Newtonian and Non Newtonian Fluids

HEY STEEMIANS!

Figure: Splashing Water
Source: https://pixabay.com/en/splashing-splash-aqua-water-rain-165192/

Non-Newtonian, eh! What does this even mean plus is this the same Newton guy who discovered gravity and stopped us from flying. But again what does Newtonian Fluid even mean! Haha. Have Patience. All your questions will be answered.

Before starting, wanna clear this confusion. FLUID. Have met many who think that FLUID is anything LIQUID. But no, that is not the case. FLUID is anything that flows (Even the air).Or if you're more of a technical definition kind of person, "Fluid is anything that has no fixed shape but responds to an external stimuli."

Prerequisite Section

A.P.S(After Pre-Script ;)) Before you flag me for using equations, I've tried to minimize them as much as possible. The ones used are only to demonstrate or make you understand the complex idea associated. Also read somewhere in the comments, where @lemouth said that if he uses equations, the number of readers decreases by almost three times. And this makes you wonder, if there's a "SAYNAYTOEQUATIONS" Group functioning around. Please, do not flag me. ;)

Shear Stress: State of stress caused by the force applied parallel to the surface. Also can be defined as the tangential component of the force acting along the surface over that given area. Shear Stress is denoted by Greek symbol τ(Tau).

• The Force responsible for this stress can also be called as the Shearing Force. Pretty Obvious!

τ=F/A, where F= Force Component Parallel to the Surface, A= Area over which the force is applied.

Figure: Shear Stress direction with the deformation
Source: https://commons.wikimedia.org/wiki/File:Shear_stress.JPG

Viscosity: It's the internal resistance of the fluid to flow. Or in a much simpler way, measure of how thicc the fluid is ;) haha. Let us take water and honey for example. If they are allowed to flow on a surface, who do you think will win the race. If you know what honey is, you know the answer is water. That means the resistance within the water is less compared to that of the honey. So, in this case, Honey is the thicc one.

• Denoted with Greek symbol η(eta), is known as the Coefficient of Viscosity, or the index of viscosity.

You probably are shaking your head over this friction thing. Let me explain it in a much simpler way. Suppose, water is flowing over a surface with a height(depth) of, let's say, 10 cm.

Now, suppose that, this 10 cm height is made up of infinite number of layers having almost negligible thickness, stacked on top of each other. Now, a stimulus is applied, the layers now start to flow, but relative to each other, meaning, they flow with different speeds. This difference in speed is, what is responsible for the resistance within the fluid. They are literally holding themselves back.

Figure: Velocity Profile along the depth with boundary layer, Ignore the boundary layer :)
Source: https://commons.wikimedia.org/wiki/File:Shear_stress.JPG

• Because of this, the layer at the beneath has almost negligible velocity, and is taken as 0 in calculations, and is termed as THE NO SLIP CONDITION. Too many layers stacked upon this layer, I'd day infinity and I still wouldn't be wrong ;)
• So, that means the layer at the top has the greatest velocity, because there are no any other layers stacked on top of it? Yeah, you got it.
• Now, let us take two adjacent layers, having relative speed du, within a distance of dy. Now, du/dy, is what is known as the shear rate. Is represented with Greek letter γ(gamma). In other terms, Change of velocity between the layers is what is known as shear rate.[Flow being one directional]

Now relation between all three terms.

Shear stress(τ)= Viscosity Index(η)* shear rate(γ)

Main Section

1. Newtonian Fluid: For these kind of fluid, the viscosity index remains constant, for a constant temperature. Meaning, there exists a direct relation between the shear stress and the shear rate. The graph is linear. If the shear stress is changed by 'x' amount, the shear rate also gets changed by 'x' amount.

Example: Water, Alcohol, Air.

The reason I did not put an emphasis on this part is because I wanted to talk about Non-Newtonian Fluids a lot more, and present you with the cool examples, and also if I did that, this post would be too bloody long.

2. Non-Newtonian Fluids: I guess, this does not need to introduction. The name gives it away, but for the sake of you know writing, those fluids, whose coefficient of friction does not remain constant, for a given temperature are known as Non-Newtonian Fluids.

• Basically, there are four different types of Non-Newtonian Behavior.
  1. Shear Thickening (Dilant): As the name suggests, the viscosity increases with increase in stress.
  2. Shear Thinning (Pseudoplastic): Opposite of Shear Thickening.
  3. Rheopectic: A type of time dependent. Viscosity Increases with increased duration of stress. Example: Printer Ink, Cream.
  4. Thixotropic: Opposite of Rheotropic. Example: Paint, Glue, Yogurt

Figure: Graph of Shear Stress vs Shear Strain, for both Newtonian and Non-Newtonian Fluids, the gradient is what is known as the coefficient of VIscocity.
Source: https://commons.wikimedia.org/wiki/File:Rheology_of_time_independent_fluids.svg

Will be Focusing on Shear Thickening and Shear Thinning. What is the point of learning if there aren't any COOL examples to observe.

a. Shear Thickening (Dilant): So, viscosity Increases with increase with shear force. One of the finest examples observed in nature is QUICKSAND, mixture of sand and water along with some proportion of clay.

• This is why you're advised not to move, when you're caught up in this. The Increased Movement leads up to no Movement at all. But the thing that it will swallow you is a myth. So, you know why you should stay Calm and Call for help, if you ever get stuck in one.

Figure: QuickSand, Shear Thickening Fluid
Source: https://commons.wikimedia.org/wiki/File:Quicksand_in_Courthouse_Wash_(9737357112).jpg

• Other Examples may Include Mixture of Cornflour and Water. (Cook friends, now you know why)
• Human Blood.

b. Shear Thinning (Pseudoplastic): Opposite of Shear thickening. Meaning Viscosity decreases with Increases Stress. You guys could even guess this.

HINT: Shake it to get it ;)

Before I lament about the pervert thinking, that has found a place, in the 21st century, the answer is KETCHUP, which makes for a fine fake blood. And if you ever have a career in detective, now you know how to distinguish between a ketchup (fake blood) and the real blood. Just shake it!

Figure: Ketchup, Shear Thinning Fluid
Source: https://pixabay.com/en/tomatoes-ketchup-sad-food-veggie-1448361/

That awkward moment, when your prerequisite section is longer than the main section, but doesn't matter as long as we learn something. And I guess, we did.

I have Included a video from YouTube in the comment section, as I was not sure about whether or not it was considered a free source. The video shows, people walking, running and dancing on a Non-Newtonian Fluid.

Comparison Section

So, the innate behavior, to find out who's won it after all.

• If we were to count the types of different types of fluids, then Non-Newtonian wins it by a fair amount of margin, but, since, air and water constitutes a larger portion of the earths' biosphere, if were to add it all, then Newtonian seems to have a upper hand.

References

1. Shear Stress
2. Viscosity
3. Newtonian and Non-Newtonian Fluids
4. Viscosity of Newtonian and Non-Newtonian Fluids

Images

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For Free Images: https://pixabay.com/

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