Why do water droplets not fall off the car after rain?

[LaLane]

This is due to the car being waxed, which generally prevents water droplets from running off. The benefit of waxing is to protect the car's paint. Below is relevant information about car waxing: The role of car waxing: As a traditional aspect of car care, waxing serves to waterproof and protect against acid rain. The protective layer of wax significantly reduces the amount of water droplets adhering to the car body, with an effectiveness ranging from 50% to 90%. Another benefit is protection against high temperatures and UV rays. As the weather gets hotter, cars that are driven or parked outdoors year-round are prone to paint aging and fading due to sunlight exposure. The film formed by waxing can reflect some of the light. Precautions for car waxing: The interval between waxing should vary depending on the driving environment and parking location of the vehicle. Before waxing, it is essential to thoroughly clean dirt and dust from the car's exterior.

[GenevieveDella]

Once after washing my car, I noticed water droplets clinging to the paint like little pearls, refusing to roll off. A friend said this was the hydrophobic coating doing its job. This water-repellent layer increases the contact angle of water droplets, preventing them from spreading out. I've previously applied liquid wax myself, and after washing, the water droplets would also bead up and roll right off, with dirt barely sticking. If an uncoated car behaves similarly, it might be due to excessive grime on the paint, where oil films turn the body into a lotus leaf effect. I deliberately tested two scenarios: a car parked outdoors for a week retained large water beads, while a freshly waxed car had smaller droplets that rolled off much faster.

[MacConnor]

The water droplets clinging to the car body result from the interplay between water's surface tension and the paint interface. The oxide layer on metal, residual tree sap, or grease causes water molecules to cluster. In a previous experiment, droplets on a phone's oleophobic coating formed rounder beads than on regular glass. The critical angle for roof water runoff is around 40 degrees—I verified this using a protractor against my garage slope. Wiping half the hood with alcohol made water spread into a film, while the other half maintained distinct droplets, creating a striking visual contrast.

[DiAdelyn]

Our car wash shop sees this situation most often. On new cars after crystal coating, water beads bounce off like marbles, with such strong hydrophobicity that water pours off in streams. For older cars, it's mostly oil film accumulated on the windshield transferring to the paint surface. We've had cases where a car wasn't washed for three months, and water droplets left muddy trails underneath. The worst are cars parked under trees long-term - tree resin mixed with dust forms a natural hydrophobic layer. Once we encountered a Mercedes with persistent water beads on the rear, which turned out to be localized oil-repellent spots caused by exhaust pipe soot spraying onto the bumper.