Tar pitch is widely regarded as one of the slowest moving liquids on Earth. While it appears solid at room temperature, it is, in fact, flowing, albeit at an incredibly low rate.
The Famous Pitch Drop Experiment
The best-known demonstration of pitch's liquid nature is the Pitch Drop Experiment. Several institutions around the world maintain these experiments, where a sample of tar pitch is placed in a funnel and allowed to drip over decades.
Based on observations from these experiments, including information like that from the Wikipedia article on the Pitch drop experiment, we know that:
- At room temperature, tar pitch flows at a very low rate.
- It takes several years to form a single drop.
This extremely slow flow rate highlights its exceptionally high viscosity.
Understanding Tar Pitch
Tar pitch is a complex viscoelastic polymer. Its appearance is brittle and solid, leading many to mistake it for a solid material. However, under gravity and over extended periods, it undeniably flows, behaving like a liquid with incredibly high viscosity. Its resistance to flow is vastly greater than everyday liquids like water or honey.
Why is its Slowness Significant?
- Demonstrates Viscosity: It serves as a powerful, visual example of how viscosity can vary dramatically between substances.
- Challenges Perception: It challenges our intuitive understanding of what constitutes a "liquid" versus a "solid," showing that flow is a matter of degree and observation timescale.
- Scientific Study: Pitch drop experiments allow scientists to study extremely slow flow phenomena.
While other substances might exhibit extremely slow flow under specific conditions (like certain glasses sometimes considered supercooled liquids), tar pitch is the most classic and widely cited example used to illustrate the concept of an extremely slow-moving liquid due to its prominent demonstration in long-term experiments.
