Definition
Viscosity is a measure of a fluid’s (liquid or gas) resistance to gradual deformation by shear stress or tensile stress. It is essentially “internal friction” between the molecules of a fluid as they slide past each other.
- How to read: “Viscosity.”
- Meaning: A fluid’s thickness or resistance to flow.
Why It Matters
Viscosity determines how fluids behave in pipes, engines, and biological systems. In rocketry, fuel viscosity affects pump performance; in medicine, blood viscosity affects heart health. Mastering viscosity is essential for fluid dynamics and chemical engineering.
Core Concepts
- Cohesive Forces: Viscosity is determined by the strength of the attractive forces between molecules. In liquids, these are primarily intermolecular bonds; in gases, viscosity arises from the momentum exchange between colliding molecules.
- Temperature Effects: In most liquids, viscosity decreases as temperature increases (molecules move faster and bonds weaken). In gases, viscosity actually increases with temperature.
- Newtonian vs. Non-Newtonian: A Newtonian fluid has a constant viscosity regardless of the stress applied (like water). A non-Newtonian fluid’s viscosity changes with the rate of shear (like ketchup or Oobleck).
- Poiseuille’s Law: Describes the flow rate of a viscous fluid through a cylindrical pipe, showing that flow is inversely proportional to viscosity.