A feel for thick and thin
Pour water and it splashes; pour honey and it crawls. Both are liquids, yet they resist flowing very differently. The study of that resistance — and of how materials deform and flow under force — is called rheology. It is one of the quiet workhorses of pharmaceutics, because almost every cream, syrup, gel and suspension you can name flows, and how it flows decides whether it is pleasant to use and easy to make.
The single number that captures “thick versus thin” is viscosity. Roughly, viscosity is internal friction: how strongly a fluid resists being made to flow. Honey has high viscosity, water low. To make this precise, we need to picture exactly what “being made to flow” looks like at the level of the liquid itself.
Sliding layers: shear stress and shear rate
Imagine the liquid as a stack of very thin layers, like a deck of cards lying flat. Slide the top card sideways and it drags the card beneath it, which drags the next, and so on down to the bottom card, which stays still. Each layer moves a little slower than the one above. The liquid is being sheared.
The sideways force per unit area you apply to the top layer is the shear stress (symbol τ, units pascals). How fast the layers slide relative to one another — the difference in velocity divided by the gap between them — is the shear rate (symbol γ̇, units per second). Viscosity is simply the ratio of the two: η = shear stress ÷ shear rate. A thick liquid needs a lot of stress to reach a given shear rate, so its η is large.
A first calculation, and a useful cousin
Worked example: finding viscosity from a measurement
A fluid is sheared between two plates.
Shear stress applied, tau = 50 Pa
Shear rate produced, gamma. = 25 /s
Viscosity:
eta = tau / gamma.
= 50 Pa / 25 /s
= 2 Pa.s (= 2000 cP)
So this fluid is about 2000x as viscous as water (~1 cP).You will also meet kinematic viscosity (symbol ν): the ordinary viscosity divided by the fluid's density, ν = η ÷ ρ. It tells you how a fluid flows under its own weight — handy for thin liquids that drain through a tube. A simple Ostwald viscometer measures it directly by timing how long a fixed volume takes to flow. The everyday word for all of this, used on a product spec, is consistency — the overall thickness and body a patient feels.