What Is Viscosity?
Viscosity describes a liquid’s resistance to flow. It depends on molecular interaction and internal friction and is a key factor affecting flow rate. Low-viscosity liquids such as water move more freely than thicker liquids. Viscosity matters in many industries and scientific fields, including the processing and transport of oils, chemicals and food. It affects both a liquid’s physical behaviour and the efficiency of the systems using it.
Shear Stress in Solids and Fluids
Solids strongly resist shear stress, while fluids offer much less resistance. Even under the smallest shear stress, fluids deform continuously.
Newton’s Factors for Shear Stress in Fluids
Newton Described These Factors:
- Plate velocity (v) is directly proportional to applied force (F).
- Force (F) on the moving plate is directly proportional to plate area (A).
- Force (F) on the moving plate is inversely proportional to the distance between plates (h).
Calculating Shear Stress
For linear velocity variation between parallel plates in a Newtonian fluid, shear stress is: τ = F/A = μ(v/h). Here:
- τ: Shear stress (Pa)
- μ: Dynamic viscosity (Ns/m²)
- v: Velocity (m/s)
- h: Distance between plates (m)
Velocity Variation and Non-Linear Shear Stress
Fluid velocity does not always vary linearly. For a Newtonian fluid, local shear stress is calculated using the velocity gradient: τ = μ(dv/dy). Here, dv/dy is the velocity change perpendicular to the flow. This simple linear relationship does not apply in the same way to non-Newtonian fluids.
Kinematic and Dynamic Viscosity
Calculating Kinematic Viscosity
Kinematic viscosity is dynamic viscosity divided by density: ν = μ/ρ. Here:
- ν: Kinematic viscosity (m²/s)
- μ: Dynamic viscosity (Ns/m²)
- ρ: Density (kg/m³)
Density and Specific Weight
Their relationship is: ρ = γ/g. Here:
- ρ: Density (kg/m³)
- γ: Specific weight (N/m³)
- g: Gravitational acceleration (m/s²)
Shear Stress and Kinematic Viscosity
For a Newtonian fluid, μ = ρν, so shear stress can also be written as τ = ρν(dv/dy).
Viscosity Units
In the SI System:
- Dynamic viscosity: Ns/m² or Pa.s
- Kinematic viscosity: m²/s
Factors Affecting Viscosity
Several factors determine liquid flow behaviour. Generally, viscosity falls as temperature rises, so warmer liquids tend to flow more easily. Hot water, for example, can flow faster than cold water. Temperature increases molecular mobility and affects fluidity. Chemical composition, molecular structure and molecule size also matter. Large, complex molecules can reduce flow and increase viscosity. Interactions between polar and non-polar molecules can likewise alter fluidity. Under high pressure, liquid molecules pack more closely, potentially increasing viscosity and reducing flow to create a denser structure.