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So recently when I was in a toilet, I noticed a strange behavior of the water that flowed down a nearly vertical pipe of my friend house. Instead of flowing straight down on the nearly vertical pipe, the water swirled around the pipe and it flabbergasted me. The pipe is made of some plastic or rubber I don't know but I need an explanation of why the water behaved such way, why it didn't just flowed down the pipe in straight flow. I'm sorry for my bad writing and explaination. English is not my mother tounge enter image description herehere is a photo of the strange behavior of the water (pls excuse the bad quality of this photo). So the clear visible line was the water flow. Notice how it just swirled around the pipe. May I know if water surface tension has anything to do with this ?

I'm sorry if my question sounds silly. I'm not an expert in this field but rather just a student from a standard high School.

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This phenomenon is due to the combined action of gravity, inertia, viscous and surface tension forces. This is observed not only when the jet flows down the pipe, but also when it runs down the inclined plane see Meandering of liquid rivulets. Note that the phenomenon of meandering was studied by Einstein. There are several theories of this phenomenon. It is assumed that the study of the thin stream meandering mechanism may help in understanding the meandering of rivers. The photo on the left shows the mode of runoff of the jet with a turn of the flow in the opposite direction. Therefore, part of the jet is twisted clockwise, and the other part is counterclockwise. The photo on the right shows a mode similar to that presented by the author of the topic. fig1

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It is not a silly question. And yes surface tension has everything to do with it. But the fact that only a very small quantity of water is flowing down is also important. Because the plastic is hydrophobic, the water is not properly wetting the tube. And spreading as is being expected by you.

The water in the wash basin ( It most certainly a wash basin. Only then is that translucent drain tube visible. ) Starts to enter the much wider drain pipe at an angle. This accounts for the spiraling.

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This phenomenon is similar to the rise of water in a narrow tube, partially lowered into the water, then surface tension also acts. Moreover, the rise of fluid in a narrow tube can be easily calculated by equating the acting forces. The phenomenon that you describe is difficult to describe.

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