Friction is the force experienced when two surfaces slide, or try to slide, across each other. We are thankful for friction when we put our foot on the brake to stop the car, or when we are walking down the sidewalk. However, friction is often seen as a negative. Friction causes wasted energy, the need for more force to get the work done , and machines to lose their effectiveness.
The loss of friction creates problems all around the world, such as those experienced by the rail system in the United Kingdom. Seasonal delays occur on a regular basis, causing interruptions and problems all along the line. Consequently, many individuals do not use the rail system, feeling it more hassle than it is worth.
When the idea of replacing horses with steam engines running on metal rails came in, engineers were not sure if the wheels could grip the rails with enough friction to travel up the hills and down into the valleys. Experiments quickly revealed that if the train was heavy enough, a locomotive with smooth steel wheels could pull a large load. Since then, the friction coefficient between the trains’ wheels and the rails has been a top concern for railway engineers. The friction coefficient determines how quickly the train can accelerate, the distance required for the train to come to a complete stop, and, most importantly, how many passengers and goods the trains can safely carry.
Dealing with the problems with the rail system in the United Kingdom involved observations and testing by many different groups and individuals. The conclusion was that somehow the leaves falling on the tracks caused a decrease in the amount of friction between the wheels and the rails. For obvious safety reasons, less friction means the trains must travel at a slower speed, thus causing the delays and headaches for everyone involved. The trains’ wheels crush the leaves as they fall on the tracks, forming a black layer of material that covers the tracks, which is as slippery as ice, which causes the decrease in friction between the tracks and the wheels of the trains.
Researchers Michael Watson, Benjamin White, Joseph Lanigan, Tom Slatter, and Roger Lewis resolved to solve the problem created by the black layer by determining its composition, how it is formed, and why it is so slippery. The team tested several hypotheses found in literature regarding the composition of the black layer. For each hypothesis, they designed chemical extractions and tests to determine the effect each compound had on friction.
Chemical tests determined that the correct hypothesis was that the black layer was comprised of iron tannate, which is produced by a reaction between dissolved iron and tannins (naturally occurring organic substances found in various plant tissues, including leaves from certain trees). However, determining that the substance was iron tannate did not necessarily indicate that this substance was the cause of the loss of friction on the tracks. Tribological tests needed to be carried out to test the hypothesis that the tannins found in the leaves were the cause. The researchers decided to focus on an aqueous extract of sycamore leaves and metallic iron and the reaction between the two. Combined, the aqueous extract and iron formed a black precipitate like that found on the railway tracks, and friction tests determined that the black precipitate creates low friction.
Three different types of tribological testing were carried out: ball-on-flat testing; rolling-sliding, ball on ring testing; and twin disc, rolling-sliding testing.
The results of the ball-on-flat testing indicated that the tannins (in both untreated leaf extract and in tannic acid solutions) significantly lowered the coefficient of friction when compared with water. The other two types of testing (rolling-sliding, ball on ring testing, and twin disc, rolling-sliding testing) resulted in the same conclusion, tannins significantly lower the coefficient of friction between the wheels of the trains and the tracks they are traveling across. The tests also indicated that the reduction in friction occurred as a result of dynamic lubrication since the experiments with higher rolling speeds and lower contact pressures showed the least amount of friction.
Having this knowledge helps the engineers involved in the railroad industry understand the cause behind the loss of friction and work towards creating a solution. In the meantime, the railroads are looking at the trees flanking the railways, determining which ones are causing problems (mainly deciduous and broad-leaved trees including ash, horse chestnut, lime, poplar, sweet chestnut, and sycamore) and how to deal with them in an environmentally friendly manner.
Further information: Michael Watson et al. The composition and friction-reducing properties of leaf layers, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences (2020). DOI: 10.1098/rspa.2020.0057