Angle of Repose

1. Powder flowability characterization : angle of repose

The angle of repose is used an indicator of the flowability of bulk solids such as powders or granulates. It is a quick method that allows to get an idea of the flowability of granular materials.

It's actually important to highlight that there are different angles of repose, depending on the methodology that has been used to measure it. The most common method is to have the material poured freely through a funnel, the material forming a heap. But it is also possible to measure an angle by sliding the material on a plate, using a revolving cylinder, or leaving the powder flow out of a smaller container through a hole.

Figure 1 : Different methods for measuring the angle of repose of powders and granulates

Those different methods are actually giving different values of the angle of repose. It is thus very important to know according to which protocol the angle of repose has been measured in order to be able to compare the flowability of materials. The funnel method (external funnel) is giving the lowest measure of the angle of repose :

External funnel method angle

< Internal funnel method angle

< Sliding method angle

The lower the angle of repose, the more flowable is a material.

2. Calculation formula of the angle of repose

How to calculate the angle of repose ?

The angle of repose of a material can only be measured through an actual experience. The methodology considered here is the most common, the external funnel method, done by pouring a certain quantity of solid from a funnel and measuring the angle of the heap of material formed.

To be noted that even for this funnel method, there are also some variations in methodology. For example, the funnel can be fixed or can be slowly moved up. A specified height of the heap can be the parameter to stop the experience, or it can be the diameter of the heap formed.

If the angle of repose method is used within a lab to characterize the flowability of different components, it is important to chose one protocol and keep it for all measurement.

The formula to calculate the angle of repose is the following :

Angle_of_repose = arctan(h/r)

With

h : height of the heap (mm)
r : radius of the heap (mm)

3. Interpretation and relation to flowability of angle of repose

The flowability of a bulk solids is estimated by 1^st measuring the angle of repose, then referring to the table below to determine in which class is the powder studied :

Flowability expected	Angle of repose
Excellent / Very Free Flow	25-30
Good / Free Flow	31-35
Fair (discharging aid may not be required)	36-40
Passable	41-45
Poor Flow / Cohesive (active discharging aid required)	46-55
Very Poor Flow / Very Cohesive	56-65
Approximatively no flow	> 66

The table was determined by the authors of the method, by testing different material flowability, and measuring the corresponding angle of repose.

Note that it is an approximate method giving trends, it can happen that a material behave differently once in the process. The characterization of the flowability and the determination of physical parameters allowing the design of hoppers would require a full shear cell testing.

Other methods to define the flowability of powders or granules :

• Carr Index
• Hausner ratio
• Powder rheometer
• Shear cells testing