Hydrophilic substrates, friction coefficients

Friction Coefficient (p) Values of PDMS A and B, in Contact with Hydrophobic and Hydrophilic Substrates for Different Friction Speeds and a Force Equal to 1 and 3 N (Ap = 0.02)... 

Significant differences between hydrophobic and hydrophilic substrates are observed at lower speed, with a low friction for hydrophobic wafer at low normal load. For example, as illnstrated in fig. 12.5 (presenting the friction coefficient of PDMS A versus speed), the friction coefficient of PDMS A (at 25 mm/min and 1N) is equal to 1.04 for the hydrophobic wafer and 1.58 for the hydrophilic wafer (factor equal to 1.5 between the two coefficients). However, when the friction speed is increased, the difference between the friction coefficients of hydrophilic and hydro-phobic systems becomes lower, and both coefficients are identical for high speed, with a value close to 1.3 (for a normal force of 1 N). 

The friction coefficient is increased as a function of speed for hydrophobic substrate and is decreased for hydrophilic substrate, as illustrated in figs. 12.5 and 12.6, which present the evolution of the friction coefficient as a function of speed for both substrates for PDMS A and B, respectively (at a normal force equal to 1 N). This... 

Higher shear rate and stress are indeed able to induce a chain orientation at the PDMS surface [24]. Such an orientation will probably modify the rheological behavior of the polymer interface, and the anisotropy of the confined interfacial layer is able to induce a lower shear resistance. Moreover, it is also necessary to interpret the speed dependence of the friction for both substrates, with an increase of the friction coefficient with speed for the hydrophobic wafer and a decrease for the hydrophilic one. 

First, the inflnence of the snbstrate chemistry was more significant compared with the case of PDMS, with PS friction coefficients close to 0.40 and 0.15, respectively, for hydrophilic and hydrophobic wafers. The friction coefficient of the hydrophilic wafer was always mnch greater than that of the hydrophobic one, whatever the experimental conditions (speed, normal force). Moreover, the effect of friction speed was totally different in the case of polystyrene compared to PDMS. A slight increase of the friction coefficient with speed conld indeed be observed for both types of substrates, attributed to viscoelastic effects. It was also shown that the normal force had a negligible influence. 

The tribological behavior of this glassy polymer is thus completely different compared with PDMS elastomers. The influence of the substrate chanistry on the friction of PDMS and PS is very different. The effect of interface chemistry on friction is thus not so evident. The friction coefficient of hydrophilic and hydrophobic substrates can be identical or different, depending on the experimental conditions. Friction speed also plays a major role through its influence on polymer interfacial rheology, especially in the case of soft polymers. 

The aim of this study was to illustrate the complex role of the interface through the study of PDMS friction in contact with both hydrophilic and hydrophobic silicon wafers. Experimental results have shown that the friction coefficient of hydrophilic substrates can be either greater than or similar to that of hydrophobic wafers, depending on the friction speed (and normal load). At low speed, a significant difference between the two substrates is observed, but the influence of the substrate chemistry becomes negligible at higher friction speed. 

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