Kinds of Fluids

• Some differentiate fluid from solid by the reaction to shear stress. It is a known fact said that the fluid continuously and permanently deformed under shear stress while solid exhibits a finite deformation which does not change with time. 
• It is also said that liquid cannot return to their original state after the deformation. 
• This differentiation leads to three groups of materials: solids and liquids. This test creates a new material group that shows dual behaviors; under certain limits; it behaves like solid and under others it behaves like liquid (see chart of Fluid mechanics). The study of this kind of material called rheology.
• It is evident from this discussion that when a liquid is at rest, no shear stress is applied.
• The fluid is mainly divided into two categories: liquids and gases. 
• The main difference between the liquids and gases state is that gas will occupy the whole volume while liquids has an almost fix volume. 
• The difference between a gas phase to a liquid phase above the critical point are practically minor.
• Hence, the pressure will not affect the volume of liquid. 
• In gaseous phase, any change in pressure directly affects the volume. The gas fills the volume and liquid cannot.