The Physics of Viscosity: Why Honey Flows Slowly, Water Splashes, and Blood Is Thicker Than Water (Literally)

The Thickness of Things

Pour water from a glass. It splashes, runs, and settles in an instant. Now pour honey from a jar. It crawls, oozes, and takes its time getting anywhere. Both are liquids. Both are mostly water, chemically speaking (honey is about 17% water). But their behaviour when flowing is dramatically different.

The difference is viscosity — a fluid’s internal resistance to flow. Water has low viscosity: its molecules slide past each other easily. Honey has high viscosity: its molecules (mostly sugars — fructose and glucose — in a concentrated solution) cling to each other and resist relative motion.

Viscosity is one of the most practically important properties in physics. It determines how blood flows through your arteries, how oil lubricates your engine, how lava erupts from a volcano, how the atmosphere circulates, how paint spreads on a wall, and how the Earth’s mantle convects over millions of years. And despite the underlying equations being known since the 1840s, the behaviour of viscous fluids — particularly when they become turbulent — remains one of the deepest unsolved problems in physics.

What Creates Viscosity

At the molecular level, viscosity has two origins, depending on whether you’re dealing with a liquid or a gas.

In liquids, viscosity arises from intermolecular attractive forces. When a liquid flows, molecules must slide past their neighbours, and the attractive forces between molecules resist this relative motion. Stronger intermolecular forces mean higher viscosity. Water has moderate viscosity because of hydrogen bonding. Glycerol (with three hydroxyl groups per molecule, forming extensive hydrogen bond networks) is about 1,000 times more viscous than water. Honey is viscous because of the high concentration of sugar molecules that form hydrogen bonds with each other and with the water molecules.

In gases, viscosity has a counterintuitive origin: momentum transfer. When layers of gas move at different speeds, faster molecules from one layer wander into the slower layer and speed it up (by transferring momentum through collisions), while slower molecules wander into the faster layer and slow it down. This molecular mixing creates a drag between layers — viscosity. Unlike liquids, gas viscosity increases with temperature because hotter molecules move faster and transfer momentum more effectively. Liquid viscosity decreases with temperature because thermal energy helps molecules overcome attractive forces.

The dynamic viscosity η is measured in pascal-seconds (Pa·s). Some values: air at 20 °C: 1.8 × 10⁻⁵ Pa·s. Water at 20 °C: 1.0 × 10⁻³ Pa·s. Olive oil: 0.08 Pa·s. Honey: 2-10 Pa·s. Peanut butter: ~250 Pa·s. Pitch (tar): ~10⁸ Pa·s. Glass at room temperature: ~10⁴⁰ Pa·s.

That range — from 10⁻⁵ to 10⁴⁰ — spans 45 orders of magnitude. Few physical quantities vary so enormously across common materials.

Laminar vs. Turbulent: The Reynolds Number

Fluid flow comes in two fundamental flavours: laminar and turbulent.

Laminar flow is smooth and orderly. Fluid moves in parallel layers that slide past each other without mixing. Honey pouring from a spoon is laminar. Blood flowing through a capillary is laminar. The flow is predictable, calculable, and quiet.

Turbulent flow is chaotic. The fluid breaks into eddies, vortices, and unpredictable swirls at every scale. A river rapid is turbulent. Smoke rising from a cigarette starts laminar near the tip and transitions to turbulent within a few centimetres. Turbulent flow is noisy, dissipative, and mathematically intractable.

The transition between them is governed by the Reynolds number:

Re = ρvL/η

where ρ is the fluid density, v is the flow velocity, L is a characteristic length scale (like pipe diameter), and η is the dynamic viscosity. Low Re (below about 2,000 in a pipe) means laminar flow. High Re (above about 4,000) means turbulent flow. In between is a transition zone where both can occur.

The Reynolds number is the ratio of inertial forces (which promote turbulence) to viscous forces (which suppress it). Thick, slow, small-scale flows are laminar. Thin, fast, large-scale flows are turbulent.

Honey from a spoon: Re ≈ 0.1. Blood in a capillary: Re ≈ 0.01. Water from a kitchen tap: Re ≈ 5,000 (turbulent). Air around a car at highway speed: Re ≈ 3 × 10⁶. Air around a commercial airliner: Re ≈ 10⁸.

Non-Newtonian Fluids: When Viscosity Gets Weird

Isaac Newton assumed that viscosity is a fixed property of a fluid — that how fast you stir it doesn’t change its thickness. For water, oil, and most simple liquids, this is true. These are Newtonian fluids.

But many common substances violate this assumption. They’re non-Newtonian, and their viscosity depends on how vigorously you deform them.

Shear-thinning fluids become less viscous when stirred faster. Ketchup is the classic example: it sits stubbornly in the bottle until you shake it, at which point it flows freely. Paint is shear-thinning: it flows smoothly under the brush but stays put on the wall. Blood is shear-thinning: red blood cells form aggregates (rouleaux) at rest, increasing viscosity, but these break apart under flow in narrow capillaries, decreasing viscosity when it matters most for circulation.

Shear-thickening fluids become more viscous when stressed quickly. A mixture of cornstarch and water (oobleck) can be stirred slowly with a spoon but behaves almost like a solid when you punch it. The mechanism: under rapid deformation, the suspended particles jam together, forming transient force chains that resist flow. Some body armour designs use shear-thickening fluids impregnated into fabric — flexible under normal conditions, rigid under ballistic impact.

Bingham plastics require a minimum stress (the yield stress) before they flow at all. Below the yield stress, they behave like a solid. Above it, they flow like a viscous liquid. Toothpaste is a Bingham plastic: it stays on your brush until you squeeze it. So is mayonnaise. And so is lava — below its yield stress, it sits like a solid; above it, it flows (slowly, because its viscosity is also very high).

Thixotropic fluids thin over time when sheared, then re-thicken when left alone. Yogurt: stir it and it becomes runny, leave it and it firms up. Some clays exhibit thixotropy, which can cause landslides: vibration (from an earthquake) liquefies the soil, which then re-solidifies after the shaking stops.

Turbulence: The Unsolved Problem

The Navier-Stokes equations — derived in the 1840s by Claude-Louis Navier and George Gabriel Stokes — describe the motion of viscous fluids with complete generality. Every swirl of smoke, every ocean current, every beat of a butterfly’s wing is, in principle, described by these equations.

But solving them for turbulent flow is essentially impossible analytically. The equations are nonlinear partial differential equations, and their solutions can be exquisitely sensitive to initial conditions — tiny perturbations can grow into completely different flow patterns. This sensitivity is a hallmark of chaos, and it makes long-term prediction of turbulent flows fundamentally limited.

We can simulate turbulence with computers (direct numerical simulation, or DNS), but the computational cost is staggering. The number of grid points needed scales as Re⁹/⁴. For atmospheric flows (Re ~ 10⁹), a full DNS would require about 10²⁰ grid points — far beyond any foreseeable computer. Engineers use turbulence models (simplified statistical approximations) for practical calculations, but these are empirical, not derived from first principles.

The Clay Mathematics Institute has offered one of its seven Millennium Prize Problems ($1 million) for a proof of existence and smoothness of solutions to the Navier-Stokes equations in three dimensions. As of 2026, the problem remains unsolved. We know the equations. We can see turbulence everywhere. We cannot prove that the equations always produce well-behaved solutions, and we certainly cannot solve them in general.

Richard Feynman called turbulence “the most important unsolved problem in classical physics.” It governs weather prediction, aircraft design, blood flow, ocean mixing, stellar convection, and the formation of galaxies. We understand it statistically (through the work of Kolmogorov and others) but not deterministically. We can model it approximately but not exactly. We live with it every day but cannot fully explain it.

What Viscosity Teaches Us

Viscosity is where physics meets the everyday. You experience it when pouring syrup, driving in cold weather, watching lava flow on a documentary, or bleeding from a paper cut. It’s not glamorous physics — there’s no quantum weirdness, no relativistic paradoxes, no exotic particles. Just molecules sliding past each other, resisting the motion, and creating patterns of flow that range from perfectly smooth to impossibly chaotic.

But I think that’s what makes it compelling. The Navier-Stokes equations are among the most practically important in all of physics, and they remain mathematically unsolved. Turbulence surrounds us — in every gust of wind, every river rapid, every stirred cup of coffee — and it’s still the last frontier of classical physics.

The universe, it turns out, is easy to describe at the very large (cosmology) and the very small (quantum mechanics). It’s the middle — the flowing, swirling, turbulent world at human scales — that remains the hardest to fully understand.