What is viscosity?

• A. The mass of a fluid.
• B. The property of a fluid that presents a resistance to flow.
• C. The density of a liquid.
• D. The boiling point of a liquid.

Answer: (B)

Viscosity is the property of a fluid that presents a resistance to flow. It arises from the internal friction between layers of liquid as they move past one another, caused by cohesive forces among the molecules. A liquid with high viscosity flows slowly (think honey), while one with low viscosity flows readily (think water).

In liquid penetrant inspection, viscosity matters because it influences how well the penetrant can enter small surface defects and how it behaves on the surface during application and cleanup. The right viscosity allows adequate penetration into cracks without excessive spreading or run-off, and it helps ensure the penetrant can be effectively removed after inspection. Temperature and formulation can alter viscosity, so operators adjust it to fit the inspection needs.

This property is different from mass or density, which describe how much material there is and how compact the material is, and from boiling point, which is the temperature at which a liquid becomes a gas.