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Why does water never move in straight lines? Even running down a windowpane, it wiggles. Is there any explanation for this?
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Ron Dippold
San Diego, California, US
There are many forces acting on water, and what you see is the result of a fierce battle between all of them. If water just ran nicely down windows, we wouldn’t need windscreen wipers! If there were no other forces, gravity would pull water straight down. But water is a polar molecule – its two hydrogen atoms are slightly positive and its oxygen atom is slightly negative. This is known as a dipole. The positive hydrogen atoms attract the negative oxygen atoms in other water molecules and vice versa. The atoms are also attracted to various charges of the surfaces they are on.
Common window glass is usually made of silica, which is one silicon atom and two oxygen atoms. The negative oxygen atoms in the silica strongly attract the positive hydrogen atoms in water. Water isn’t as attracted to non-polar molecules, such as oil, which is why oil and water are hard to mix and keep mixed. This is one reason windscreens usually have a coating to keep water from direct contact with the surface of the glass. There are even designs for hydrophobic (water-repelling) surfaces that use small, non-polar microstructures to make water very averse to sticking to them.
Water is buffeted by many invisible forces and the path it takes is the result of whatever force wins in that moment
Besides the influence of charge, any normal surface, such as a window, has various imperfections. Water might flow into a tiny valley or around a tiny bump, which will make it take an apparently random path. When water tends to attract other water – called cohesion – you get beads of water. Surface tension also encourages this. If the water tends to cling to the surface, then it spreads out, and this is called “adhesion” or “wetting”.
Even with the most hydrophobic or flat surfaces, air is still an issue, as wind blows the water around somewhat unpredictably, especially on a moving car’s windows. The wind is mostly towards the back of the car, but there are dozens of tiny vortices driven by the edges and corners of the car’s body. Even if you just release a drop of water inside a perfectly still tower, the air beneath it acts as an upward resistance similar to wind. Because water isn’t solid, the drop wobbles around unpredictably, which can slightly alter its course, like a skydiver tilting their chute.
In summary, every drop of water you see is buffeted by many invisible forces, and the path it takes is the result of whatever force wins in that moment. If some water molecules on the left of the drop bond more strongly to the surface than to the other water molecules and spread out, then perhaps the drop will move left a bit. A tiny air vortex might nudge it up and right a bit. Gravity mostly wins by volume, but there are always a few drops left clinging until they evaporate. Or perhaps your windscreen wipers will just cruelly fling those drops into the great unknown.
Mel Earp
Macclesfield, Cheshire, UK
In physics, most, if not all, of the equations portray an idealised, simplified scenario. The maths is just too complex otherwise. The idealised situation would indeed suggest that raindrops should run down a windowpane vertically, but that is never the reality.
Let’s start with the window glass. Despite what we can see and feel, it isn’t perfectly flat. It has small bumps and ridges, and the raindrops respond to the lateral (side) forces that the imperfections impart. Perhaps more significant is that the surface of the glass will be covered in various contaminants, such as leftover cleaning materials or dust and dirt deposits. Even when we aren’t able to see them, they are there, and they won’t be distributed uniformly across the glass.
Then there are the raindrops. They will contain dust and dirt, which won’t be evenly distributed in the drops, creating an imbalance. More significantly, an individual drop won’t hit the glass in a perfectly vertical direction. And when two drops are close enough, the electrostatic attraction that keeps water molecules together will be enough to pull them into one larger drop, in a non-uniform manner. In short, it would be quite a surprise, given all the forces acting on a drop, if it did travel perfectly vertically down a windowpane.
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