Vacuum or pressure to move spaghetti through a hole

[Low-Q]

Hello,

I have a somewhat dumb question. It is based on the fact that I can create a small vacuum in my mouth to move that "string" of spagetti into my mouth.
I know it is possible, but I do not understand how spaghetti is possible to suck in if the shape is even and round like a "piston".
I have learned that pressure is acting angular to any surface. With the spaghetti, that surface will cause the pressure to act 90° to it, and (in my thoughts) not be able to create a force that pulls or push the spaghetti into my mouth. But still it does. Why?

Vidar

 

[Drakkith]

Good question. My first thought is that spaghetti is far from being "airtight". There is gas bouncing around inside it as well as outside of it, and I wouldn't be surprised if that helped. Unfortunately I don't know if that's actually a good answer or not.

 

[Delta2]

I don't think we need to consider the pressure distribution along the whole surface of a spaghetti string. I believe the lips play important role. The combined effect of the lips+ of vacuum created in mouth makes the spaghetti being pulled like someone is pulling it from one edge.

Regardless if my reply is satisfying or not, I believe you should put the spaghetti inside your mouth with the fork, doing minimal sucking, its not polite to suck spaghetti all the way from one end to the other :)

 

[CWatters]

The partial vacuum inside your mouth doesn't "pull" the spaghetti into you mouth. It's the air pressure outside that pushes it in. If there was a vacuum outside you wouldn't be able to suck the spaghetti into your mouth.

The spaghetti doesn't have to be air tight. It's soft so the pressure inside the spaghetti is the same as the air pressure that surrounds it. You could say the air pushes the spaghetti and the spaghetti pushes itself into your mouth.

 

[CollinsArg]

The partial vacuum inside your mouth doesn't "pull" the spaghetti into you mouth. It's the air pressure outside that pushes it in. If there was a vacuum outside you wouldn't be able to suck the spaghetti into your mouth.

I've heard this theory many many times, and I still can get it; how is it that the outside air pressure is that strong? if I make a vacuum the pressure is really strong, how is it that it doesn't feel that strong normaly without any vacuum? then, the air in my mouth is more pressed than the outside air. Do you know about any experiment that prove this?

 

[Cutter Ketch]

I've heard this theory many many times, and I still can get it; how is it that the outside air pressure is that strong? if I make a vacuum the pressure is really strong, how is it that it doesn't feel that strong normaly without any vacuum? then, the air in my mouth is more pressed than the outside air. Do you know about any experiment that prove this?

Hmmm ... this post surprised me. Air pressure is an extremely well known and common part of every day life. I know of several "experiments" the most obvious being that air pressure is what a barometer measures. It is what those front lines on the weather map are showing. You test it every time you blow up a balloon or suck on a straw. It is caused by the weight of the atmosphere. We are at the bottom of a vast ocean of air, and exactly the same as how the pressure increases when you dive to the bottom of a pool here at the surface of the Earth we experience 14lb /sq. in. of atmospheric pressure.

The reason you don't feel it is because you are built for it. 1 atmosphere is our normal. If we were thrown into a vacuum we would be very uncomfortable. (to say the least)

 

[Cutter Ketch]

Returning to the OP. The question was about shape.

Consider a circular opening in a wall with a pressure difference across the wall. If the opening is plugged with a small disk, we easily see the force pushing the disk through the opening. The question is if we plug the opening with a long cylinder how does it get pushed through the hole. The forces on the sides of the cylinder do not push the correct direction.

I think the OP's instinct is correct. If the hole is plugged with an infinite straight cylinder there is no force pushing the cylinder through the hole. Therefore the reason we can slurp spaghetti is either because it is not infinite, or because it is not straight.

It is easy to see how any finite straight cylinder still gets pushed through due to the pressure on the ends. However that doesn't answer the question for a long strand of spaghetti.

I think the trick is to consider a bent piece of spaghetti. Suppose we have a cylinder plugging the opening, but the cylinder is bent to have a straight section through the opening and then sharp 90 degree bends on either side. Now once again we can easily see the unbalanced force. Of course real spaghetti won't make exactly that shape, but any amount of bend has a similar result.

So unless your spaghetti stays straight and is infinitely long it still gets sucked through the hole.

 

[CWatters]

Consider a circular opening in a wall with a pressure difference across the wall. If the opening is plugged with a small disk, we easily see the force pushing the disk through the opening. The question is if we plug the opening with a long cylinder how does it get pushed through the hole. The forces on the sides of the cylinder do not push the correct direction.

It is easy to see how any finite straight cylinder still gets pushed through due to the pressure on the ends. However that doesn't answer the question for a long strand of spaghetti.

Most people cook their spaghetti so it's soft when they eat it. Therefore the pressure inside the spaghetti is the same as outside it. So there is a pressure drop at the point where it enters the mouth..

 

[CWatters]

The reason you don't feel it is because you are built for it.

That's not really the answer. It's because the pressure inside us is the same as the pressure outside.

When the pressure outside reduces (eg we go up in an aircraft) the pressure inside us changes to match. The human body is mostly water so it doesn't have to expand much to rebalance the pressure. However some parts of us, like the sinuses, are filled with air, and that has to expand much more to rebalance the pressure, for that reason we can sometimes feel pain in the ears or bloated until some air escapes.

 

[Low-Q]

Thanks for your replies. I didn't expect this much response to an apparently silly question :-)
Well, I still don't get it. I can understand that if I replaced the spagetti with a thin iron rod (Shorter one but same diameter). The outside air pressure will push the rod in my mouth by the force excerted on the rods flat end, but the spagettis end does not push the spagetti in. So what force does?

The mouth will make the spagetti thinner between the lips, and even if the spagetti is swollen up by the vacuum inside the mouth, I think it won't explain the force that sucks the spagetti in. I'm lost at this point - still after your replies. Maybe the swallen spagetti push back on the firm lips, but that wouldn't explain how to move the spagetti piece in both directions. At this point, in spite of being 44 years old, I feel very immature, hoho :-)), but this physics really bugs me.

 

[Chestermiller]

This has nothing to do with axial pressure forces on the spaghetti. There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.

 

[Cutter Ketch]

This has nothing to do with axial pressure forces on the spaghetti. There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.

That sounds reasonable. However, I don't do that and the spaghetti still goes in.

 

[Cutter Ketch]

That's not really the answer. It's because the pressure inside us is the same as the pressure outside.

When the pressure outside reduces (eg we go up in an aircraft) the pressure inside us changes to match. The human body is mostly water so it doesn't have to expand much to rebalance the pressure. However some parts of us, like the sinuses, are filled with air, and that has to expand much more to rebalance the pressure, for that reason we can sometimes feel pain in the ears or bloated until some air escapes.

And yet if you drop the pressure further all sorts of bad things happen. Primarily water vaporizes (and other dissolved dissolved gasses come out of solution) forming bubbles and gas pockets. They say it doesn't officially boil, but basically enough vaporizes until you blow up like a balloon and the elastic tension equalizes the pressure. You swell up painfully to something like twice your size. You can't hold breath in your lungs because the pressure difference actually would do serious damage, so you'd better exhale before being tossed into space. The heart can't effectively pump blood due to the gas and the distended veins. You pass out in ten to fifteen seconds. You die in something like 90 seconds. So while we can live happily in a range of pressures from Death Valley to Mt Everest (well, most the way up Mt. Everest anyway) we are not built to function at or near zero pressure. We are constructed to live on Earth in an atmosphere of pressure give or take.

 

[Cutter Ketch]

Most people cook their spaghetti so it's soft when they eat it. Therefore the pressure inside the spaghetti is the same as outside it. So there is a pressure drop at the point where it enters the mouth..

View attachment 110495

I see. I believe you are saying that the spaghetti has a low enough bulk modulous so that it expands as it enters the mouth. If it does I can see how that would do it.

 

[CWatters]

Most people cook their spaghetti so it's soft when they eat it. Therefore the pressure inside the spaghetti is the same as outside it. So there is a pressure drop at the point where it enters the mouth

I see. I believe you are saying that the spaghetti has a low enough bulk modulous so that it expands as it enters the mouth. If it does I can see how that would do it.

It doesn't need to expand. Water sucked up through a straw doesn't expand (at least not significantly). Your lips act just like a very short drinking straw. There is a pressure difference between the ends.

 

[Nidum]

There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.

+1

The same principle has applications in industry . Most common use is for feeding fine cables and filaments .

 

[houlahound]

Warning to the bro's:

I tried reproducing this experiment at a Japanese restaurant using small bore rice noodles in a watery soup base while on a first date. Varied the shape of my mouth cavity to optimize the effect.

I went home alone - thanks PF

:-(

 

[OmCheeto]

This has nothing to do with axial pressure forces on the spaghetti. There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.

I tried to produce an experiment, both last night and this morning, as your assertion struck me as incorrect.

Last nights experiment ended poorly, as I wasn't able to insert my noodle into my "non-mouth" orifice, and, like the time I was going to disprove "bananas are yellow", I ended up eating my experiment.

This morning's attempt ended up as usual, with me:

a: hurting myself: spaghetti is hot!
b: getting very angry: this syringe does NOT mimic my mouth!
c: thinking: I need two jello type apparatuses
d: digging through the cupboards: only to find my jello-molds are the wrong size...​

But, I did find some test-tubes. And I know where to buy jello.

Videos of experiment attempt #3 at 11!

 

[houlahound]

Has anyone factored in sauce viscosity. It will effect the sheer tensor.

Assuming we are not doing this sauce less.

 

[OmCheeto]

I tried to produce an experiment, both last night and this morning...

With Jello!

Last nights experiment ended poorly

again...

, as

Jello makes for a very poor noodle.

again

I wasn't able to insert my noodle into my "non-mouth" orifice, and, like the time I was going to disprove "bananas are yellow", I ended up eating my experiment.

Though this time, I decided eating A LOT of Jello for breakfast was a very poor idea.

Yesterday's attempt ended up as usual, with me:

a: shooting jello out of some fish tank tubing, which bounced off the ceiling, and leaving remnants scattered around my kitchen*
b: getting very angry
c: thinking, I need to think about this some more
d: discovering that it's nearly impossible to get gelatinized jello out of a test tube, without destroying your jello noodle
​

Videos of experiment attempt #4 at 11!

*This might make an interesting high speed video for @Andy Resnick . It was quite explosive, once the pressure exceeded the adhesive force of 'jello' to 'fish tank tubing'.

 

[Andy Resnick]

This has nothing to do with axial pressure forces on the spaghetti. There is a small annular gap between your lips and the strand of spaghetti. Air is being sucked into your mouth through the annular gap. You can hear this air flow when you are sucking in the spaghetti. The air that is being sucked in through the annulus creates a viscous shear stress on the spaghetti, tangent to its surface. This provides the force to drive the strand into your mouth.

Excellent explanation. And, if the spaghetti is wet (sauce or otherwise), the argument still holds- viscous drag of the moving fluid provides the force.

 

[houlahound]

I will test this theory with dry spaghetti sticks and dry lips.

 

[Andy Resnick]

*This might make an interesting high speed video for @Andy Resnick . It was quite explosive, once the pressure exceeded the adhesive force of 'jello' to 'fish tank tubing'.

Bravo! To paraphrase a Great American Composer, humor *does* belong in science! :)

 

[OmCheeto]

Excellent explanation. And, if the spaghetti is wet (sauce or otherwise), the argument still holds- viscous drag of the moving fluid provides the force.

I'm still not convinced.
I borrowed CWatters' image, and thought about a limp noodle, and did some measurements:

Since the pressure differential creates a net inward force that is greater than the downward force on the noodle, I still think it will go in.

Another thing I tried before my failed jellomakesabadnoodle experiment, was cook a piece of spaghetti inside one of my syringes.

(before cooking)

It was a complete success, up until the moment I tried to partially eject the noodle, at which point, the noodle shot across the kitchen.
But I think that is reasonable evidence that only a pressure difference is required for trans-lip noodle transport.

ps. It should be noted that I have NOT succeeded in proving anything.
pps. I think I will buy some gummy worms tomorrow, as noodles and jello are no longer my friends.

 

[houlahound]

Has this work been peer reviewed?

 

[Drakkith]

Has this work been peer reviewed?

OmCheeto always peer reviews his work. Usually he reviews it by peering at it in a threatening manner until it gives the results he wants.

 

[OmCheeto]

OmCheeto always peer reviews his work. Usually he reviews it by peering at it in a threatening manner until it gives the results he wants.

This sounds about right.

Experiments #4 & 5 this morning, like the ones before, ended badly.
Though, like all failed experiments, I learned many strange and wonderful things:

A. With much practice, you can get a spaghetti noodle back into the hole in the end of a syringe. I had previously assumed it was impossible. Hence; "Never assume anything!"
B. The adhesive force between jello and the surface of a syringe exceeds the cohesive force of jello. Though, I think I already knew this.
C. If you google things, you will find out why, leaving jello out to liquify, is a fool's endeavor: Jelly has a melting temperature below 35°C. [ref: Jelly Facts]
D. Since I'm Celsius/Centigrade illiterate, except for two temperatures, I discovered that our bodies have a/an "SOT" of ≈37°C.
E. Never, ever, give up

 

[Andy Resnick]

I'm still not convinced.

But you omitted the viscous drag from the inwardly-moving fluid. I'm not in my office so I don't have a good reference handy but as best I can do here, the shear stress per unit length acting on the noodle will be approximately given using the lubrication approximation, resulting in a viscous drag on the noodle as F ≈ 4/3πr h ΔP ≈ 0.05 N using the numbers you posted above and 0.1mm for the film thickness. Plus, you can increase the pressure difference and use your tongue to assist.

 

[Chestermiller]

But you omitted the viscous drag from the inwardly-moving fluid. I'm not in my office so I don't have a good reference handy but as best I can do here, the shear stress per unit length acting on the noodle will be approximately given using the lubrication approximation, resulting in a viscous drag on the noodle as F ≈ 4/3πr h ΔP ≈ 0.05 N using the numbers you posted above and 0.1mm for the film thickness. Plus, you can increase the pressure difference and use your tongue to assist.

From an axial force balance on the air between your lips and the noodle, I get $F=\pi rh \Delta P$ for the drag force, for the case in which the noodle is held on place and not moving (i.e., noodle held stationary by applying axial tension from outside). So your estimate for a moving noodle with less external axial tension seems reasonable.

 

[OmCheeto]

But you omitted the viscous drag...

I discovered this last week, that I don't really know what "viscosity" is.
So, the rest of your post is just "wibbly wobbly", to me.

, I get F=2πrhΔPF=2πrhΔPF=2\pi rh \Delta P for the drag force...

Delta "Physics Forums"?

hmmmm...

 

[houlahound]

Just solve for "x".

 

[houlahound]

The relevant physics starts at about 3.00.

 

[Chestermiller]

This is a continuation of my previous post. There I showed that, if the noodle is held in place manually so that it is prevented from being sucked into the mouth, the tension that must be applied to the noodle from the outside is $F=\pi r h \Delta P$, where r is the radius of the noodle, h is the annular gap between the noodle and lips, and $\Delta P$ is the pressure difference between the inside and outside of the mouth.

If we ease up on the tension applied manually to the noodle from the outside, the noodle can begin to slip axially into the mouth with velocity V. I carried out a lubrication flow analysis on this problem, and obtained the following relationship between the outside noodle tension F, the noodle velocity V and the pressure difference $\Delta P$:$$F=\pi r h \Delta P-\frac{2\pi r \eta L}{h}V$$where L is the length of the lip channel and $\eta$ is the viscosity of the fluid in the gap (air or water or oil or sauce). This result, of course, agrees with the previous finding for the case in which the tension is high enough to hold the noodle stationary (V=0). Eventually, if we ease up on the noodle tension enough, the tension will drop to zero, and we will obtain the maximum suck velocity of the noodle:

$$V=\frac{h^2}{2\eta L}\Delta P$$

 

[houlahound]

You have assumed a smooth, incompressible noodle?

 

[Chestermiller]

You have assumed a smooth, incompressible noodle?

The present hydrodynamic lubrication model recognizes the fact that the most important physical mechanism responsible for the noodle being sucked through the lips and into the mouth is provided by the viscous drag and pressure flow of the fluid in the annular gap between the lips and noodle. When the liquid advances axially through the gap, it drags fluid with it. Additional fluid flow is provided by the pressure difference between air outside and the air inside the mouth. Both these components of the fluid flow contribute to the axial shear force on the surface of the noodle. The model takes all this into account.