INTRODUCTION
The widespread use of powders in the pharmaceutical industry has generated a variety of methods for characterizing powder flow. Four commonly reported methods for testing powder flow are (1) angle of repose, (2) compressibility index or Hausner ratio, (3) flow rate through an orifice, and (4) shear cell. Powdres are generally considered to be composed of solid particles of the same or different chemical compositions having diameters less than 1000 microns. Pharmaceutically, the largest use of powders is to produce tablets and capsules, in addition to mixing and compression properties, the flowability of a powder is of critical importance in production.Success or failure in many pharmaceutical operations can be directly linked to the flow properties of the powder being processed. Flowability is critically important when assessing how material moves around the plant. Industry needs the ability to develop formulations withtailored flowability, and first step is identifying suitable techniques for powder characterization.

Principle
Depending upon the attractive forces between the powder particles, powders may be free flowing or cohesive (sticky). Flow properties of a powder depend upon its particle size, particle size distribution, particle shape, powder density, surface texture, moisture content and electrostatic attractive forces between particles. Powder flow is important in manufacturing of tablets, filling of capsules, bottles, vials, and in powder mixing. If the granules do not flow properly from hopper to die cavity, it will cause weight variation problems in manufacturing of tablets.

When a powder is allowed to flow onto a flat surface, a conical heap is formed. The particles will slip and roll over each other, until the gravitational force is balanced by the cohesive forces between the particles. The angle formed by the heap with the horizontal surface is known as angle of repose. The lesser the angle of repose the better is the flow property of the powder. From the radius and the height of the heap, angle of repose can be determined. The tangent of angle of repose is a measure of the frictional forces between the flowing powder particles.Angle of repose of a powder can be found out by open end cylinder or fixed funnel method.

θ = Tan^-1 (h/r),
θ = angle of repose,
h = height of heap,
r = radius of heap