1.1 Basic fluid properties
One of the most important primary properties of a fluid (liquid or gas) is its viscosity — its
resistance to flow or to objects passing through it. Conceptually, viscosity might be thought of
as the ‘thickness’ of a fluid. In essence it is an internal frictional force between the different
layers of the fluid as they move past one another. In a liquid, this is due to the cohesive
forces between the molecules whilst in a gas it arises from collisions between the molecules.