Physicists explain the mesmerizing movement of raindrops on a car windshield

(ORDO NEWS) — Watching raindrops on a car windshield is not just a way to connect with your inner self. You can also learn some physics.

When the car is racing in the rain, some drops of water roll down the windshield, others roll down, and some seem to get stuck in place. “It’s very hypnotizing, isn’t it?” says fluid mechanics researcher Sungyong Lee of the University of Minnesota at Minneapolis.

Lee and Alireza Hushanginejad, a fluid mechanics researcher at Cornell University, used mathematical equations to describe the forces acting on raindrops. This work has identified several factors that determine the behavior of droplets, the couple reports March 4 in the journal Physical Review Fluids.

Raindrops on the tilted windshield of a moving car simultaneously experience the forces of gravity and the wind that sweeps over the car. Lee and Hushanginejad say that the direction a raindrop travels depends on its size.

For large raindrops, gravity wins by pulling them down. Small drops are dominated by the wind, which pushes them up the slope.

For medium sized raindrops, the forces balance and the drops sit still. The smallest droplets also stay put because the wind doesn’t provide enough force to overcome the water’s tendency to stick to the glass.

Other factors also influence the behavior of raindrops. As the speed of the car increases, and hence the speed of the wind, large drops of rain rise up the windshield.

Decreasing vehicle speed has the opposite effect. And if the car is moving slowly enough, there won’t be enough wind to make the raindrops move up. Meanwhile, windshields with steeper angles allow smaller raindrops to succumb to gravity than smaller ones, the results show.

To simplify the calculations, the researchers studied an idealized version of raindrops based on only two measurements. This means the results won’t match real raindrops perfectly, Lee says, but they can still provide insight into why water drops do what they do.

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