Viscosity, such as in honey, results in a slower flow rate compared to a less viscous fluid like water. The term "viscosity" is derived from the Latin word for mistletoe, viscum, as mistletoe berries produce a viscous glue also called viscum. Common symbols for viscosity include the Greek letters mu (μ) and eta (η). The inverse of viscosity is fluidity.
Viscosity measures a fluid’s resistance to flow:
The SI unit for viscosity is newton-second per square meter (N·s/m²). It is also commonly expressed as pascal-second (Pa·s), kilogram per meter per second (kg·m⁻¹·s⁻¹), poise (P or g·cm⁻¹·s⁻¹ = 0.1 Pa·s), or centipoise (cP), with water at 20 °C having a viscosity of about 1 cP or 1 mPa·s.
In American and British engineering, viscosity may also be measured in pound-seconds per square foot (lb·s/ft²) or pound-force-seconds per square foot (lbf·s/ft²).
Viscosity can be reported in two ways: absolute or dynamic viscosity, which measures a fluid’s resistance to flow, and kinematic viscosity, which is the ratio of dynamic viscosity to a fluid’s density. Two fluids with identical dynamic viscosity values may have different densities, resulting in different kinematic viscosity values. Consequently, dynamic and kinematic viscosity are measured in different units.
At 20 °C, the dynamic viscosity of water is 1.0016 millipascal-seconds (mPa·s) or 1.0 centipoise (cP). Its kinematic viscosity is 1.0023 centistokes (cSt), 1.0023×10⁻⁶ m²/s, or 1.0789×10⁻⁵ ft²/s. The viscosity of liquid water decreases significantly as temperature increases, with water’s viscosity at 80 °C being 0.354 mPa·s. Conversely, water vapor viscosity increases with rising temperature.
Water has a relatively low viscosity, higher than most liquids with molecules of similar size due to hydrogen bonding between water molecules.
Newton’s law of friction is crucial for understanding viscosity:
τ = μ dc / dy = μ γ
where:
Rearranging the terms provides the formula for dynamic viscosity:
μ = τ dy / dc = τ / γ
A Newtonian fluid adheres to Newton’s law of friction, meaning its viscosity is independent of the strain rate. Non-Newtonian fluids do not follow this law and can deviate in various ways:
Viscosity is measured using viscometers and rheometers. A rheometer is a specialized type of viscometer. These devices measure either the flow of a fluid past a stationary object or the movement of an object through a fluid, with the viscosity value representing the drag between the fluid and the object’s surface. Accurate measurements require laminar flow and a small Reynolds number.
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