When does any material flow?

Materials can flow in any number of ways. The flow and motion of materials, especially liquids is dealt in classical physics through quantum mechanics or relativity. The study of liquids comes under fluid dynamics which is again divided into hydrodynamics (flow of water) and aerodynamics (flow of air).

Fluids consist of both liquids and gaseous components as they constantly change shape when a force is exerted on them. Gases are compressible as they can either spread out or compress to fill in the volume of space available. But liquids are non-compressible as they do not compress or spread out and fill the volume of space completely.

Most of the materials flow because its particles and molecules do not form a rigid and definite arrangement at normal temperature and pressure. Changes in temperature and pressure can be the main reasons for the flow of materials. Liquids flow but stay together in the same form and shape because of the existence of attraction between the molecules and particles. Since these attractions are weak in liquids, there is greater space between the particles. Thus they flow with changes in temperature and pressure. In the case of most gaseous components, flow and movement occur due to the pressure difference. When there is a region of high pressure (more molecules and particles at a lesser space), they attempt to break free and move to a region with low pressure (fewer particles and molecules). This difference in pressure is the basis of the movement of gaseous components.