Page 19 - Handbook of Adhesion Promoters

12 Mechanisms of Adhesion

Figure 2.8. (a) Typical schematic of the functional relationship between the adhesive force and the humidity, and
(b) anomalous schematic of the counter-intuitive monotonic decrease of the adhesion force with increasing
humidity. The inset shows the tip shape. [Adapted, by permission, from Yang, L; Hu, J; Xiao, H; Quan, W,
Particuology, in press, 2017.]
a small tip-substrate separation distance (less than 0.3 nm) with an increase in relative
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humidity (Figure 2.8).
It is known from practice that in autumn trains occasionally overrun station or pass
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signals, which are dangerous occurrences. Incident analysis showed that the incidents
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more likely occur in the morning (5:00-10:00) and at night (20:00-24:00). This sug-
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gested that the dew effect may be important. It was found that the wet leaves in the con-
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tact area with rails produce very low friction coefficients (below 0.1). Wet leaves tend to
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produce a low friction/traction coefficients of around 0.05. The effects of iron oxides,
temperature, pressure, and the presence of leaves were identified as the primary reasons
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for low traction.
Figure 2.9 illustrates the rea-
sons for inferior adhesion which
were suggested by Bikerman long
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time ago. (Interesting to note
that two of these reasons (impuri-
ties and reaction between compo-
nents) were mentioned in the
railway case problems.) These
inhibitors of adhesion are very
Figure 2.9. Model of weak boundary layer. [Adapted, by common in practice in the form of
permission, from Bikerman, JJ. The Science of Adhesive Joints. air pockets, dirt particles, oily sur-
Academic Press, New York, 1961.] face, and reaction products (corro-
sion, products of oxidation, products of degradation on wood surface, skin formation,
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etc.).
The examples given in this section may seem slightly remote from the main topic of
this chapter but they should not be rejected from consideration because they may help to
explain some practical observations other than included in this section.