Viscosity is a measure of how strongly a fluid resists gradual deformation. Fluids with higher viscosity such as honey are thought of as “thicker” than less viscous fluids like water. The range of viscosities in liquids is enormous. Superfluids, such as Helium-II, have zero viscosity. Honey is between 2,000 and 10,000 times more viscous than water. Some fluids are so viscous they appear to be solid and yet, over time, slowly flow.
One of the most viscous liquids known is pitch, also known as bitumen, asphalt, or tar. Demonstrating its flow and measuring its viscosity is the subject of the longest continuously running scientific experiment, begun in 1927 at the University of Queensland in Australia. Prof. Thomas Parnell heated pitch (which dramatically decreases its viscosity), then poured it into a funnel with a sealed bottom. After three years (to allow the pitch to settle), the bottom of the funnel was removed and the funnel placed in a bell jar with a beaker below it. The pitch slowly flows out of the funnel, forming a large drop. About every decade a drop falls off into the beaker.More