Viscosity Explained: Causes, Units and Examples Guide

Viscosity is a fluid's resistance to flow — the internal friction that makes honey pour slowly and water pour quickly. It arises because adjacent layers of a moving fluid attract each other and drag on one another, so the faster layers are slowed down and the slower layers are pulled along. In liquids this drag comes from intermolecular attractions; in gases it comes from molecular collisions.

Viscosity matters wherever fluids move: it decides how oil lubricates an engine, how blood flows through your veins, how lava from a volcano spreads, and how easily crude oil can be pumped through pipelines. Engineers must measure it to design pumps, paints, medicines and even ketchup bottles.

Key Concepts

• Viscosity (η, eta): SI unit pascal-second (Pa·s)
• Dynamic vs kinematic viscosity: dynamic viscosity η measures the drag itself; kinematic viscosity ν = η / ρ (ρ = density) has units m²/s and matters when gravity drives the flow.
• Laminar vs turbulent flow: in laminar flow, fluid moves in smooth parallel layers; above a critical speed, flow turns turbulent and chaotic.
• Temperature effect: heating a liquid weakens intermolecular attractions, so liquid viscosity falls sharply with temperature. Gas viscosity, by contrast, rises slightly with temperature because faster molecules collide more.
• Newtonian vs non-Newtonian fluids: Newtonian fluids (water, air) keep constant viscosity at any shear rate; non-Newtonian fluids (paint, blood, cornflour paste) change viscosity when stirred or pressed.

Formulas

• Newton's law of viscosity: τ = η (dv/dy), where τ is shear stress (Pa), and dv/dy is the velocity gradient (s⁻¹) between adjacent layers. Valid for Newtonian fluids in laminar flow.
• Stokes' law (drag on a sphere): F = 6π η r v, where F is drag force (N), r the sphere radius (m), v its speed (m/s). Applies at low speeds (low Reynolds number) in a viscous fluid.

Worked Example

A small steel ball of radius 2.0 mm falls at a steady (terminal) speed of 0.050 m/s through glycerine of viscosity η = 1.41 Pa·s. What is the viscous drag force on the ball, and how does it compare with the ball's weight? Take the ball's density as 7800 kg/m³.

1. Stokes' law: F = 6π η r v = 6 × 3.1416 × 1.41 × 0.002 × 0.050 = 2.66 × 10⁻³ N.
2. Ball volume = (4/3)π r³ = (4/3) × 3.1416 × (0.002)³ = 3.35 × 10⁻⁸ m³.
3. Weight = density × volume × g = 7800 × 3.35 × 10⁻⁸ × 9.8 = 2.56 × 10⁻³ N.
4. Drag (2.66 × 10⁻³ N) ≈ weight (2.56 × 10⁻³ N), which confirms the ball falls at constant terminal velocity — the two forces balance.

Common Misconceptions

• "Viscosity is the same as density." Wrong. Mercury is very dense but flows easily (low viscosity); honey is less dense but highly viscous. Density is mass per volume; viscosity is resistance to flow.
• "Thick liquids always flow slower everywhere." Non-Newtonian fluids break this rule: ketchup (a shear-thinning fluid) flows faster when shaken or squeezed, and quicksand-like cornflour paste stiffens when struck.

Real-World Applications

• Engine oils: graded by viscosity (e.g. 10W-30) so they stay thick enough to protect parts when hot yet flow easily on a cold start.
• Medicine: doctors watch blood viscosity because thicker blood (high red-cell count or dehydration) forces the heart to work harder.
• Food industry: viscosity of syrups, sauces and dough is measured for consistent texture and pumpable processing.

In short, viscosity is internal friction between fluid layers, measured in Pa·s. Newton's law and Stokes' law describe it, and it falls with temperature for liquids.