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The Pitch Drop

How a Material That Shatters Like a Solid Can Flow for Decades

Big Question: How can a material resist deformation strongly enough to feel solid in your hand yet continue flowing under a small force for years?

Quick Answer

Pitch is an extraordinarily viscous material. Its resistance to flow is so large that ordinary observation times make its deformation nearly invisible. Under a sustained force such as gravity, however, it continues to creep. The University of Queensland’s Pitch Drop experiment, begun in 1927 by Thomas Parnell, was designed specifically to make that slow flow visible. UQ reports that only nine drops have fallen and that the pitch is roughly 100 billion times more viscous than water.

University of Queensland — Pitch Drop Experiment →

The deeper lesson is not simply “pitch is a liquid.” Real materials can respond differently to slow loading and sudden loading. Under rapid impact pitch can fracture; under persistent stress it can flow. Material behaviour depends on stress, temperature and timescale.

What You Will Learn

• Why visual appearance is a weak test of state and flow.
• What viscosity measures.
• Why a huge viscosity can hide motion from human perception.
• Why brittle fracture and slow flow can coexist in one material.
• How stress and strain rate differ.
• Why temperature changes pitch flow strongly.
• What a long-duration experiment can reveal that a short experiment cannot.
• Why the exact “drop time” is not a perfect material constant.
• How environmental changes affect the historical experiment.
• How to separate observation, inference and model.
• How timescale reasoning transfers to glaciers, polymers, mantle rock and biological tissue.
• Why “solid versus liquid” is sometimes too coarse a question.

Part 1 — The Naive Model: If It Holds Its Shape, It Is a Solid

At Primary level, classifying materials by shape and volume is useful. Solids keep shape; liquids flow and take the container’s shape.

That model works for many everyday situations—but it silently assumes an everyday observation time.

If a material takes ten years to change shape noticeably, a ten-second classroom observation will misclassify its mechanical behaviour.

Part 2 — Viscosity Is Resistance to Flow

For a simple Newtonian fluid, shear stress τ and shear rate γ̇ are related by:

τ = ηγ̇

where η is dynamic viscosity.

A larger viscosity means the same applied stress produces a smaller rate of deformation. Water changes shape rapidly. Pitch changes shape extraordinarily slowly.

The equation does not say the high-viscosity material never flows. It says its flow rate can be extremely small.

Part 3 — Slow Does Not Mean Zero

This is the central measurement problem.

Suppose a material deforms by only a tiny fraction of a millimetre in an hour. Your eye may report “nothing happened.” Repeat the observation over months and the accumulated displacement becomes measurable.

below the detector’s resolution ≠ physically absent.

Long-duration experiments convert tiny rates into large total changes.

Part 4 — The University of Queensland Experiment

In 1927, Professor Thomas Parnell heated pitch and poured it into a glass funnel with its stem sealed. He allowed the material to settle for three years. In 1930 the stem was opened.

Gravity has been pulling on the pitch ever since.

The first drop took about eight years to fall. By 2014, the ninth drop had made contact with the material below. The experiment is still running.

UQ Physics Museum — Famous Pitch Drop Experiment →

Part 5 — The Naive Model Fails Twice

The first failure is obvious: “it looks solid, so it does not flow.”

The second is subtler: “if it flows, it cannot fracture like a solid.”

That second claim is also too simple. Sudden loading asks a material to deform on a very short timescale. Molecular rearrangements may not occur quickly enough, stress concentrates, and brittle fracture can occur.

Slow loading gives the structure much more time to rearrange and flow.

Part 6 — Timescale Is a Hidden Variable

Scientists often compare an observation timescale with a material’s internal relaxation timescale.

If the observation is much shorter than relaxation, the material can appear elastic or rigid. If the observation is much longer, viscous deformation can dominate.

This is one reason polymers, asphalt, biological tissues and geological materials cannot always be understood using a single “solid” or “liquid” label.

Part 7 — Stress Is Not the Same as Deformation Rate

Gravity supplies a continuing body force. That force creates stress within the pitch.

The deformation rate depends on how the material responds to that stress.

Two materials can experience similar stresses yet deform at radically different rates because their viscosities or rheological structures differ.

Part 8 — Why Temperature Matters So Much

Viscosity is strongly temperature-dependent. Heating usually increases molecular mobility and reduces viscosity; cooling does the opposite.

The UQ experiment was not originally maintained in a perfectly controlled thermal environment. UQ notes that seasonal conditions and later air-conditioning changes affected the flow rate.

This matters because the time between drops is not a pure universal constant of “pitch.” It reflects both the material and its environmental history.

Part 9 — Why One Drop Interval Cannot Be Treated as a Perfect Measurement

A tempting calculation is to divide one drop time by another and infer a viscosity change directly.

But the geometry changes as pitch leaves the funnel. The remaining head pressure changes. Temperature changes. Drops can touch material below before detaching. The apparatus itself has a long history.

The experiment therefore demonstrates high viscosity brilliantly, but precise parameter estimation requires a model of the changing geometry and environment.

European Journal of Physics — The Pitch Drop Experiment (1984) →

Part 10 — Observation vs Inference

• Observation: pitch slowly changes shape and forms descending drops.
• Observation: room-temperature pitch can feel hard and fracture under sharp impact.
• Measurement: the flow occurs over multi-year timescales and is sensitive to temperature.
• Inference: pitch has extremely high resistance to flow but non-zero long-term deformation.
• Model: viscous or viscoelastic descriptions connect stress, deformation rate and timescale.
• Boundary: the historic apparatus is not a perfectly controlled modern rheometer.

Part 11 — The Better Model: Ask Three Questions

1. What stress is applied?
2. How quickly is the material allowed to respond?
3. At what temperature?

Those three questions are often more informative than asking for one permanent label such as “solid” or “liquid.”

Part 12 — Transfer: Glaciers

Ice in your hand is a solid. A glacier is also ice, yet over long times and high stress it deforms and flows downhill.

The microscopic mechanisms differ from pitch, so a glacier is not simply “another pitch drop.” But the reasoning transfer is valid:

a material can be rigid on a short timescale and flow on a long one.

Part 13 — Transfer: Earth’s Mantle

Mantle rock supports seismic shear waves, showing solid-like elasticity on earthquake timescales. Over millions of years it also creeps, allowing convection and plate motion.

Again, the constitutive physics is not identical to pitch. The transferable skill is timescale-aware classification.

Part 14 — Transfer: Polymers and Asphalt

Polymer-rich materials can creep under sustained load. Asphalt pavements can slowly deform under traffic and heat. Cold materials may crack under rapid stress yet soften or flow under warmer, slower loading.

The Pitch Drop therefore becomes a gateway into rheology rather than an isolated curiosity.

How Do We Know?

• Long-term photography measures displacement that the eye cannot detect in real time.
• Drop timing establishes persistent flow over decades.
• Temperature records help explain changes in rate.
• Controlled rheometers measure stress versus deformation rate directly.
• Repeated tests at different temperatures quantify viscosity changes.
• Impact tests reveal short-timescale fracture behaviour.
• Models can be checked against the observed geometry and flow rate rather than accepted from appearance alone.

The Failed Model → The Better Model

Naive model	Why it fails	Better model
It holds its shape, so it is not flowing.	Observation time may be too short.	Measure deformation rate over a suitable timescale.
If it flows, it cannot shatter.	Rapid and slow loading probe different response regimes.	Material behaviour depends on stress rate and relaxation time.
One drop interval equals one exact viscosity.	Temperature and geometry change.	Infer viscosity with controlled conditions and an explicit model.
“Solid” and “liquid” are always sufficient labels.	Real rheological behaviour can span both idealised categories.	Describe the constitutive response and timescale.

Checkpoint Questions

1. What does viscosity measure?
2. Why can pitch look motionless while still flowing?
3. Why can pitch fracture under sudden impact?
4. Why is timescale a hidden variable in material classification?
5. How does temperature affect viscosity?
6. Why is the historic Pitch Drop not a perfectly controlled rheometer?
7. What observation directly defeats the claim that pitch never flows?
8. Why does a single drop interval not define a universal material constant?
9. How does the same reasoning transfer to glaciers?
10. What three questions should replace a simplistic “solid or liquid?” classification?

Answer Key

Open after attempting the questions

1. Resistance to flow or deformation rate under stress.
2. Its deformation rate is below easy short-term visual detection.
3. Short-timescale molecular relaxation can be too slow, allowing stress concentration and brittle failure.
4. The same material can show different dominant behaviour over seconds, years or geological time.
5. Heating generally lowers viscosity strongly for pitch-like materials.
6. Its temperature, geometry and boundary conditions changed through its long history.
7. The measurable descent and formation of drops over years.
8. The apparatus and thermal conditions are not constant.
9. Ice can be rigid locally yet creep and flow over long periods.
10. Applied stress, response timescale and temperature.

Unfamiliar Transfer Challenge

A newly discovered material supports a 1 kg block for five minutes without visible deformation. A month later the block has sunk 2 mm into it. A hammer strike chips the surface.

Do not classify it from one observation. Propose the next measurements you would need: deformation versus time under several stresses, temperature dependence, recovery after unloading, and rapid-impact behaviour. Then decide whether a viscous, elastic, plastic or viscoelastic model—or a combination—is justified.

Deep Science Window — Deborah Number

Rheologists often compare a material relaxation time λ with the observation time T using a dimensionless ratio called the Deborah number:

De = λ / T

When De is large, elastic or solid-like response can dominate the observation. When De is small, flow has enough time to develop. The exact interpretation depends on the material model, but the ratio formalises the key lesson: behaviour depends on timescale.

Deep Science Window — Constitutive Laws

Newton’s law of viscosity is one constitutive law. Hooke’s law for elastic solids is another. Viscoelastic models combine springs and dashpots mathematically to represent materials that store and dissipate energy over different timescales. Real pitch may require a more detailed constitutive description depending on temperature and loading history.

Evidence Boundaries

• Hard to the touch ≠ zero flow.
• Flowing over years ≠ behaves like water over seconds.
• Brittle fracture ≠ proof of permanent solid-like behaviour.
• One historic drop interval ≠ exact universal viscosity.
• Timescale dependence ≠ all material categories are meaningless.
• Pitch analogy ≠ identical microscopic mechanism in glaciers, mantle rock or polymers.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: viscosity, stress, strain rate, creep, relaxation time, temperature dependence.

CONNECT: high viscosity to tiny deformation rate, tiny rate to long observation time, and loading timescale to fracture versus flow.

EXPLAIN: why pitch can shatter under a blow and still drip under gravity.

APPLY: analyse an unfamiliar slow-deforming material without relying on appearance.

CHECK: ask whether the experiment’s timescale, temperature and stress match the claim being made.

Research Sources and Further Reading

• University of Queensland — Pitch Drop Experiment
• University of Queensland Physics Museum — Famous Pitch Drop Experiment
• European Journal of Physics — The Pitch Drop Experiment