Viscosity — Why Honey Flows Slower Than Water
Viscosity explained: internal friction in fluids, temperature effects, laminar vs turbulent flow, Stokes' law, and the Reynolds number.
Quick answer
Viscosity is a fluid's internal friction — how strongly its layers resist sliding past each other. High viscosity means slow, syrupy flow; low viscosity means easy, watery flow.
Unit: pascal-second (Pa·s)
Example: A steel ball falls slowly through honey (η ≈ 10 Pa·s) but plummets through water (η ≈ 0.001 Pa·s) — a ten-thousand-fold difference in internal friction.
What you will learn
- Define viscosity as internal friction between fluid layers
- Explain how temperature changes viscosity in liquids vs gases
- Distinguish laminar from turbulent flow using the Reynolds number
- Apply Stokes' law to a sphere falling through a viscous fluid
Internal friction: why honey dawdles
Pour honey and pour water. Same gravity, same tilt — wildly different behavior. The difference is viscosity: the internal friction between a fluid's own layers as they slide past one another.
Picture fluid flowing through a pipe as a stack of thin sheets. The sheet touching the wall barely moves (it sticks), while the center sheet races ahead. Each sheet drags on its neighbors — that drag is viscosity at work. More drag between sheets means a blunter velocity profile and slower overall flow for the same push.
Temperature changes everything
Temperature rewrites viscosity dramatically — but in opposite directions for liquids and gases:
- Liquids get runnier when heated. Warm honey pours; cold honey crawls. Heat loosens the molecular attractions that resist sliding. Motor oil ratings like 5W-30 exist precisely to guarantee flow on cold mornings and protection at engine temperatures.
- *Gases get slightly more viscous when heated.* Faster molecules collide more often and ferry momentum between layers more effectively.
Pressure barely affects liquid viscosity but matters for gases at high pressure. These trends are why engineers always ask at what temperature a viscosity value was measured.
Laminar vs turbulent: the Reynolds number
Watch dye injected into slow water flow and it draws a clean line: laminar flow, layers sliding in order. Crank up the speed and the line explodes into swirls: turbulent flow, chaotic eddies mixing momentum across the stream.
The referee is the Reynolds number, a dimensionless ratio of inertial to viscous forces:
In pipes, usually means laminar, turbulent. Because it is dimensionless, the same number predicts behavior from blood capillaries to oil pipelines — a triumph of dimensional reasoning.
Stokes' law: drag on a slow sphere
For a small sphere creeping through a viscous fluid, George Stokes found the drag force is beautifully simple:
Drag grows linearly with viscosity, radius, and speed — double any one and the drag doubles. Millikan's famous oil-drop experiment used exactly this law to measure the electron's charge.
Mini-example: a steel ball () falls at through oil (). Drag: — under a millinewton, which is why the ball drifts down gently instead of plummeting.
Stokes' law holds only at low Reynolds number (slow, small, viscous). Faster motion switches to quadratic drag — the realm of air resistance.
Two drag regimes: slow and syrupy → Stokes' linear law (F ∝ v); fast and airy → quadratic drag (F ∝ v²). Knowing which regime you are in is half of fluid dynamics.
Variables and units
| Symbol | Name | Unit | Meaning |
|---|---|---|---|
| Viscosity | Pa·s | Dynamic viscosity (internal friction) | |
| Stokes drag | N | Viscous drag on a slow sphere | |
| Radius | m | Sphere radius | |
| Speed | m/s | Speed through the fluid | |
| Reynolds number | dimensionless | Dimensionless ratio predicting laminar vs turbulent flow |
Common mistakes
They are independent. Mercury is far denser than water but less viscous than honey — it splashes almost like water. Density is mass per volume; viscosity is resistance to flow.
Flow rate also depends on pressure, pipe radius (flow scales with radius⁴ in laminar flow — Poiseuille's law), and length. A very viscous fluid can still flow fast through a wide pipe under high pressure.
Speed alone does not decide. The Reynolds number combines speed, size, density, and viscosity — a tiny slow microbe swims in a laminar world while a fast river is turbulent, but slow honey in a wide channel can be laminar too.
Real-world applications
Engine oil is graded by viscosity (like 5W-30) so it flows when cold yet protects when hot.
Blood viscosity affects circulation — dehydration thickens blood and raises the heart's workload.
Paint is formulated to be thin when brushed (high shear) but thick at rest so it does not drip.
Lava viscosity decides eruptions: runny basalt lava flows in rivers, sticky rhyolite lava explodes.
Frequently asked questions
What is viscosity in simple terms?
Viscosity is a fluid's resistance to flow — its internal friction. Honey has high viscosity and pours slowly; water has low viscosity and pours easily. The SI unit is the pascal-second (Pa·s).
Does heating a fluid make it thicker or thinner?
For most liquids, viscosity drops sharply as temperature rises — warm honey pours far faster than cold honey. Gases behave oppositely: their viscosity increases slightly with temperature because faster molecules transfer more momentum.
What is the difference between laminar and turbulent flow?
Laminar flow is smooth, layered flow where fluid slides in orderly sheets. Turbulent flow is chaotic, with swirls and eddies mixing everything. Which one occurs depends on the Reynolds number.
What does the Reynolds number tell you?
It predicts whether flow will be laminar or turbulent: Re = ρvD/η. Below about 2000 in a pipe, flow is typically laminar; above about 4000, typically turbulent. In between is a messy transition zone.