page_title: Viscosity
section_title: Selected substances
hierarchical_section_title: Viscosity / Selected substances
caption_attribution_description: Picture of the Pitch Drop Experiment at the University of Queensland, with 9-volt battery for size comparison
context_page_description: The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
  Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For instance, when a fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. In such a case, experiments show that some stress is needed to sustain the flow through the tube. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion: the strength of this force is proportional to the viscosity.
  A fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. Zero viscosity is observed only at very low temperatures in superfluids. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity; such fluids are technically said to be viscous or viscid. A fluid with a high viscosity, such as pitch, may appear to be a solid.
context_section_description: Observed values of viscosity vary over several orders of magnitude, even for common substances (see the order of magnitude table below). For instance, a 70% sucrose (sugar) solution has a viscosity over 400 times that of water, and 26000 times that of air. More dramatically, pitch has been estimated to have a viscosity 230 billion times that of water.
caption: In the University of Queensland pitch drop experiment, pitch has been dripping slowly through a funnel since 1927, at a rate of one drop roughly every decade. In this way the viscosity of pitch has been determined to be approximately 230 billion (2.3×1011) times that of water.[61]
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