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Is there a reason why water seems to curl around itself when it is being poured? Like when pouring water from a gallon to a bottle and there is a sort of braiding pattern which can be extended if the gallon is raised and the distance between the two increases. Also, is there a way to predict it, or are there too many variables?
Question Date: 2019-04-16
Answer 1:

This is a great question! The effect you are describing is the result of a delicate balance of several chemical and physical processes that are simultaneously competing with one another. Water has an intrinsic property called cohesion, which is the tendency of like molecules to be attracted to one another and "stick" together. This attraction is governed by the shape of the molecules involved, which affects the shape of the distribution of electrons belonging to the water molecules. Since the distribution of electrons is irregular, certain parts of a water molecule will be attracted to one another more than others. This kind ofelectrical attraction allows a substance to hold some shape -- in particular, water will tend to form spherical droplets because of its high surface tension (tendency to shrink to a shape which minimizes the surface area of the substance). However, because gravity is also acting on the system, the pouring water exhibits a braiding pattern rather than forming spheres.

While there are many variables involved in describing this behavior, it is possible to model it by applying the Navier-Stokes equations for a Newtonian fluid. At the end of the day, water is a real fluid, but it can be modeled as a Newtonian fluid, which follows Newton's law of viscosity (stress and strain are related by constant viscosity). In other words, the fluid does not become thicker or thinner under different forces --think honey or ketchup, respectively. The Navier-Stokes equations describe the flow of viscous fluid. They are equations derived from the assumption that mass, momentum, and energy are conserved in the system. Thus, the equations relate and balance functions like density, flow velocity, pressure, body accelerations acting on continuum (e.g. gravity in the case of your pouring water scenario), and stresses (e.g. the shape of the vessel from which you poor the water will impose some initial stress on the water could possibly have some considerable effect on the shape the water takes as it travels away from the vessel).

The Navier-Stokes equations are "coupled differential equations," which means they have some dependence on one another and relate various functions and their derivatives (rates and accelerations) to one another. Theoretically, you might be able to solve these analytically (exactly) using the tools of calculus, but typically they are quite complicated, and are solved numerically using a variety of approximation techniques.

I realize there's a lot of jargon that I just threw at you. I think the best way to learn about some of this stuff in more detail is to try Googling some of these terms and read a little bit about each topic, using the answers you find on Science Line as a guide for a "bigger picture" understanding of your question. I hope this helps!

Answer 2:

Hi Kathy, there is in fact a reason! Water molecules are very unique in that it is made up of two hydrogen atoms and one oxygen atom. The positive charges on the hydrogens and negative charges on the oxygen cause them to stick together; this means water can stick to itself, called cohesion.

You are right by saying it seems to curl around itself because that’s exactly what it’s doing! Another characteristic of water is its tendency to for spheres. If acted on by an outside force that causes it to change shape, it will eventually return to a spherical shape. That being said, when poured out of a container into another, the water molecules are being flattened in many directions and are also springing back into their original shape and flattening in the other direction. They continue to “oscillate” like this as they travel to their next destination. This produces the braid like structure you mentioned. This phenomena is reliant on both physics and chemistry, how cool!

Answer 3:

Water has a behavior called surface tension, which are caused by electromagnetic force from the fact that water molecules are electric dipoles due to the oxygen holding onto the hydrogens' electrons more tightly than the hydrogens do. This has the result that water has a glue-like property that causes it to bind to itself and other objects. It's why water forms drops that run down the sides of objects, for example. The braiding pattern is also due to this surface tension.

Note, by the way, that all liquids do this, but in water it is particularly obvious. Try pouring olive oil out of a bottle into a bowl, and then do the same thing with water into a different bowl, and you will see the difference between the two liquids.

Answer 4:

You are a good observer! The water molecules are being pushed around by other water molecules. 'Everything' makes a difference in the patterns that form, as you are observing. There must be lots of good predictions about the patterns of water flow, but I don't know them. Dropping a rock into a pool of water makes a fun pattern of circles in the water. I'm sure that one is well-studied. How heavy is the rock?, for example.

Answer 5:

In the case of water I think the exact pattern would be a result of edge effects near the spout (that means the effect of the shape of the spout on how the water flows) and surface tension of water (surface tension means that the water will want to hold onto itself and minimize its contact with the air, to a certain extent). You could most likely model something similar with a computer, but I don’t know the exact contributions from the different forces. (If you are interested, the equation you would solve to figure out how the fluid moves is called the Navier-Stokes equation.)

Interestingly, non-newtonian fluids (fluids that have a non-constant viscosity) can exhibit weird behaviors in this kind of situation. For example, fluids which exhibit extensional thickening (that means they get stiffer or thicker as you pull on them) can actually flow out of a beaker without tilting it! This phenomenon is called the open siphon. You can see it here.

Answer 6:

The shape of this water braid depends in part on gravity and in part on the polar interaction between water molecules. As water molecules rush out of the container, gravity is pulling down on them while polar forces (which are really just a type of electrostatic forces) are pulling them in all directions because these molecules are surrounded by more molecules like themselves.

It may be the interplay of these two types of forces, gravity and polar interactions, that change the shapes of these water "braids". As you observed, the distance between the repeats can be increased when the container is raised. This may be a result of increasing gravitational potential while keeping the polar forces relatively constant (we're not increasing or decreasing the strength of polar interactions between the water molecules as we raise the container higher).

To test your hypothesis that correlates container height with the "wavelength" (distance, as you put it) of the braids, you may want to build a basic mechanical setup that can change the height of the container and can pour the water with consistency. To keep your constants "constant" and only test the variables you want to test, I would recommend that you use the exact same container, the same amount of water, and do your tests at the same location in your house/school. I would not recommend pouring the water using your hands because it is very difficult to control your motion so that every single pour is exactly the same as the last one.

Hope this helps!

Answer 7:

When water is poured into a sink, as long as the water is not aimed precisely at the center of the sink, it will carry "angular momentum", which determines how the water is "curled". The exact pattern of the curl can in principle be modeled and predicted using hydrodynamics.

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