Experiment Part D: Angle of Repose

Title

Angle of repose

Objectives 

  1) To measure the angle of repose.

  2) To determine factors that influence the angle of repose of the sand.

  3) To study the effect of glidant on the angle of repose.

Date of Experiment

30th September 2014

Introduction

Angle of repose is related to the flow properties of particles/solids as it is a mean of quantifying the interparticulate friction between particles that will affect the flow of particles. Angle of repose is measured upwards from the horizontal base of the pile of granular material that is held stable after the cylinder is removed. Angle of repose is important to be used to design the storage and transportation of granular materials.

Apparatus and Materials

100g of various sizes of sand, glidants(magnesium stearate), ruler, cylinder, weighing bolt, newspaper.

Procedures

1. 100g of 150, 355, 500, 850 micron and various sizes of sand are prepared.

2. The sand is placed in the cylinder with the rubber base at the bottom. 

3. The cylinder is removed and the sand is allowed to flow out and form a heap.

4. The height, slope and diameter of the heap are measured.

5. The angle of repose is calculated.

6. The experiment is repeated using other sand or with the addition of glidant.

Questions

1.       What are the angles of repose for the sands that you use?

100g of granular materials without glidant (result):

Granular materials (micron)	Diameter (cm)	Height (cm)	Angle of repose, θ(˚)
150 µm	2.6	3.2	56.56
355 µm	2.6	2.9	53.47
500 µm	2.6	2.8	52.36
850 µm	2.6	2.4	47.45
Various sizes of sands	2.6	2.6	50.00

100g of granular materials with glidant (result):

Granular materials(micron)	Diameter (cm)	Height (cm)	Angle of repose, θ(˚)
150	2.6	3.0	54.54
355	2.6	2.8	52.36
500	2.6	2.6	50.00
850	2.6	2.3	46.11
Various sizes of sands	2.6	2.5	48.75

2.        What are the factors that can influence the angle of repose of a powder?

    (a)    Particle size – Particle size decreases, total surface area of particles increases, VDW forces of attraction increases, angle of repose increases.

     (b)   Particle shape - Spherical particles have smaller angle of repose due to the tendency to roll.

   (c)    Cohesiveness – Fine particles may reveal cohesiveness due to the electrostatic effect which increases the angle of repose.

3.       What other methods can you use to measure the angle of repose?  

    (a) Tilting box method – The material is placed within a box with a transparent side to observe the granular test material. The box is slowly tilted and stopped when the material begins to slide in bulk. The angle of the tilt is measured.   

    (b)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. Pouring is stopped when the pile reaches a predetermined height or width.

    (c) Revolving cylinder method – The material is placed within a cylinder with at least one transparent face. The cylinder is rotated at a fixed speed. The granular material will assume a certain angle as it flows within the rotating cylinder.

Discussion

In the experiment, various sizes of sand are used to determine their angle of repose either with the presence of absence of glidant. Sand of 150mic, 355mic, 500mic, 850mic and various sizes are used to determine the angle of repose with and without the presence of various concentrations of glidants (0.5%, 1.0%, 2.0% and 3.0%).

This experiment shows that there is a relationship between the size and irregularity of granular materials that results in different in angle of repose. The smaller the size of particles, the higher is the angle of repose. This is because smaller particles have greater cohesive forces of attraction between them that holds them together due to the higher total surface area of particles.  Meanwhile, gravity plays a dominant role for the bigger particles and leads to a smaller angle of repose. 

Furthermore, sand with various sizes or said to have higher irregularity has higher angle of repose. This is because mixture of various sizes of sand tends to have more surface interaction or friction between the particles. In addition, the angle of repose will decrease with respect to the addition of glidant. Glidant reduces the frictional forces between particles which increases the flowability of particles and decreases the angle of repose.

Conclusion

1.The smaller the particle size, the bigger is the angle of repose.

 2. The greater the irregularity of the granular materials, the bigger is the angle of repose.

3.The angle of repose decreases with the addition of glidant at a certain range of concentration.

References

1. www.usc.edu/CSSF/History/2006/Projects/J0620.pdf

2.http://books.google.com.my/booksid=V3VIaBDTpJ8C&pg=PA159&lpg=PA159&dq=glidant+increase+angle+of+repose?&source=bl&ots=rI_EfAKm2m&sig=wRw8LlZO7j7ORy1gt9LfmPUQ8g&hl=en&sa=X&ei=IBpfVOyODIKauQTYi4KIAQ&ved=0CCcQ6AEwAQ#v=onepage&q=glidant%20increase%20angle%20of%20repose%3F&f=false