Viscosity

After briefly going over streamlines, pathlines, and streaklines in Fluids, we moved on to cover vicsosity. It’s a word we hear all the time, but what exactly is viscosity? Before learning the actual definition, the way I always thought about viscosity is how easy it is for a fluid to flow.

For example, ketchup is a far more viscous than water. But why? The engineering definition of viscosity that we defined in class was as follows. Viscosity is a quantitative measure of the resistance of a fluid to a flow. So basically it’s like friction, but for fluids.

There’s even Newton’s Law of Viscosity. Where 𝜏=μ(du/dy). Any fluids following this law are Newtonian fluids, and any that don’t follow this law are known as non-Newtonian fluids. For example, Dilatants and Pseudoplastics are non-Newtonian fluids.

Pseudoplastics follow “shear-thinning”, meaning that when a shear force is applied, the fluid thins. Examples of this are ketchup and whipped cream. Have you ever shaken a bottle of ketchup to thin it out so it comes out of the bottle? That’s because ketchup is a pseudoplastic.

Dilitants follow “shear-thickening”. meaning that when a shear force is applied, the fluid thickens, e.g starch, sand-suspensions, etc.

That’s all I have for today. Man, I’m basically teaching 231 concepts in my blog. Lehigh should hire me.

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