The main cause behind viscous drag

In summary, the main cause behind viscosity and viscous drag is the internal friction between different fluid particles. This friction is due to the transfer of momentum between regions with different velocities. The force responsible for this friction is not gravity, but most likely electromagnetic interaction between adjacent molecules. The other two forces, adhesion and cohesion, also play a role in molecular attraction and contribute to viscosity. However, there currently is no microscopic theory for dissipative processes, such as viscosity. The details of viscosity are more closely related to thermodynamics and the fluctuation-dissipation theorem. Additionally, in experiments, the thin fluid layer in contact with a solid is considered to be stagnant due to the no-slip boundary condition imposed by the Navier-Stokes equation
  • #1
T C
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Poster has been reminded to avoid personal speculation when asking questions
What's the main cause behind viscosity and viscous drag? Let's see it in the molecular level. Drag occurs because one molecule attracts another. And the foremost reason for which one mass can attract other is gravity. But, it's too weak in comparison to other forces in nature and at the molecular level, it's presence is negligible. The other two forces responsible for molecular attraction is adhesion and cohesion force. So, no other candidate.
This is just my thought and I want to hear from others about this matter.
 
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  • #2
Viscosity of a fluid is due to internal friction between different fluid particles moving with different velocities, transferring momentum from regions where the velocity is larger to regions where the velocity is smaller.
 
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  • #3
Viscosity is certainly not due to gravitational interaction between the molecules. Most likely it is due to electromagnetic interaction between adjacent molecules.

P.S Gravity between the fluid and the Earth might play a role.
 
  • #4
Delta2 said:
Viscosity is certainly not due to gravitational interaction between the molecules. Most likely it is due to electromagnetic interaction between adjacent molecules.
I have already rejected gravity as the force. And the other two forces that are available in the whole universe is Electromagnetic and nuclear. Nuclear has no role and that's very easy to understand. Therefore the only candidate left is electromagnetic. Now, electromagnetic has many kind of expressions including adhesion and cohesion. Kindly tell me whether this expressions of electromagnetic forces is responsible for the drag or not.
ergospherical said:
Viscosity of a fluid is due to internal friction between different fluid particles moving with different velocities, transferring momentum from regions where the velocity is larger to regions where the velocity is smaller.
In all kind of viscosity related experiments, the thin fluid layer in contact with the inner wall of the tube or container has been considered to be stagnant. Can you explain what kind of force causing that stagnancy?
 
  • #5
T C said:
Now, electromagnetic has many kind of expressions including adhesion and cohesion. Kindly tell me whether this expressions of electromagnetic forces is responsible for the drag or not.
Well if it is one of those two it is cohesion of course since we are talking about molecules of the same fluid.
 
  • #6
Delta2 said:
Well if it is one of those two it is cohesion of course since we are talking about molecules of the same fluid.
And viscous drag is also applicable when something solid passes through the fluid. And that means Adhesion. Right?
 
  • #7
T C said:
And viscous drag is also applicable when something solid passes through the fluid. And that means Adhesion. Right?
I think so yes.
 
  • #8
So, sure, there are four fundamental forces, two of them act outside of an atom and one of those only over long distances, which makes the other one the principal actor in many physical interactions. But that doesn't necessarily mean it is useful to try and describe a particular interaction using that force.
 
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  • #9
Delta2 said:
Viscosity is certainly not due to gravitational interaction between the molecules. Most likely it is due to electromagnetic interaction between adjacent molecules.

T C said:
I have already rejected gravity as the force. And the other two forces that are available in the whole universe is Electromagnetic and nuclear. Nuclear has no role and that's very easy to understand. Therefore the only candidate left is electromagnetic. Now, electromagnetic has many kind of expressions including adhesion and cohesion. Kindly tell me whether this expressions of electromagnetic forces is responsible for the drag or not.

The electromagnetic force is conservative and therefore cannot account for viscosity, which is a dissipative process (dissipation of momentum). AFAIK, there is no (at least, not yet any) microscopic theory for dissipative processes.

It's the same problem trying to explain friction in terms of electromagnetism.
 
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  • #10
T C said:
Summary:: i want to know the main cause behind viscous drag in fluid. IMO it's the adhesion, cohesion force.

This is just my thought and I want to hear from others about this matter.
I think you will find the details have more to do with thermodynamics than the exact nature of the forces. In particular the fluctuation-dissipation theorem says that Brownian motion and viscous drag are related...
https://en.wikipedia.org/wiki/Fluctuation-dissipation_theorem#Classical_version

This shows up in Nyquist noise in resistors and various other transport coefficients
 
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  • #11
Thanks! But I am more interested to understand the basic forces behind viscosity.
 
  • #12
T C said:
In all kind of viscosity related experiments, the thin fluid layer in contact with the inner wall of the tube or container has been considered to be stagnant. Can you explain what kind of force causing that stagnancy?
This is correct. When solving the Navier Stokes equation for a viscous fluid one imposes the boundary condition ##\mathbf{v} |_{S} = \mathbf{0}## at any interface ##S## with a fixed solid. The physical motivation for this boundary condition is the presence of attractive molecular forces between the fluid and the solid.
 
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  • #13
T C said:
Thanks! But I am more interested to understand the basic forces behind viscosity.
Basic classical mechanics and Newton's laws can't explain everything. Statistical mechanics goes beyond those basics. Study of statistical mechanics will lead you to thermodynamics. So, to understand better, you must broaden your horizon.
 
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  • #14
ergospherical said:
The physical motivation for this boundary condition is the presence of attractive molecular forces between the fluid and the solid.
And it's known as adhesive force. And, IMO it's electron bonding that gives rise to such forces. So, at the end of the day, it's electromagnetic.
 
  • #15
Two things:
1 ~All of the interactive forces between stuff we deal with are electromagnetic so this tells us nearly nothing. OK gravity may be slightly involved.
2 In order create "friction" or "drag" one must be in contact with many degrees of freedom (to dissipate and spread energy) which is why I referred to the FD Theorem
The forces themselves are not dissipative. You might want to research the "arrow of time" https://en.wikipedia.org/wiki/Arrow_of_time
Its a good but not simple question.
 
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  • #16
The way I learned it is basically ergospherical’s description in posts 2and 12.
 
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  • #17
There is no doubt that viscosity is the result of momentum transfer from high velocity area to low velocity area. But, the main point is HOW. At molecular level, friction or similar kind of forces can't exist. What can exist is molecular collision and momentum transfer by that process. At molecular level, electromagnetic forces can exist in only two ways; first electrostatic attraction/repulsion due to opposite or similar charges and/or bonding due to exchange of electrons at the outer shell. Most probably adhesion and cohesion occurs due to electron exchange.
In this video, how Tesla view viscosity has been described and it's the basic principle behind Tesla turbine. The basic thought comes to mind after viewing this video where viscosity has been defined at 4.18.
 
  • #18
This has been answered (see #15). As mentioned ( in #2 thanks to ##e^{i\pi}## ) the presence of the rest of the system allows momentum and energy to "dissipate". This statistical mechanism is detailed in the fluctuation-dissipation theorem.
The actual forces themselves are invariant under time reversal and will not provide the answer. But it is a very good question.
 

1. What is viscous drag?

Viscous drag is a force that resists the motion of an object through a fluid, such as air or water.

2. What is the main cause of viscous drag?

The main cause of viscous drag is the friction between the fluid and the surface of the object moving through it.

3. How does the viscosity of a fluid affect viscous drag?

The higher the viscosity of a fluid, the greater the resistance to motion and therefore the greater the viscous drag.

4. Can the shape of an object affect viscous drag?

Yes, the shape of an object can greatly impact the amount of viscous drag it experiences. Objects with streamlined shapes experience less drag compared to those with irregular shapes.

5. How can viscous drag be reduced?

Viscous drag can be reduced by using a streamlined shape, reducing the speed of the object, or by using a more viscous fluid. Additionally, surface modifications, such as adding a smooth coating, can also help reduce viscous drag.

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