Viscosity 101

• Viscous stresses in a fluid result from relative velocity of different fluid particles.
• These stresses depend on spatial gradients of flow velocity.
• For small velocity gradients, viscous stresses depend only on first derivatives of velocity.
• Viscosity tensor maps velocity gradient tensor onto viscous stress tensor.
• There are 81 viscosity coefficients in total.
• Newtonian fluids have a linear dependence on first derivatives of velocity.
• Viscous stresses must depend on spatial gradients of flow velocity.
• The resulting SI units are (length)^2/time.
• Viscous stresses must depend on spatial gradients of flow velocity.
• The general relationship can be written in Cartesian coordinates.
• Maxwell used the term fugitive elasticity for fluid viscosity.
• Some liquids react like elastic solids when subjected to sudden stress.
• Some solids flow like liquids under small stress, making them viscoelastic.
• Viscoelastic solids can have shear viscosity and bulk viscosity.
• Extensional viscosity is a linear combination of shear and bulk viscosities.
• It is used for characterizing polymers.
• Earth materials in geology can also be viscoelastic.
• Viscoelasticity describes both elasticity and viscosity.
• It is a reaction to deformation and rate of deformation.
• Granite can flow like a liquid under small stress.
• Viscosity is the measure of a fluid's resistance to flow.
• It is measured with viscometers and rheometers.
• The SI unit of dynamic viscosity is the newton-second per square meter (N·s/m2).
• The most frequently used systems of US customary units are the British Gravitational (BG) and English Engineering (EE).
• Viscosity tends to increase with temperature in gases and decrease with temperature in liquids.
• Above the liquid-gas critical point, the mechanisms of momentum transport interpolate between liquid-like and gas-like behavior.
• The viscosity of a system depends on how the molecules constituting the system interact.
• Viscosity in gases arises principally from the molecular diffusion that transports momentum between layers of flow.
• An elementary calculation for a dilute gas at temperature and density gives the value of viscosity.
• The quantity, the mean free path, measures the average distance a molecule travels between collisions.
• Viscosity is a measure of a fluid's resistance to flow.
• It is affected by temperature, pressure, and the intermolecular interactions between molecules.
• There are different methods for measuring viscosity, including rotational viscometry and capillary viscometry.
• The viscosity of liquids can be calculated using empirically derived expressions based on existing viscosity measurements.
• Viscosity of gaseous mixtures can be calculated using the Chapman-Enskog approach.
• This approach uses the individual component viscosities, their respective volume fractions, and intermolecular interactions.
• The dependence of viscosity on intermolecular interactions enters through collisional integrals.
• These integrals may not be expressible in terms of elementary functions.
• Viscosity is an important factor in many industrial and scientific applications.
• It is used to determine the flow behavior of fluids and to optimize their performance.

Take this quiz to test your knowledge on viscosity, the measure of a fluid's resistance to flow. Learn about the different types of viscosity, including shear viscosity, bulk viscosity, and extensional viscosity, and how they are used to characterize materials such as polymers and Earth materials in geology. Discover the relationship between viscosity and temperature, pressure, and intermolecular interactions, and how it is measured using viscometers and rheometers. Challenge yourself with questions on the viscosity of gases and liquids,