Shami asks for lifting ban on saliva for shining cricket balls: Mechanics of swing, explained

A thin layer of air forms along the ball’s surface after it is released by the bowler. But the so-called “boundary layer” must separate from the surface at some point. Where this separation occurs on either side of the ball determines the air pressure on that side.

Bowlers generally tilt the seam towards one direction or the other. In a new ball, the raised seam disturbs the airflow on the side towards which it is tilted. This turbulent flow on the seam side sticks to the ball’s surface longer and travels faster than the smooth laminar flow on the other side.

According to Bernoulli’s principle, faster travelling air on the seam side results in a decrease of air pressure on that side, making the ball swing in that direction. In theory, a new ball that is held straight — with the seam perfectly aligned to the direction it is travelling — will not swing because the airflow on both sides of the ball is even.

Movement of the cricket ball during a ‘swing’.

What is reverse swing and why does it occur?

What has been discussed so far is conventional swing which generally occurs with a new ball. Things get more interesting when the ball gets older, its lacquer wears off, and its surface gets more abrasive. This is when the ball starts to reverse, that is, move in the direction opposite to the seam side.

From the moment they are handed the ball, the fielding team makes efforts to shine one side. Consequently, as the ball gets older, one side is smoother than the other. That said, the so-called shiny side is nonetheless rougher than a new ball. This means that this side still generates sticky, turbulent airflow.

However, the rougher side generates even more turbulent airflow than the shinier side, which can be further aggravated with the help of the seam. This extra turbulent airflow is not as sticky as the turbulent airflow on the other side. It shears off — meaning that the point of separation for the boundary layer moves towards the front of the ball. As a consequence, the air pressure on the seam side is now relatively higher than on that on the shiny side. This makes the ball swing on the side opposite to the seam.

To aid reverse swing, bowlers also try to ensure that the shiny side is slightly heavier than the other side. This gives the ball a natural tendency to fall in that direction.

Where does saliva figure in all this?

Cricketers have traditionally used saliva to shine one side of the ball, smoothen the surface, and make it a tad heavier on that side. The idea is to create as great of a contrast between the two sides of the ball as possible, which helps generate reverse swing. Cricketers have chewed on all kinds of mints and candy to aid the shining process — sugary saliva is heavier, and thus considered more effective for shining a cricket ball.

Can something else be used in saliva’s place? In theory, yes. But saliva is the most readily accessible shining agent that works the way cricketers want it to.

Take sweat, for example. Rubbing the ball with sweat is legal but sweat is not as good a polishing agent as saliva, presumably because it does not contain mucus.

Moreover, the ball tends to soak up sweat — too much sweat can soften the ball which is not something bowlers want. Softer balls do not bounce as much, they do not turn as quickly, or retain much pace after pitching. In short, they are easier for a batter to deal with.

Then there is the fact that unlike saliva, sweating is condition-dependent. Players simply do not sweat enough in cold conditions for it to be an effective shining substance.

Other substances like vaseline have also been touted, although they are not as effective. Notably, vaseline is lighter than sweat. As former Indian pacer Ashish Nehra once said: “Vaseline can help keep the shine, but it does not make the ball heavier on one side”.