By increasing the air pressure and resulting flow rate, the air exerts greater force on the sand and reduces the gravitational contact forces between the particles. By increasing the air flow further, the drag forces on the particles will eventually counterbalance the gravitational forces and the particles become suspended in the upward stream. The point where the bed starts to behave as a fluid is called the minimum fluidization condition. The increase in bed volume is insignificant when compared with the non-fluidized case (Fig. 1c).