Researchers explain how wet surfaces can exhibit more friction than dry ones

During their experiment the relative humidity was varied from 0.6 per cent to 80 per cent.

Published : Dec 29, 2022 10:30 IST

Experiments measured the friction between a silicon sphere and a silicon wafer as humidity was varied.

Experiments measured the friction between a silicon sphere and a silicon wafer as humidity was varied. | Photo Credit: American Physical Society

While, in general, wetting a floor reduces friction and increases the risk of slipping, there are cases where adding water to a surface increases friction, such as when licking one’s fingers to turn a book page.

The earliest friction studies had considered relatively large objects, such as a block of wood sliding down an inclined surface. More recent experiments have focussed on the nanoscale. These have studied frictional forces on needle-like probes and identified mechanisms operating at a single microscopic bump, or “asperity”, on a surface. But, said the physicist Liang Peng of the University of Amsterdam, the question how the frictional forces at the two scales, nano and macro, were related had remained unanswered.

Peng’s team took a relatively large probe, a 3-mm-wide silicon sphere, and pressed it down on a flat silicon wafer with a “normal force” of 50 millinewtons. This resulted in a 20-micrometre-wide contact area consisting of several asperities. The researchers then measured the horizontal force needed to move the sphere across the wafer at 100 nanometres/s. The relative humidity was varied from 0.6 per cent, where the surfaces remained dry, to 80 per cent, where water was present but not completely coating the surfaces. They also tested with the surfaces fully immersed in water.

The researchers found that the coefficient of friction (horizontal force divided by vertical force) had its minimum value of 0.3 when the surfaces were completely dry. As the humidity increased, the coefficient peaked at 0.6 when the humidity was 20 per cent and then slowly decreased as the air became more humid. For the fully immersed case, the coefficient was around 0.5; completely wet surfaces were nearly twice as sticky as the completely dry ones.

The researchers first tried to understand this behaviour by considering capillary adhesion, a phenomenon well known from single-asperity studies. In a humid environment, water vapour can condense on the asperity to form a “water bridge” that connects to another asperity on a nearby surface. The bridge resists any motion (such as sliding) that pulls the asperities apart. For the team’s multiasperity system, a multibridge model reproduced the friction peak at around 20 per cent humidity. But, it could not explain the observation with surfaces fully immersed, where the friction was more than in the dry situation. There are no bridges in both the extreme cases.

To explain the mystery, the researchers proposed hydrogen bonding: the weak electrostatic attraction that can form between the water molecules and the silicon surfaces. To test this explanation, the team repeated the experiments with heavy water, in which the isotope deuterium replaced the hydrogen atoms. Since heavy water has stronger hydrogen bonding, it produced a higher coefficient of friction (0.58) than normal water (0.51) in the fully immersed trials. Our experience of slippery wet floors is not wrong, Peng said. Our shoes and the floor are relatively rough, so the asperities are large, and they tend to lock together, causing a resistance to sliding. If you put in some water, those asperities can separate, and the resistance becomes less.

The results of the work were published in a recent issue of Physical Review Letters.

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