Apologies for the long post.
That is a misunderstanding of hydrophobicity then. If there is no force exchanged what is holding them apart? There are no bonding forces between them (hydrogen or Van der Walls) but they do have electrostatic interactions.
Is there a gap of air trapped between the oil floating on top and the water? If there really was an air layer trapped between the water and the wall (there isn't) then the water molecules would be interacting with the air molecules which would be interacting with the wall. The water molecules would still be stopped at the wall of the pipe. You can have gas molecules adhered to the surface, but that is more like a surface treatment of the material than a film of air between them. At the wall the water molecules still stop due to their interactions with the gas molecules which are stuck to the surface.
The water molecules do bounce in and out of being the ones "stopped" at the wall, so any particular molecule will eventually flow through.
Hydrophobicity is defined as how much the water, prefer itself, instead of a substrate.
The more hydrophobic the substrate is, the more the water will minimize the contact with it.
Hence, the usual test of hydrophobicity, is made by measuring the angle of contact, between a water droplet and a substrate.
The more the droplet adopt a round shape, the more the surface is hydrophobic.
The more the droplet wet the surface, spreading onto, the more the surface is hydrophilic.
Water is a tiny polar molecule, composed by 1 O atom and 2 H atoms.
Which mean that the electronic cloud density, around the molecule constantly change.
One of the H atom of a molecule, can have its electronic density, shifted toward the O atom.
Making himself more +, there is less electronic cloud density around the nucleus.
The other H atom instead will be more -, there is more electronic density around the nucleus.
This is what a polar molecule do, changing dynamically the electronic density around the molecule, exposing more - or more + areas, on its surface.
So in water, the major force keeping the fluid together are, the H-bonds, that are enhanced by the strong dipole momentum of the molecule.
Now, i mainly dealt with hydrophobicity in biological cellular membranes.
Lipids, or in general any hydrophobic material, do not expose polar groups, nor the electronic density around the molecule, vary too much.
That's why we call them apolar, because the hydrogen atom electrons, are not pulled or pushed toward the molecular bond.
That's why there is no transient dipole creation, the whole molecule is more or less neutral, like long hydrocarbon chains for example.
Not having any dipole is not a big issue, one can still have a functional group like OH, that care an electronegative atom like Oxygen.
Well guess what, the chain of C-H bonds we find in lipids, for example, is more or less symmetric and neutral, from an electrostatic point of view.
The force that keeps together gasoline and other hydrocarbon structures, is the dispersions forces.
Which translate basically, to the atom's own dipole itself, which is a small force compared to H bonds.
The strength of the dispersion forces increase, when increasing the numbers of atoms.
Hence, a hydrophobic molecule is somehow neutral, it does not attract, nor it is attracted by a polar molecule.
Even the opposite, apolar molecules can't stand polar molecules, doing everything they can to minimize the contact interface boundary.
Still, i suppose that the energy at the boundary of a polar/apolar mediums would be lower, compared to the boundary of poplar/polar, apolar/apolar mediums.
Tho, i haven't dived too much into the polar/apolar boundary interfaces topic.
By the way, this is what some superhydrophobic materials look like:
And this is the physics behind the superhydrophobicity of such materials, discovered from the self-cleaning propriety of Lotus leaves, flowers:
At the molecular level no commercially available pipes are smooth. Water molecules are tiny!
Smoothness is WAY more important than material choice and none of the tubing we use is very smooth. I mean molecularly smooth, e.g. electropolished stainless steel still isn't perfectly smooth.
This is because the resistance to flow is due to the viscosity of the fluid and not interactions with the wall.
Yes, but it was already established that a smooth structure would reduce the turbulences at the wall boundary.
Having a smooth wall lower the shear stress applied to the liquid when travelling into the pipe.
So it allows a better transition to a laminar flow, when one move away from the pipe walls.
The fact that the viscosity dictate the resistance of the flow, doesn't change the fact, that there is a real contact of the fluid, with the walls of the pipe.
The contact with the wall, is the cause of the shear stress applied to the liquid, when different layers of the liquid move at different speeds.
In few words, how much the liquid fight back when it is deformed, before the liquid layers slips on top of each others, is what define the viscosity.
So the resistance to the flow, is a function of the viscosity and of the length, radius of the pipe.
Obviously, larger, short pipes will have fewer issues with the flow, than longer, thin pipes.
It won't be and it isn't something to take into account. If it took any less energy or needed a lower RPM to flow water through the pipe at the same rate it means that
water would flow better though that pipe. There might be measurable differences in a very tiny tube with very precise measuring equipment, but in a custom water loop there are absolutely no differences. Even on that scale smoothing the walls has a much larger effect than changing the materials.
Even then, the reason we sometimes electropolish the inside of steel transfer lines is not to improve flow, but to prevent molecules getting trapped in the line and contaminating the next sample.
It is counter intuitive for sure, we don't think of water flowing through a pipe as being stopped at the walls, but it is. There are lots of scientific applications where we would love to solve this, but we cannot.
Edit:
If you want to look into it more look up "Boundary Layer" in fluid dynamics.
The thing that was counter intuitive for me, was thinking that removing or lowering the bonding energy of the water to the wall pipe, would allow an efficient flow.
As we said previously, the layer of the liquid, that is in the closest contact with the pipe wall, is almost static, stuck to the wall.
From the pipe walls, the liquid layers will begin to slide, one on top of each other, until reaching the maximum speed at the centre of the pipe.
If we apply a hydrophobic coating, that lower the bonding energy, between the pipe wall and the water.
Even the water molecules, that were stuck to the wall, are now able to move, reducing the liquid shear stress, so enhancing the flow.
What came out from the papers about the topic, as said previously, is quite the opposite.
Having this "no-slip" condition, where the water molecules are less bound to the pipe wall.
Have a negative effect when transitioning to laminar flow, when going away from the wall of the pipe.
There are some visible effects of enhanced rate of flow, but unfortunately only when the pipes are less than 0.5cm diameter and lower.
As usual, the physical effects is available only in the tiny world, once one switch to macroscopic, it looses all the proprieties.
It was fun to look at it.
From what i got, hydrophobic material are mainly used against icing and to keep away the water, moisture, self-cleaning surfaces.
And as usual, fluids mechanics are beautiful, but it makes you cry pretty easily.
Attached here, some papers about hydrophobic stuff.