squeezed out of the contact. 

Since the difference in the chemical composition of the terminal groups might be solely responsible for the increased frictional response [22-27], we considered exactly how this difference might influence the frictional response. First, differences in the... 

Figure 2. Frictional response of CF3-terminated ( ) and CH3-terminated ( ) tridecanethiol SAMs measured by AFM as a function of decreasing applied load.

Figure 11 shows friction versus load plots of CF3-, /-Pr-, and CH3-terminated SAMs. Like the CF3-terminated film, the /-Pr-terminated film exhibits a frictional response that is greater than that of the CH3-terminated film. Since the lattice spacing, conformational order, and chemical composition of the films are the same, this higher frictional response can be rationalized in terms of the additional steric interactions that accompany the larger size of the /-Pr group. The results are consistent with a model in which the frictional responses of the /-Pr and CF3-terminated films are influenced by the increased sizes of their terminal groups relative to those of a CH3-terminated film. Studies to further explore these systems are currently underway in our laboratories. 

The pull-off force, which is the negative load at which the sample loses contact with the tip, was taken as a measure of the adhesion interactions between the tip and sample. All measurements were conducted in air at ambient temperatures and humidities (24° C and 40%, respectively) using a silicon nitride AFM tip with a radius of 500 A and a normal spring constant of 0.58 N M" Due to the hydrophobicity of the two films, capilliarity effects on the measured pull-off forces and frictional responses are expected to be negligible [22-27]. 

We review the basic characteristics of frictional response within the framework of a model of a monolayer embedded between two plates one of which is externally driven. The model accounts for the coupled lateral and normal motions of the driven plate. We concentrate on two main aspects (a) the response of the embedded system to the external drive, and (b) how to modify and control the frictional force. For describing the response of the embedded system, we apply the concept of shearons, which are collective modes of well defined spatial/temporal patterns in the particles density. Shearons, which are shown to dominate the frictional response of the driven system, are found to be useful in tuning the frictional behavior and predicting new methods to control friction, in addition to the established mechanical ones. 

Fig. 8.7 The effect of a friction-modifying additive on the total engine friction response to oil temperature for an SAE 5W-30 engine oil (Courtesy of Texaco Inc.)...

The deformation or ploughing modes can also be well described for plastic and possibly even brittle fracture systems using modem numerical techniques. As with the elastomeric systems the models basically include geometric terms, such as 6, some load and various parameters such as an interface shear stress but more importantly a relatively accessible bulk deformation or dissipation property of the material. For the case of elastomers, an appropriate viscoelastic loss tangent is sufficient and for a ductile polymer some pressure dependent yield stress. There are many examples in the literature where good correlations have been obtained between a bulk mechanical test and a frictional response. Properly, it has been seen as the domain of others, perhaps polymer scientists, to seek to provide interrelationships between molecular structure and deformation dynamics and the consequent bulk material responses. 

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