Vacuum or pressure to move spaghetti through a hole

[Chestermiller]

Is my understanding correct that it should be possible to distinguish models experimentally, as follows (unfortunately, I have no relevant equipment or access to a lab):

Have a chamber with a rubber orifice which can be lubricated with different fluids, and is also connected to a vacuum pump. Then testing against rods and noodles of different thickness, ideally measuring the force exerted when the pump is running and the outside end of noodle and rod is attached to a force measuring device:

- boundary flow model would suggest that for a given lubricant, the effect is proportional radius (really circumference, but same difference for proportionality); and that different lubricants (high viscosity oil, low viscosity oil, sugar syrup, etc.) could make a significant difference.

- pressure dominant case would predict force proportional to radius squared, with minimal difference by lubricant, especially for radius not too small (only impact is friction, which would be less relevant as radius got larger).

Unfortunately, while using my mouth, I can easily determine there is more force for bigger radius, I have nowhere near the precision to distinguish proportional to r versus r².

I'm a modeller, and, as a modeller, I am very confident that these issues can be resolved without resorting to experiments (at least initially to get the lay of the land).

I feel very frustrated. I've been trying to get responders to focus exclusively on the noodle, using simple free body diagrams and force balances (and perhaps a smidgen of stress analysis), but no one seems willing. I contend that the uncooked noodle can be analyzed as a rigid- or slightly deformable rod, and that the limp noodle can be analyzed as either an inextensible- or slightly extensible rope.

I'm prepared to present such analyses myself, but I wanted to give others a chance. Anybody?

Please excuse my frustration.

 

[Baluncore]

Examination of Chestermiller's equation (1) in post #73 shows that there is a critical sauce viscosity above which the maximum sauce flow velocity within the channel is always lower than or equal to the velocity of the upper plate = spaghetti surface velocity. It suggests that with a thick viscous sauce the spaghetti must be propelled by the axial pressure difference within the spaghetti rather than by the drag of the moving sauce. It raises the question of the relative strength of the two propulsion forces. Indeed, for a thick sauce, the sauce would be drawn through the channel by drag from the spaghetti.

 

[Chestermiller]

I have carried out the solid mechanics analyses of the noodle I alluded to in my previous post, and have come to the following conclusions:

1. For a horizontal rigid noodle, it is not possible to suck the noodle into the mouth using a significant pressure difference and with a constant noodle velocity (i.e., no acceleration) without friction at the lips. At constant velocity, the frictional resistance is necessary to balance the pressure difference. Otherwise, the noodle will be constantly accelerating.

2. For a vertical rigid noodle or a dangling limp noodle, it actually is possible to suck the noodle into the mouth using a significant pressure difference and with a constant noodle velocity even without friction at the lips. The gravitational force on the noodle compensates for the acceleration of the noodle required in case 1. This does not mean that there is no significant viscous frictional drag at the lips in actual practice (with both the frictional drag and the weight of the noodle balancing the pressure difference). We simply need to approximately quantify the magnitude of the frictional drag (using reasonable estimates of the parameters involved, such as viscosity, noodle velocity, and annular gap) to compare it with the contribution of gravity to the overall pressure difference.

P.S., If anyone is interested in the solid mechanics analysis for these cases, I can provide it.

Chet

 

[PAllen]

I have carried out the solid mechanics analyses of the noodle I alluded to in my previous post, and have come to the following conclusions:

1. For a horizontal rigid noodle, it is not possible to suck the noodle into the mouth using a significant pressure difference and with a constant noodle velocity (i.e., no acceleration) without friction at the lips. At constant velocity, the frictional resistance is necessary to balance the pressure difference. Otherwise, the noodle will be constantly accelerating.
Chet

I'm not sure why you keep stressing the constant velocity case. Crude experiments I can do readily establish that for the rigid cylinder case with constant pressure difference, and common lubricants, the cylinder does steadily accelerate. I did not reach the point where terminal velocity was reached due to friction increasing proportional to speed, but presumably such point is readily reached for a long cylinder.

To me, this (plus your gravity example) establishes that the rigid case is primarily a matter of pressure difference modified by obvious factors like friction.

My mind is still open on the flaccid noodle case, especially for practical parameters.

 

[Andy Resnick]

I'm a modeller, and, as a modeller, I am very confident that these issues can be resolved without resorting to experiments (at least initially to get the lay of the land).

I feel very frustrated. I've been trying to get responders to focus exclusively on the noodle, using simple free body diagrams and force balances (and perhaps a smidgen of stress analysis), but no one seems willing. I contend that the uncooked noodle can be analyzed as a rigid- or slightly deformable rod, and that the limp noodle can be analyzed as either an inextensible- or slightly extensible rope.

I'm prepared to present such analyses myself, but I wanted to give others a chance. Anybody?

Please excuse my frustration.

FWIW, I very much appreciate the time and effort you have invested in this thread.

 

[Chestermiller]

I'm not sure why you keep stressing the constant velocity case.

I just had the sense that, when I suck in a noodle (through my actual lips), it travels close to constant velocity. Maybe I'm wrong, or maybe I'm subconsciously continuously adjusting the suction so that the velocity is close to constant.

Crude experiments I can do readily establish that for the rigid cylinder case with constant pressure difference, and common lubricants, the cylinder does steadily accelerate. I did not reach the point where terminal velocity was reached due to friction increasing proportional to speed, but presumably such point is readily reached for a long cylinder.

To me, this (plus your gravity example) establishes that the rigid case is primarily a matter of pressure difference modified by obvious factors like friction.

My mind is still open on the flaccid noodle case, especially for practical parameters.

I'm going to present my solid mechanics analysis of the limp noodle case to show how it works. I'm also going to make a rough calculation of the viscous drag force at the lips and compare it to the dangling weight.

 

[Chestermiller]

I'm reporting back on some calculations I've done to estimate the drag force on the noodle by the lips, and compare this with the weight of the noodle dangling from the lips. The calculations also give the minimum level of vacuum required to suck the noodle in. To get the dangling weight of the noodle, my results are similar to those of @OmCheeto in a previous post. The calculated weight is based on the following:

Noodle radius = 0.1 cm
Noodle density = 1 gm/cc
Noodle length = 30 cm

This gives a noodle mass of about 1 gram, and a noodle dangling weight of about 1000 dynes (0.01 N).

To crudely estimate the drag force by the lips, I used the following formula:
$$F=2\pi r L \eta \frac{V}{h}$$
where L is the length of the lip channel and h is the annular gap between the lips and the noodle. The calculation was based on the following parameter values:

L = 1 cm
$\eta=0.01\ \frac{gm}{cm\ sec}$ (i.e., water at room temperature, 0.01 Poise)
V = 6 cm/sec (noodle suction speed)
h = 0.001 cm

Using these values, I get a drag force of about 40 dynes. Thus, the drag force would be a small fraction of the dangling weight of the noodle and, in practice, can be neglected (This confirms @PAllen's contention).

The suction required to draw the noodle into the mouth (in cm of water) would be about equal to the length of the noodle, and would thus be equal to about 30 cm of water. This would correspond to about 3000 Pa, or 12" of water. This would be the minimum vacuum to suck the noodle in.

 

[BillTre]

I just read this thread and since I was making some spaghetti, so I did some experiments.

This may partly dis-entangle some variables:
wetness
diameter
water vs. sauce
horizontal vs. vertical noodle
straight vs, bent flexible noodle

Dry noodles (diameter = 1.93mm):
Using noodles that are about 1.93 mm diameter (dry):
1) I can not suck in a dry noodle with dry lips. It felt like too much friction.
2) I can suck in a "dry noodle if I wet my lips and the noodle with saliva (very thin film on the noodle, most likely less than 0.1 mm). Much of the wetness seems to get sucked into the noodle. Could only suck in an inch or two (2.5-5.0 cm).
This was most easily done in a vertical orientation (head tilted forward so mouth opening is approximately horizontal).
Horizontal dry noodles have to propped up outside the mouth or it runs into things inside the mouth.
Vertical dry noodles also work.
3) If I only wet a short distance of the noodle I can suck it in until my lips hit the dry part of the noodle

Wet noodles (diameter = 2.8 mm):
4) wet only from the cooking water; works well in either a horizontal or vertical orientation.
5) Wet with sauce (sauce from a jar, quick dinner), sucked (seemingly) more easily than with just water. Warning: In the vertical orientation I almost got some sauce in my eye (remember to wear your googles when experimenting!)
6) noodle, cooked but laying around and tacky on the surface: could suck it in, but much more difficult (in either horizontal or vertical orientation). Similar to the dry noodle

Cooked, wet hot dog (diameter = ~19.4 mm):
7) These sucked in very easily (horizontal or vertical).
They were very pliable and had a somewhat greasy surface.

I guess in the vertical orientation I could put a weight on the end to get a real idea of the applied force, but food is now gone.

Conclusions:
Horizontal vs. vertical (noodle or head) orientation does not seem to matter much.
Straightness of noodle doesn't seem to matter much.
Larger diameter may help.
Minimal wetness is needed to reduce friction (at least).
Sauce coating seems to work best.

I'll let you guys figure out the meaning of all this.

 

[Chestermiller]

NEWS FLASH FROM FOX NEWS: TODAY IS NATIONAL SPAGHETTI DAY

 

[Andy Resnick]

I just read this thread and since I was making some spaghetti, so I did some experiments.
<snip>

Gasp- perform experiments instead of endless noodling? :)

 

[BillTre]

NEWS FLASH FROM FOX NEWS: TODAY IS NATIONAL SPAGHETTI DAY

Yumm!

 

[Chestermiller]

The figure below shows a schematic for the Limp Noodle case. This is the situation we will be analyzing.

The noodle extends from inside the mouth, through the lips, over the lower lip, and then dangles vertically downward. The pressure outside the mouth is atmospheric pressure $p_a$, and the pressure inside the mouth is $p_v<p_a$. Both pressures act strictly perpendicular to surfaces that they contact.

Now let's consider the state of stress within the noodle. In this development, we will be working with absolute pressures and stresses, rather than gauge pressures and gauge stresses. We also adopt the usual sign convention that tensile stresses are regarded as positive and compressive stresses are regarded as negative.

Inside the mouth, the noodle is in a state of isotropic compression throughout, caused by the surrounding air at pressure $p_v$. Both the radial and axial components of tensile stress within the noodle are equal to $-p_v$. In the figure, only the axial component of tensile stress is shown. We use the symbol $\sigma$ to represent the axial component of tensile stress in our analysis. So, $\sigma=-p_v$ for the entire portion of the noodle inside the mouth.

Outside the mouth, the axial tensile stress within the noodle varies with elevation z in order to support the weight of the noodle. However, at the very bottom of the noodle at zero elevation (z = 0), the axial tensile stress is equal to $\sigma =-p_a$, because the noodle is in contact with room air acting normal to its bottom surface.

We will next carry out a differential force balance on the noodle to quantify how the axial tensile stress $\sigma$ within the noodle varies along the contour coordinate s (see figure); the contour coordinate s is the cumulative distance measured along noodle contour, starting at the very bottom of the noodle.

In this development, we treat the noodle in the same manner that we frequently treat ropes in many mechanics problems. The bending rigidity of the noodle is neglected, and the primary focus of the force balance is in the tangential direction along the noodle contour; the forces acting normal to the contour (e.g., at the bottom lip) are of less interest here, and are not considered. Unlike mechanics problems, the weight of the noodle is included in the force balance. Our objective is to determine the minimum pressure difference required to just support the weight of the noodle, such that the noodle will be in static equilibrium. Any additional pressure difference will cause the noodle to accelerate along the contour into the mouth.

If we perform a differential force balance in the tangential direction on the section of noodle between contour locations $s$ and $s+\Delta s$ (see figure), we obtain: $$A[\sigma(s+\Delta s)-\sigma(s)]=(\rho A \Delta s)\left(g\frac{dz}{ds}\right)\tag{1}$$where A is the cross sectional area of the noodle, $\rho A \Delta s$ is the mass of noodle between contour locations $s$ and $s+\Delta s$, $\rho$ is the density of the noodle, and $g\frac{dz}{ds}$ is the component of gravity in the local tangential direction along the contour. If we divide Eqn. 1 by A$\Delta s$ and take the limit as $\Delta s$ approaches zero, we obtain:$$\frac{d\sigma}{ds}=\rho g\frac{dz}{ds}\tag{2}$$This equation can immediately be integrated between the two ends of the noodle (subject to the boundary conditions on $\sigma$) to yield:
$$-p_v+p_a=\rho g H\tag{3}$$where H is the maximum elevation of the noodle (relative to the bottom of the dangling portion, see the figure). Therefore, for static equilibrium of the noodle, $$\Delta p=(p_a-p_v)=\rho g H\tag{4}$$

This completes the description on how to determine the pressure difference required for the Limp Noodle case.

 

[Andreas C]

The figure below shows a schematic for the Limp Noodle case. This is the situation we will be analyzing.View attachment 111083
The noodle extends from inside the mouth, through the lips, over the lower lip, and then dangles vertically downward. The pressure outside the mouth is atmospheric pressure $p_a$, and the pressure inside the mouth is $p_v<p_a$. Both pressures act strictly perpendicular to surfaces that they contact.

Now let's consider the state of stress within the noodle. In this development, we will be working with absolute pressures and stresses, rather than gauge pressures and gauge stresses. We also adopt the usual sign convention that tensile stresses are regarded as positive and compressive stresses are regarded as negative.

Inside the mouth, the noodle is in a state of isotropic compression throughout, caused by the surrounding air at pressure $p_v$. Both the radial and axial components of tensile stress within the noodle are equal to $-p_v$. In the figure, only the axial component of tensile stress is shown. We use the symbol $\sigma$ to represent the axial component of tensile stress in our analysis. So, $\sigma=-p_v$ for the entire portion of the noodle inside the mouth.

Outside the mouth, the axial tensile stress within the noodle varies with elevation z in order to support the weight of the noodle. However, at the very bottom of the noodle at zero elevation (z = 0), the axial tensile stress is equal to $\sigma =-p_a$, because the noodle is in contact with room air acting normal to its bottom surface.

We will next carry out a differential force balance on the noodle to quantify how the axial tensile stress $\sigma$ within the noodle varies along the contour coordinate s (see figure); the contour coordinate s is the cumulative distance measured along noodle contour, starting at the very bottom of the noodle.

In this development, we treat the noodle in the same manner that we frequently treat ropes in many mechanics problems. The bending rigidity of the noodle is neglected, and the primary focus of the force balance is in the tangential direction along the noodle contour; the forces acting normal to the contour (e.g., at the bottom lip) are of less interest here, and are not considered. Unlike mechanics problems, the weight of the noodle is included in the force balance. Our objective is to determine the minimum pressure difference required to just support the weight of the noodle, such that the noodle will be in static equilibrium. Any additional pressure difference will cause the noodle to accelerate along the contour into the mouth.

If we perform a differential force balance in the tangential direction on the section of noodle between contour locations $s$ and $s+\Delta s$ (see figure), we obtain: $$A[\sigma(s+\Delta s)-\sigma(s)]=(\rho A \Delta s)\left(g\frac{dz}{ds}\right)\tag{1}$$where A is the cross sectional area of the noodle, $\rho A \Delta s$ is the mass of noodle between contour locations $s$ and $s+\Delta s$, $\rho$ is the density of the noodle, and $g\frac{dz}{ds}$ is the component of gravity in the local tangential direction along the contour. If we divide Eqn. 1 by A$\Delta s$ and take the limit as $\Delta s$ approaches zero, we obtain:$$\frac{d\sigma}{ds}=\rho g\frac{dz}{ds}\tag{2}$$This equation can immediately be integrated between the two ends of the noodle (subject to the boundary conditions on $\sigma$) to yield:
$$-p_v+p_a=\rho g H\tag{3}$$where H is the maximum elevation of the noodle (relative to the bottom of the dangling portion, see the figure). Therefore, for static equilibrium of the noodle, $$\Delta p=(p_a-p_v)=\rho g H\tag{4}$$

This completes the description on how to determine the pressure difference required for the Limp Noodle case.

This post should be on the home page of physicsforums: "Physicsforums.com: the ONLY place on the internet you'll find this sort of analysis on noodles"

 

[Andy Resnick]

The figure below shows a schematic for the Limp Noodle case. This is the situation we will be analyzing.Therefore, for static equilibrium of the noodle, $$\Delta p=(p_a-p_v)=\rho g H\tag{4}$$

This completes the description on how to determine the pressure difference required for the Limp Noodle case.

That makes sense- it's the same hydrostatic pressure required to draw a 'fluid rope' up a height H. Nice!

 

[Swamp Thing]

Here is a thought experiment that is a bit different from the noodle situation, but may help to validate or falsify a given model.

Take a rigid transparent plastic tube about one centimeter in diameter. Connect it to a suitable large syringe. Use the syringe to extrude a "rope" of clear RTV rubber through the plastic tube so that the tube is full of the RTV, plus you have some rope hanging out from one end of the tube. Allow the RTV to cure and set into a rope with one end stuck inside the plastic tube.

Before extruding, you need to add some colored particles to the RTV, so that you can observe any internal deformations that might happen inside the RTV during the actual experiment.

Now pass the tube through a hole in a plastic jar and seal it in place with part of the tube inside and part outside the jar. The extruded RTV rope hangs outside the jar. Note that the RTV is stuck in place within the tube, so there is no fluid flow at the junction - in fact, there is no gap at all between the RTV and the tube. The RTV is stuck firmly in place.

At this point we create a vacuum within the jar. As the pressure falls, we watch the marker particles that we embedded in the RTV. Although the RTV won't slide through the tube, it is quite possible that the pressure differential would distort the RTV such that particles in the tube would deflect towards the inside of the jar. The displacement would likely be proportional to the pressure differerence. Furthermore, particles near the center would probably deflect the most, while particles near the wall of the tube would remain essentially fixed.

If this does happen, then it confirms that there is indeed a static force that is trying to push the RTV inwards, without needing to invoke any fluid flow at the interface. On the other hand, if the particles show absolutely no deflection, then we are left with no option but the "fluid flow --> shear force" theory.

For what it's worth, my personal intuition expects to see the RTV being deformed when we apply the pressure differential. If the pressure is large enough, this would be enough to shear the bond and propel the rope into the jar. But the "sauce flow model" would predict that the RTV would just not budge, no matter how high a pressure we applied.

 

[Chestermiller]

Here is a thought experiment that is a bit different from the noodle situation, but may help to validate or falsify a given model.

Take a rigid transparent plastic tube about one centimeter in diameter. Connect it to a suitable large syringe. Use the syringe to extrude a "rope" of clear RTV rubber through the plastic tube so that the tube is full of the RTV, plus you have some rope hanging out from one end of the tube. Allow the RTV to cure and set into a rope with one end stuck inside the plastic tube.

Before extruding, you need to add some colored particles to the RTV, so that you can observe any internal deformations that might happen inside the RTV during the actual experiment.

Now pass the tube through a hole in a plastic jar and seal it in place with part of the tube inside and part outside the jar. The extruded RTV rope hangs outside the jar. Note that the RTV is stuck in place within the tube, so there is no fluid flow at the junction - in fact, there is no gap at all between the RTV and the tube. The RTV is stuck firmly in place.

At this point we create a vacuum within the jar. As the pressure falls, we watch the marker particles that we embedded in the RTV. Although the RTV won't slide through the tube, it is quite possible that the pressure differential would distort the RTV such that particles in the tube would deflect towards the inside of the jar. The displacement would likely be proportional to the pressure differerence. Furthermore, particles near the center would probably deflect the most, while particles near the wall of the tube would remain essentially fixed.

This system is mechanistically very much different from the system involving a noodle (although it can readily be modeled using solid mechanics). The only need for an experiment would be to confirm the model predictions quantitatively. In post #102, I solved the solid mechanics of the noodle problem without invoking any noodle tension variation caused by viscous resistance at the lips (i.e., it was assumed that the lips are frictionless).

If this does happen, then it confirms that there is indeed a static force that is trying to push the RTV inwards, without needing to invoke any fluid flow at the interface. On the other hand, if the particles show absolutely no deflection, then we are left with no option but the "fluid flow --> shear force" theory.

For what it's worth, my personal intuition expects to see the RTV being deformed when we apply the pressure differential. If the pressure is large enough, this would be enough to shear the bond and propel the rope into the jar. But the "sauce flow model" would predict that the RTV would just not budge, no matter how high a pressure we applied.

In post #97, we already confirmed that, rather than driving the noodle into the mouth, the viscous forces actually do the opposite, providing a small amount of additional "frictional" drag to resist the noodle movement into the mouth.

Please read the entire thread to see the evolution of our thinking on this problem, rather than focusing on incorrect assessments early on in the thread. We can thank @PAllen for finally getting us going on the right track in posts #78, 80, 82, 85, 89, and 94.

 

[Andreas C]

That makes sense- it's the same hydrostatic pressure required to draw a 'fluid rope' up a height H. Nice!

Someone has to try sucking a really really long "noodle" with a vacuum pump or a vacuum cleaner or something to test it. I estimate the density of boiled spaghetti is something like... 1500-2000kg/m^3? They don't float in water but I don't think they're very dense. Let's pick 2*10^3 kg/m^3. If g=10, and Pa=101*10^3 N/m^2, then you wouldn't be able to suck noodles about... 5 meters long... Yeah, that's a pretty long noodle... I'm not sure how you test that...

 

[Chestermiller]

Someone has to try sucking a really really long "noodle" with a vacuum pump or a vacuum cleaner or something to test it. I estimate the density of boiled spaghetti is something like... 1500-2000kg/m^3? They don't float in water but I don't think they're very dense. Let's pick 2*10^3 kg/m^3. If g=10, and Pa=101*10^3 N/m^2, then you wouldn't be able to suck noodles about... 5 meters long... Yeah, that's a pretty long noodle... I'm not sure how you test that...

How about that "someone" be you? How about we make that your assignment?

 

[Andreas C]

How about that "someone" be you? How about we make that your assignment?

Uh... Sure, but... I don't really have any of the right tools for that... Where do you find a 5 meter long noodle?

 

[Baluncore]

Uh... Sure, but... I don't really have any of the right tools for that... Where do you find a 5 meter long noodle?

I do however notice that “two minute noodles” come in a pack that is often extruded, dried and then packed as a single length. By identifying and tagging the free end it would be possible to rehydrate the noodles without stirring, then to draw a single noodle thread back from the bundle.

But my concern here becomes one of quality control, that of the Chinese noodle product versus the Italian pasta product.

 

[Chestermiller]

Uh... Sure, but... I don't really have any of the right tools for that... Where do you find a 5 meter long noodle?

Get yourself a pasta press.

 

[Andreas C]

Baluncore, I'm afraid they don't sell that in my country. In fact even finding noodles isn't easy. Spaghetti is where it's at.

 

[Andreas C]

Get yourself a pasta press

Nah I'll skip on that. 100-something euros are more than I intend to spend for noodle experiments... If anyone can find massive pasta, he should try it. Or we could find a more readily available analogue.

 

[PAllen]

Someone has to try sucking a really really long "noodle" with a vacuum pump or a vacuum cleaner or something to test it. I estimate the density of boiled spaghetti is something like... 1500-2000kg/m^3? They don't float in water but I don't think they're very dense. Let's pick 2*10^3 kg/m^3. If g=10, and Pa=101*10^3 N/m^2, then you wouldn't be able to suck noodles about... 5 meters long... Yeah, that's a pretty long noodle... I'm not sure how you test that...

You would also need to do this from high railing of some kind, with the noodle hanging off. Otherwise most of the noodle's weight would be supported, e.g., a long noodle coiled in a plate could still readily be sucked in.

 

[BillTre]

You would also need to do this from high railing of some kind, with the noodle hanging off. Otherwise most of the noodle's weight would be supported, e.g., a long noodle coiled in a plate could still readily be sucked in.

Or try it with a variety of weights on the end of the noodle.

 

[Andreas C]

Or try it with a variety of weights on the end of the noodle.

Yes, I had a little self facepalm moment when I realized the solution could be so simple... But experiments involving meter long pasta are definitely more exciting than hanging weights on noodles.

 

[Andreas C]

I guess I'll try it then. I'll start by finding the density of spaghetti and take it from there. Probably not this week though.

 

[Andreas C]

Oh wait no, that's not going to work... It's not consistent with the assumptions, is it?

 

[Stephen Tashi]

With all due respect, this is silly

All physical models are silly in the sense of omitting details. For example, it's "silly" to model friction in a practical situation as a constant force. Textbook problems in elementary mechanics (which frequently involving frictionless contact) are silly in that sense. The interesting question, to me, whether elementary mechanics , applied to this particular problem, is "silly" in the sense that it cannot explain why the noodle is sucked into the mouth.

In the case of a rod (instead of a curved strand of spaghetti) elementary mechanics (meaning the mechanics that explains resultant forces in terms of other specific forces) seems unable to explain the component of force that pushes the sphagetti into the mouth except by saying that the force is the resultant of the forces on the ends of the rod. The forces on the side surface of the rod make no contribution.

If we now consider the curved shape of a strand of spaghetti, the original post can be interpreted as asking how pressure acting perpendicular to the side of the of spaghetti and the ends of the spaghetti can explain why there is is a component of force that moves the spaghetti into the mouth. (In this case, neither end of the strand of spaghetti need be in a vertical plane, so forces on those faces aren't entirely in the direction of "out of" or "into" the mouth.)

The original post asked:

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.

If we just assert the force that pushes the spaghetti into the mouth is the cross section of the strand times the pressure differential, this is reasonable model, but it doesn't explain how that model is deduced. To explain that, we need to go beyond elementary mechanics into fluid mechanics and the mechanics of materials.

Of course, there is a long internet tradition of answering a different question that was asked!

 

[Baluncore]

Is there an increase in diameter of the pasta as it passes between the lips ?
If so, how much of that increase is due to the differential pressure and how much due to tension ?