How does dirt not come off of cars on the highway?

Question

Seems like it should fly off but I’ve just seen several dirty cars on my commute going 70+ mph.

In: Chemistry

Answer 1

There are two things at play here: 1)The dust/dirt mixed with even a small amount of water allows the dust particles to adhere better to the car. 2) When the car is in motion, there is an effect which creates a “boundary layer” on the surface of the vehicle. That boundary layer acts like a little bubble around the car, and inside the bubble, the airspeed is very slow/near zero, which decreases the “scrubbing” power of the air passing around the vehicle.

Here’s an image that illustrates the effect in a very simple diagram: [Boundary Layer illustration](https://www.researchgate.net/profile/Ruiying_Li3/publication/322640200/figure/fig3/AS:614368119119887@1523488182613/A-sketch-illustrating-the-fundamental-wake-features-behind-a-square-back-car-The.png)

Edit: fix image link

Answer 2

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