Wet floors increase the risk of slipping and falling.
But in some cases, such as when licking your fingers to help turn a book, water on the surface can increase friction.
Traditionally we have explained this situation by the phenomenon of capillarity.
But now scientists have discovered that hydrogen bonds between water and surfaces are the main cause of this friction in some cases.
The team pressed a 3mm diameter silicon ball onto a flat silicon wafer and then measured the coefficient of friction between the two at different humidity levels.
The coefficient of friction was 0.3 when completely dry, peaking at 0.6 at 20% humidity,
It is 0.51 at fully humid.
The capillary phenomenon can only explain the friction at the peak, but cannot explain why the friction is greater when fully wet than when dry.
The capillary phenomenon relies on a “water bridge” between the bumps on the solid surface, but there is no “water bridge” at all when fully wet.
The researchers went on to speculate that hydrogen bonding is responsible for this friction
Hydrogen bonding in this case refers to the electrostatic attraction formed between water molecules and the silicon surface.
To test their suspicions, they used heavy water with stronger hydrogen bonds instead of normal water, at which point the coefficient of friction was measured to be 0.58 when fully wet – compared to 0.51 for the control group.
This difference in results is consistent with the researchers’ prediction using hydrogen bonding.
The related paper was published in Physical Review Letters.
