Friction coefficient dependence on electrostatic tribocharging - PubMed (original) (raw)

Friction coefficient dependence on electrostatic tribocharging

Thiago A L Burgo et al. Sci Rep. 2013.

Abstract

Friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The literature presents a wealth of information on the electronic contributions to friction in metals and semiconductors but the effect of triboelectricity on friction coefficients of dielectrics is as yet poorly defined and understood. In this work, friction coefficients were measured on tribocharged polytetrafluoroethylene (PTFE), using three different techniques. As a result, friction coefficients at the macro- and nanoscales increase many-fold when PTFE surfaces are tribocharged, but this effect is eliminated by silanization of glass spheres rolling on PTFE. In conclusion, tribocharging may supersede all other contributions to macro- and nanoscale friction coefficients in PTFE and probably in other insulating polymers.

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Figures

Figure 1

Figure 1. Determination of CoRR of glass beads rolling on tribocharged PTFE surfaces.

(a) The potential map for each plate used. (b) CoRR versus average surface potential of tribocharged PTFE plates. Vertical error bars are mean standard deviations from ten replicate measurements while the horizontal bars are standard deviations of average potential for all the pixels on each plate.

Figure 2

Figure 2. CoRR measurements of different glass beads.

Comparison between the effects of PTFE tribocharging on silanized and neat glass spheres. No tribocharging effect is observed on silanized spheres.

Figure 3

Figure 3. Triboelectrification of PTFE with rolling glass beads.

(a), (b) and (c) are the sum images of two subsequent frames (1 s apart) where the beads from the first frame are false-colored red, those from the second frame are green and the yellow areas are thus the result of particle superposition in two consecutive frames, revealing the slow-moving or immobilized particles. (d) Picture of glass beads after 300 minutes shaking over PTFE on a planetary table; (e) the respective electrostatic map, where the areas covered with beads are less negative than most bare areas. The electrostatic map shown in (f) was obtained by scanning the PTFE film but after removing the glass beads; potential on the bead-trapping areas is largely negative (<−3 kV) (See also Supplementary Video online).

Figure 4

Figure 4. Tribocharging effect on friction angles of PE pellets on PTFE.

(a) Control measurement using unshaken pellets (top) and the distribution of values obtained by averaging the results of 13 shaking runs using 30 pellets each (bottom). Potential maps of: (b) PE pellets on clean PTFE prior to shaking, (c) pellets shaken for 300 seconds on PTFE and (d) PTFE after removal of PE pellets. Error bars are standard deviations of the average.

Figure 5

Figure 5. Lateral Force Microscopy (LFM) of neutral and tribocharged PTFE sheets.

Topography (left) and lateral force images (right) obtained by LFM on (a) cleaned and (b), (c) tribocharged (average potential indicated on each sample, was measured with a macroscopic Kelvin probe) PTFE samples. Loop of friction signal profiles traced in LFM images are shown in (d). See also Supplementary Fig. S4.

Figure 6

Figure 6. Force-distance (Fd) curves on tribocharged PTFE.

Fd curves for approach and retraction of a silicon nitride tip from neat, uncharged PTFE and tribocharged PTFE. Average potential measured over the polymer with a macroscopic Kelvin electrode is −192 V.

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