r/ChemicalEngineering • u/Importance-21 • Jul 31 '25
Research Can someone explain how viscosity is considered a fluid property?
I understand that viscosity has to do with how thick or resistant a fluid is to flow, but I’m a little confused on why it’s called a fluid property. What exactly makes it a property of the fluid and not flow? Would love a simple breakdown or analogy if anyone has one. Thanks!
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u/A_Losers_Ambition Jul 31 '25
Fluid viscosity is basically the internal friction between the molecules of a fluid.
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u/NCPinz Jul 31 '25
Viscosity is a property of the fluid no matter if the fluid is moving or not, so flow has nothing to do with it.
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u/YogurtIsTooSpicy Jul 31 '25
For non-Newtonian fluids, the viscosity does indeed change with respect to flow characteristics, so in that sense OP’s intuition is correct.
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u/nrubhsa Aug 01 '25
Fair. The viscosity of a Non Newtonian fluid is still a property of the fluid itself. It just happens to be dependent on state conditions.
Also, non-Newtonian fluids aren’t intuitive! 🙃
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u/Altruistic_Web3924 Aug 01 '25
Fluid composition is responsible for the intermolecular forces that directly affect its viscosity. Saying that it’s a flow characteristic solely because it is dependent on flow is no different than saying it’s an internal energy characteristic because it is dependent on temperature (which applies to both Newtonian and non -Newtonian fluids).
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u/vovach99 Jul 31 '25
Why do you differ fluid properties and flow properties? That's the same
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u/dlmobs Jul 31 '25 edited Jul 31 '25
I disagree. Fluid properties are consistent no matter the flow rate and is specific to the fluid itself. Calling it a flow property makes it sound like it’s applicable to the flow rate and not the type of fluid. “Flow property” is also not academically correct.
Edit: Grammar
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u/farfel07 Jul 31 '25
Viscosity can change with flow rate. Even for fluids we normally consider Newtonian, that is just a simplification of the truth.
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u/dlmobs Jul 31 '25
Viscosity can change with flow rate, but it is not a property of the flow rate. It’s still a fluid property.
Specific heat can change with temperature, but it’s not a temperature property, it’s a material property.
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u/Joecalledher Jul 31 '25
Lookup the etymology of the word fluid and you might discover how silly this question is.
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u/ferrouswolf2 Come to the food industry, we have cake 🍰 Jul 31 '25
Okay, I understand your question. You’re asking, essentially, “what is the sound of one hand clapping?”.
If the fluid is at rest, can it be said to have viscosity or resistance to flow? It’s not flowing!
The short answer is that all models are wrong but some models are useful. All words are models and aren’t always perfect. We, as engineers and humans, must do the best we can with what we’ve got.
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u/antiquemule Jul 31 '25
No. The last sentence is irrelevant to what we are discussing here.
For example, a fluid has a boiling point (at a given pressurure). That's it, It doesn't matter if it is actually boiling or not.
It's exactly the same with viscosity.
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u/admadguy Process Consulting and Modelling Jul 31 '25
The short answer is that all models are wrong but some models are useful
Epistemologically this is the only correct answer.
What u/ferrouswolf2 said.
If you understand this truly, OP's question is answered.
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u/antiquemule Aug 01 '25
I disagree. There is no "model" here, at least in the sense that Box, a statistician, meant when he coined the aphorism.
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u/admadguy Process Consulting and Modelling Aug 01 '25
It is all models, some capture more of the physics than others.
Also, just because the one who coined it was a statistician doesn't mean it cannot be applied to other fields. FWIW models don't necessarily have to be only statistical.
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u/ferrouswolf2 Come to the food industry, we have cake 🍰 Aug 01 '25
What about non Newtonian fluids? Isn’t the viscosity a function of shear rate?
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u/antiquemule Aug 01 '25
Just like boiling point is a function of pressure... Aren't these two examples exactly the same?
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u/admadguy Process Consulting and Modelling Jul 31 '25 edited Aug 01 '25
A possibly inaccurate but not too off way is to think of all fluids as binghamplastic of varying degrees and continuous even in flow. More viscous would be to visualise that they momentarily regain the binghamplasticity and need to overcome that continuously in flow. It's a matter of scale, both time and space.
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u/NickSenske2 Jul 31 '25
Here’s a fun one, there’s actually two kinds of viscosity. What most people think of is a fluid’s resistance to shear, but there’s also a resistance to a change in volume called the bulk viscosity
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u/CramponMyStyle Jul 31 '25
Viscosity belongs to the fluid itself because it comes from how the molecules in that liquid stick to each other. In honey, those molecules really like to hold hands with each other, making it viscous. In water, they barely hold hands at all, so it flows super easily.
τ = μ(du/dy)
"Wait, if viscosity depends on how fast the fluid is moving (du/dy), how can it be a material property?"
μ is the proportionality constant, not the result. The equation shows that shear stress is proportional to shear rate, and viscosity is that proportionality factor. The fluid owns the relationship. Different fluids have different μ values because of their molecular structure.
So even for non-newtonian fluids, viscosity describes how a fluid responds to applied stress, which is fundamentally determined by what the fluid is made of.