Though I maintain that the answer will eventually boil down to a mathematical summation, I do believe the question has great pragmatic interest on a molecular scale (including a few military application I can postulate) but I don’t feel that these can be readily explored on a layman level, nor would they satisfy the teeming Millions, who frankly are not going to be mollified by an answer that only applies to spaghetti.
“But what of gummi worms?” they will cry, and then run down a shopping list of objects that only share broad principles of material properties "What of Lumbricus terrestris (the earthworm) or wet shoestrings or… well, let’s just say that Cecil may not suck, but my experience with the Unwashed Masses in the Emergency Room is that they do suck – with infinite variety, but little discretion. Eventually some wannabe sci-geeks with altogether inadequate empirical experience will declare that sexual intercourse is impractical, and probably apocryphal, due to their analysis of theoretical pump action and suction effects.
Or somesuch. (The problem with Darwin is that he is too darn slow)
What the Teeming Milions really want --nay, demand-- is a generalized mechanism.
So such I shall provide.
First, let us establish that a rigid bar can be sucked into a mouth. Diamond, plutonium, candy cane – it matters not-- but Jello, likewise, can be sucked, as can a stream of water from a water fountain or bathwater from a tub. This suggests that rigidity is a red herring. The force of the air on the end of a candy stick is not what pushes it into the mouth.
I can prove this.
Let us posit a near-zero friction liquid (some friction is needed to preserve conventional fluid properties. If you want to mess around with sucking experimental assemblies containing, say, Helium superfluids, well, it isn’t medically recommended (see Jearl Walker’s essay on his experiences with cryogenic fluids in the human oral cavity in the inestimable Halliday, Resnick textbook “Fundamentals of Physics” – which he now co-edits) or just take my word for it: bad idea
Take a hollow ball. Drill a hole in it that fits your rigid rod (say a candy stick) reasonably well, but not perfectly – it should not establish a perfect seal. Fill the ball with your low-friction fluid and insert the rod part way, displacing some fluid, and leaving no air. Create a second identical assembly for use as a control. We will assume sufficient surface tension to barely retain the the liquid, and keep it from pouring out of the stick/ball assembly when they assembly is placed in a horizontal orientation. C----
Place the free end of your test rod in you mouth. Hold the control rod in an identical orientation in a test stand. Suck on the test rod. It pulls into your mouth, but the external air pressure differential is not being transmitted through the distal end, because that end is covered by your ball, and any net force on the ball would squeeze more fluid out of the ball, before it was transmitted to the rod, yet you can suck the ball-tipped rod into your mouth as readily as an untipped rod.
A physics or mathematical analysis will show that --sucked or not-- the forces on the fluid in the ball are balanced, except for inertial pseudo-forces due to the low velocity of the assembly being sucked into the mouth (in any suitable frame of reference). To control for the pseudo inertial effects, move the control assembly in exactly synchrony to the sucked assembly – heck, link the two assemblies with frame of diamond or infinitely rigid unobtainium to guarantee identical motion. Sucked or not, any leakage will be identical. Sucking one end of a rigid rod doesn’t appreciably alter the balance of forces on the other.
If there is not net change in force on the ball, and the low-friction lubricating oil (oe water or whatever) is not squeezed out of the ball when you suck the other end, the force on the candy cane must be more proximal to the mouth. It must be exerted on the sides of the rod.
If a rigid rod isn’t really being pushed by unbalanced forces on the distal end, the lay conception is utter crap, and we can dismiss it for either a rigid rod or flexible spaghetti. Who cares if the spaghetti flops when you poke the end. You can suck it just as well by putting a water-filled ball on the distal end. The distal end is being pulled not pushed.
Let’s “sciencify” it even more, and use a suction pump to suck a free-floating globule of water into a rubber hose. If the globule were being pushed distally, it would visibly compress in the direction of motion, due to the unbalanced force on one side. If the net force were spread over its entire surface, then it would shrink symmetrically as its volume disappeared into the tube. This experiment was done, albeit as an amusing demo for school children, rather than as a well-funded research program by the pasta conglomerates – guess what happened? At moderate velocity, the globule remained roughly spherical as it disappeared into the tube. It didn’t flatten from being pushed in the direction of motion.