Unravelling the secrets of swing in cricket with the physics of friction

Cricket teams must understand dew's impact on friction, spin, and swing. Captains must be aware of the risks of bowling first in day-night games. Batters have an advantage under dew, but must maintain moisture on gloves and soles. The amount of friction increases with the thickness of a water film before decreasing, which scientists attribute to hydrogen bonds. Fast bowlers can use higher speed to force a wet ball to grip. Lower temperature and moisture increase swing. Sunlight causes less turbulence in the air, aiding bowlers.

Cricket matches under day-night conditions are associated with shifts in humidity and moisture. However, most cricket teams’ captains have not displayed awareness that choosing to bowl first in a day-night game – because dew is expected later – carries specific risks.

Fans of the sport are aware of the ‘dew factor’: water vapour condenses on the ground in the evening, creating a slippery surface. As a result, spinners have a harder time getting the ball to grip and fast bowlers have more trouble producing swing and seam. Fielding on a slippery ground is also obviously harder.

As a result of these changes in ground conditions, batters appear to have an advantage under dew, as they face less swing, less spin, and less lateral movement of the ball. (Sometimes, fast bowlers release the ball at a certain angle into its flight path. As a result, air flow is turbulent on one side of the ball and streamlined on the other. This causes a sudden pressure difference that causes the ball to deviate from its path in a motion called its swing.) Batters also expect the ball to skid off the bat under dew, and expect opportunities to maintain a higher run rate with less effort.

For these reasons, most captains winning the toss in day-night matches prefer to bowl first and bat second.

A closer analysis of the physics of friction suggests that the belief that dew always increases slipperiness is scientifically flawed. Friction is reduced only when the water film in between is thick enough to reduce the amount of physical contact between two surfaces. When the thickness is below a certain threshold, it increases the overall friction because the water molecules interact more strongly with the two surfaces due to adhesive forces.

A recent study led by Liang Peng of the University of Amsterdam found that the coefficient of friction doubled when humidity was increased by 20% and decreased only thereafter. The scientists attributed this to hydrogen bonds that formed as a result of electrostatic forces.

Thus, in moist weather, the coefficient of friction increases, advantaging the bowler. This may have been why India’s batters lost three early wickets in the match against Australia in Chennai.