
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: 20190416   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 NavierStokes 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
NavierStokes 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 NavierStokes 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 gluelike 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 wellstudied. 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 NavierStokes equation.)
Interestingly, nonnewtonian fluids (fluids
that have a nonconstant 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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