Why Do Raindrops on Glass Suddenly Slide After They Merge?

On a sloping car window, a small raindrop does not slide away as soon as it lands. The edge where water meets glass is called the contact line. Microscopic roughness, dust and chemical variations on the glass can pin this line in place. Gravity pulls the drop downwards, but its front and rear edges can maintain different contact angles and resist motion within a certain range. This is contact-angle hysteresis.

When two drops touch, surface tension rapidly draws them into one larger drop. Coalescence does not create water, but it changes the balance of forces: the drop’s weight grows with its volume, while the resisting pinning force acts mainly near the contact line. A larger drop is therefore more likely to have a downhill gravitational force greater than the resistance. Once that threshold is crossed, the previously fixed contact line moves and the drop appears to slide suddenly.

As it travels, the drop may collect smaller drops, gain weight and accelerate. Its speed also depends on the slope, cleanliness and wettability of the glass, as well as air resistance. So ‘merged drops must always move faster’ is not a law. The sounder claim is that merging often pushes a drop past the threshold for motion, while the surface determines how high that threshold is.

https://www.phfscience.nz/digital-library/modeling-the-effects-of-contact-angle-hysteresis-on-the-sliding-of-droplets-down-inclined-surfaces/
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