What exactly is a powder?From sugar to rice and from sand to cement different types of powders are engaged in our day to day lives. Yet a proper definition is lacking in expressing what a powder is, due to its complex behaviour. For example, powder is not a solid, but they can withstand some deformation like solids. Powder is neither a fluid nor a gas, yet they can flow as fluids and be compressed as gases [1]One can broadly define powder as a material, consisting of a collection of discrete particles, in which the particles are sufficiently numerous for the properties of the material to be controlled by the statistical behaviour of the totality of particles; rather than by the properties of the individual particles [2]. But this definition is too broad and inadequate to be utilized in understanding and designing particulate systems. It lacks the two phase flow properties of a powder.Therefore a more suitable definition for powder is proposed in [2] as, a dispersed two phase system consisting of particles of various sizes with a fluid as a continuous phase. The particles form a network, as a result of inter-particle forces, which can be expanded under certain circumstances, but which tends to return to the packed state under the influence of gravity. Powders have a certain mechanical strength and certain elasticity. There is a strong interaction between the gas phase and the dispersed solid phase, which influences the expansion capability of the powder. The higher the fluid viscosity and/or the fluid adsorption, the larger the expansion and the greater the improvement in flow properties.A particle does not necessarily be a solid particle. It can be an oil droplet in water etc. Hence a particle is a dispersed phase within a fluid. Rigid particles have a slightly different behaviour compared to compressible particles (deformable particles). [internet book] But in most of the cases, solid particles are concerned when studying about particulate systems.