In our experience with original Aquapel, noticeable water clearing occurred at approximately 40–50 km/h. The actual threshold depends on the car, the rain and the condition of the glass and coating. A simple question helps explain the difference: the drop is already round — but how easily can it move?
Contact angle: the shape of a drop
The contact angle is measured at the boundary between water and the surface. It describes how much the drop spreads. This is an angle at the drop itself, not the inclination of the windscreen.
A rounder drop may look impressive, but its shape alone does not tell you how much force is needed to move it.
Hysteresis: retention at the edge
When the contact area changes, advancing and receding contact angles are distinguished. Their difference is called hysteresis. It helps describe the retention of the contact line — the boundary where water, air and the surface meet.
The onset of movement and subsequent sliding are also different. Studies include surfaces with little retention at the start but appreciable resistance to an already moving drop. So even one favourable measurement does not describe all water dynamics.
Roll-off angle: when water starts moving on a tilted surface
In this test, the sample is gradually tilted. With the same drop volume and method, a smaller angle means an easier onset of movement.
The value depends on drop size, surface condition and measurement method. A large drop on one sample cannot fairly be compared with a small drop on another. And the tilt angle cannot be converted directly into kilometres per hour.
Airflow: conditions on the car
On a car, water is moved by the airflow around the body. Glass inclination and curvature, drop position, wind and rain all matter. Air speed at a particular point on the glass is not automatically the same as the speedometer reading.
Fine drizzle may remain longer than larger drops. Once several drops merge, they may start moving where they previously stayed still. So there is no single figure for every kind of rain and every car.
| Parameter | What question does it answer? |
|---|---|
| Contact angle | What shape does the drop take? |
| Hysteresis | How do the conditions for the advancing and receding edge differ? |
| Roll-off angle | At what tilt does a given drop begin to move? |
| Performance under airflow | How does water clear under a specific airflow? |
What this means when choosing Aquapel
We care about the result on an ordinary journey: water clears, the glass remains clear and the wipers work normally. The reference from our experience helps you picture the effect, while the conditions on a particular car explain the differences.
Compare the same cleaned glass in similar weather. Do not increase speed for a demonstration or switch off wipers needed for visibility.
View Aquapel packsHow to applyHow to compare rain repellents