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What if the tubing was hydrophobic...?

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Would the fluids flow slightly quicker?
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Would the fluids flow slightly quicker?
If you mean hydrophobic coated tubing, it would alleviate the work done by the pump.
The fewer the bonds of the water to the material are, the more effortless the flow is.

Nevertheless, one still needs energy to move the liquid, hence you would be stuck with the max flow rate of the pump.
Tho, i dunno if one would not burn the pump in the process, or at least get out of it, its max rpm.

The same is true with a very hydrophilic coating, applied on the heat exchanging surfaces.
It would allow a better bonding of the water to the material, enhancing the heat transfer.
 

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Closest you would get to that kind of fluid dynamics is glass. Years ago I worked with some experimentation that used magnetic "dust" in a solution with water molecules. You could apply polarity and control the flow without using a pump/impeller. Pumps alter the flow dynamic and create static/turbulent points. You can also use a multi-modular tubing system to create a "perfect" flow. It removes any minor turbulence that is introduced to the system by uncontrollable means. Ever see one of those fountains that will shoot a blast of water that looks like a solid piece of glass (no air bubbles/deformations)? They use a multi tube system to get that effect.
 

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The liquid to inner wall interaction in the tube has basically no effect on flow. The molecules at the wall are stopped even with a hydrophobic tube and laminar flow, so resistance to flow (friction due to flow) is a function of the fluid interacting with itself (viscosity), not the tube wall. How smooth the wall is does make a difference, but the material itself does not.

The standard tubing we use where I work is PTFE (Teflon). It is chemically inert but also very hydrophobic. Water does not flow any better though it than steel tubing (which we use for high temps and/or pressures).
 

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The liquid to inner wall interaction in the tube has basically no effect on flow. The molecules at the wall are stopped even with a hydrophobic tube and laminar flow, so resistance to flow (friction due to flow) is a function of the fluid interacting with itself (viscosity), not the tube wall. How smooth the wall is does make a difference, but the material itself does not.

The standard tubing we use where I work is PTFE (Teflon). It is chemically inert but also very hydrophobic. Water does not flow any better though it than steel tubing (which we use for high temps and/or pressures).
According to the papers about the hydrophobicity, it seems that there is no contact between the water molecules, and the material itself.
Rather, the water slips on a thin air layer, deposited and bound to the material surface, instead of a static water layer, as you said, in the case of hydrophilic material.
Which in turn, change the dynamics of the flow of the water into the pipe, which also depend on the diameter of the pipe.

It's not about the smoothness, pipes are already built to be smooth, it is not the point here.

Tho, you are right, water will not flow better in a Teflon pipe, than in a glass pipe.
From what i understood, in general, once one supplied enough energy to move the water.
There is not much difference in the overall water flow, between hydrophobic and hydrophilic material.

So not sure, at this point, that the pump rmp, would be affected, as said previously.
Still, it is something to take into account instead, when dealing with water processes, with limited amount of energy available.
 

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According to the papers about the hydrophobicity, it seems that there is no contact between the water molecules, and the material itself.
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.

It's not about the smoothness, pipes are already built to be smooth, it is not the point here.
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.

Tho, you are right, water will not flow better in a Teflon pipe, than in a glass pipe.
From what i understood, in general, once one supplied enough energy to move the water.
There is not much difference in the overall water flow, between hydrophobic and hydrophilic material.
This is because the resistance to flow is due to the viscosity of the fluid and not interactions with the wall.

So not sure, at this point, that the pump rmp, would be affected, as said previously.
Still, it is something to take into account instead, when dealing with water processes, with limited amount of energy available.
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.
 

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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:
2516516



And this is the physics behind the superhydrophobicity of such materials, discovered from the self-cleaning propriety of Lotus leaves, flowers:
2516517


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.
 

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First a correction:

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.
This is very wrong. If there 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, depending on the thickness of the air layer, could encompass the entire boundary layer. This would make the flow resistance have the apparent viscosity of air instead of water, which would be a huge change! Hydrophobicity doesn't work that way, but if we could trap an air layer somehow it would do amazing things.

What if you had 4 smaller pipes that was ~about the same size as 1 bigger pipe?
It would be much worse, you can start to notice the difference at tiny diameters because the wall effects are such a large percentage of the volume (potentially 100%) but the higher the diameter the better for flow restriction, by a lot too.

I wonder if this thought process was spurred by something like this video?
(1) Hydrophobic Projectiles Slice Through Water With No Drag - YouTube

If so, it is a reasonable connection. However, this is actually a very different phenomenon from flow in a pipe and the reason hydrophobicity is important is not because it lets the water slip over the surface, at least not directly. Instead it keeps the water from wetting the sphere so the air stays entrained in that low pressure zone created as the water is pushed out of the way by the front of the sphere. Otherwise water is so sticky that surface tension would push the air out right away. With a sphere the surface is not parallel to the flow for more than an instant and the shape is important because it creates the negative pressure zone which keeps the air trapped as long as flow continues. The only direct water sphere interaction is at the very front of sphere, where it is a perpendicular interaction not a boundary layer. It still has to push the water out of the way, which is why it still falls slower through the water than it does air even if it is much faster than through water with drag from a water boundary layer too.
 

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I agree with @Asmodian, on pipes of 5mm of diameter, the liquid flow is enhanced only by 10/15%, compared to non-hydrophobic coated pipes.
Tubes half coated were showing strange effect, worth to investigate further.
It maybe provides a greater effect, when we come down to microfluidics, capillary stuff.


And having 4 smaller tubes instead of one, for the same volume, would require more work:
Change in radius alters resistance to the fourth power of the change in radius.
For example, a 2-fold increase in radius decreases resistance by 16-fold!

When i was looking for the hydrophobicity in pipes, i also found some papers, looking at hydrophobic object going through water.
I agree, the physical effect is more pronounced, than the one observed in pipes.
 
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