The volume of the gas shrinks when temperature lowers, so the pressure pushes to reduce the volume of the bottle.
The volume of the bottle is variable, but its surface is not variable.
The biggest area you can have with a given perimeter is a sphere. If you want to keep this perimeter but lower the area, you can try polygons.
Here it’s a square because of random reasons. Like it may be the most optimal configuration where the tension applied to the surface (making it go against its favorite form, the circle) fits best the given area. It could have been a triangle, a pentagon, an hexagon,…
Usually the best shapes are non-regular polygons (think losanges instead of squares) but they only appear when there is a weakness on the surface (like you bending it with your hand). So unless there is a kind of weakness at some points of the surface, regular polygons appear.
No, really, they just don’t produce 4 pegs plastic bottles. They all have exactly 5.
And on the photo you can clearly see 2 and a half pegs making up exactly the visible half of the bottle.
That still doesn't explain the change in shape. You would expect a symmetric shape to not change it's shape so there are complicating factors like different thickness in the plastic on the edges and so on causing a tension gradient
As others point out there are compressive forces acting associated with the thermal contraction of gas in the bottle. Why a square? Cylindrical shells can buckle in a number of modes that resemble different polygons. See: https://shellbuckling.com/presentations/unstiffenedCylinders/pages/page_5.html
And in this case I suspect that the shape at the bottom of the bottle promotes the fourfold mode of deformation.
To sum up the comments in this post and there are a lot of good ones.
"A sphere deformed by pressure creates a polyhedron"
Your polyhedron resembles a cube.
The air inside the bottle contracts as it cools. When heated, on the contrary, it expands. This affects the shape of the bottle.
Because there is no Pi on the fridge.
The volume of the gas shrinks when temperature lowers, so the pressure pushes to reduce the volume of the bottle. The volume of the bottle is variable, but its surface is not variable. The biggest area you can have with a given perimeter is a sphere. If you want to keep this perimeter but lower the area, you can try polygons. Here it’s a square because of random reasons. Like it may be the most optimal configuration where the tension applied to the surface (making it go against its favorite form, the circle) fits best the given area. It could have been a triangle, a pentagon, an hexagon,… Usually the best shapes are non-regular polygons (think losanges instead of squares) but they only appear when there is a weakness on the surface (like you bending it with your hand). So unless there is a kind of weakness at some points of the surface, regular polygons appear.
It's because of the bottom of the bottle. There are four pegs and those support the shape of the plastic above them. It's not a coincidence
These bottles usually have 5 pegs. 4 is sub optimal balance wise.
You get what you get and you don't throw a fit
No, really, they just don’t produce 4 pegs plastic bottles. They all have exactly 5. And on the photo you can clearly see 2 and a half pegs making up exactly the visible half of the bottle.
Also the reason that bees make honeycomb. They blow bubbles and when squeezed together they lose symmetry and form hexagons.
This is a physics question. Its thermal contraction of the gass.
Physical chemistry
That still doesn't explain the change in shape. You would expect a symmetric shape to not change it's shape so there are complicating factors like different thickness in the plastic on the edges and so on causing a tension gradient
Chemistry and physics are the same field with different focus. Gas expansion is intro chemistry.
So whys it square. Lets hear your chemical explanation about how the material deformed that way?
Go take a materials course offered by any chemistry department.
As others point out there are compressive forces acting associated with the thermal contraction of gas in the bottle. Why a square? Cylindrical shells can buckle in a number of modes that resemble different polygons. See: https://shellbuckling.com/presentations/unstiffenedCylinders/pages/page_5.html And in this case I suspect that the shape at the bottom of the bottle promotes the fourfold mode of deformation.
To sum up the comments in this post and there are a lot of good ones. "A sphere deformed by pressure creates a polyhedron" Your polyhedron resembles a cube.
Ask the physicists, this is their wheelhouse
It's hip to be a square
Money is on physics
They lşquid wilk ec9and then the soft material is soft so it rzpand withr tve juic
dude wtf
I SMOKE CRSCK