Скачать или смотреть To determine angle of repose and influence of glidant on angle of repose

Apparatus: Beaker, funnel, spatula, glass rod, stop watch, burette stand, graduated measuring cylinder, etc.
Chemicals: Calcium carbonate and starch granules, lubricants, etc.
Good flow properties are critical for development of any pharmaceutical tablet, capsule or powder formulation.
It is essential that an accurate assessment of flow properties be made in early development process as possible so that optimum formulation can be identified easily.
This avoids costly & time consuming studies of poor substances.
Most of the tests are related to flow of substances through different devices.
Interparticle forces or forces between particles as well as flow characteristics of powders are evaluated by angle of repose.
Angle of repose is defined as maximum angle possible between the surface of pile of sample and horizontal plane.
The angle of repose, or critical angle of repose,  of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping.
At this angle, the material on the slope face is on the verge of sliding. The angle of repose can range from 0° to 90°.
The morphology of the material affects the angle of repose; smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands.
When bulk granular materials are poured onto a horizontal surface, a conical pile will form.
The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material.
Material with a low angle of repose forms flatter piles than material with a high angle of repose.
It is essential that an accurate assessment of flow properties be made in early development process as possible so that optimum formulation can be identified easily.
Angle of repose is calculated by equation-
Tan θ= h/r

Θ= tan -1 h/r

Here, h= height of pile, r= radius of circle and Θ= angle of repose.
Angle of repose affects particle size distribution, as larger the particle size, it will flow freely & vice versa.
Applications:

The angle of repose is sometimes used in the design of equipment for the processing of particulate solids. For example, it may be used to design an appropriate hopper or silo to store the material, or to size a conveyor belt for transporting the material. 
Methods in determining the angle of repose
1. Tilting box method:
This method is appropriate for fine-grained, non-cohesive materials with individual particle size less than 10 mm. The material is placed within a box with a transparent side to observe the granular test material. It should initially be level and parallel to the base of the box. The box is slowly tilted until the material begins to slide in bulk, and the angle of the tilt is measured.
2. Fixed funnel method:
The material is poured through a funnel to form a cone. The tip of the funnel should be held close to the growing cone and slowly raised as the pile grows, to minimize the impact of falling particles. Stop pouring the material when the pile reaches a predetermined height or the base a predetermined width. Rather than attempt to measure the angle of the resulting cone directly, divide the height by half the width of the base of the cone. The inverse tangent of this ratio is the angle of repose.
3. Revolving cylinder method
The material is placed within a cylinder with at least one transparent end. The cylinder is rotated at a fixed speed and the observer watches the material moving within the rotating cylinder. The effect is similar to watching clothes tumble over one another in a slowly rotating clothes dryer. The granular material will assume a certain angle as it flows within the rotating cylinder. This method is recommended for obtaining the dynamic angle of repose, and may vary from the static angle of repose measured by other methods.
A glidant is a substance that is added to a powder to improve its flowability.
A glidant will only work at a certain range of concentrations.
Above a certain concentration, the glidant will in fact function to inhibit flowability.
In tablet manufacture, glidants are usually added just prior to compression.
Examples of glidants include magnesium stearate, fumed silica (colloidal silicon dioxide), starch and talc.
A glidant's effect is due to the counter-action of factors that cause poor flowability of powders.
Like correcting surface irregularity, reducing interparticular friction and decreasing surface charge.
The result is a decrease in the angle of repose which is an indication of an enhanced powder's flowability.