PRACTICAL 4 : ANGLE OF REPOSE

AIM

1) To measure the angle of repose of the sand.
2) To study the factor that can influence the angle of repose of the sand.
2) To study the effect of glidant on the angle of repose.

INTRODUCTION

        Basically, the movement/slideness  of particle are due to  the angle of inclination that is more than frictional force between particles. In Addition, the object in motion will stop sliding when  angle of inclination is decrease to its limit, this due to the adhesion and cohesion When bulk granular material is poured on a horizontal surface of conical pile, it will form the internal angle between the surface of the pile and the horizontal surface is known as angle of repose. Angle of repose or the critical angle of repose is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90°.

       In this experiment, we were measuring the angle of repose of the sand with 355 micron, 500 micron, 850 micron and various sizes without and with the addition of glidants (0.5%,1%,2%,3%  of magnesium stearate). The experiment is done with a view to assessing the angle of repose of a substance and the factors that can influence the angle of repose.


APPARATUS AND MATERIAL

100g of 355, 500, 850 micron and various size of sand
Glidants
ruler


PROCEDURE 

1. The 355 micron sand (without addition of the glidant yet) was poured in a level surface allowing it to build from the top.
2. The height of the pile from the peak to the ground eas measured by using the ruler
3. The horizontal distance from the middle of the pile to the edge was measured by using the ruler.
4. The equation tan-1 (height/width) had been used to find the angle of repose.
5. The procedure 1-5 was repeated with the addition of glidant.
6. The procedure 1-6 was repeated by using 500 micron, 850 micron and various size of sand


RESULTS

CALCULATION

QUESTIONS

1. What is the angle of repose for each of the materials?











2. What are the factors that influence the angle of repose of the materials?

a) Particle size - As particle size increases, the angle of repose decreases.
b) Particle shape- Spherical particles have a smaller angle of repose compared to irregular shape particle due to a greater tendency to roll.
c) Cohesiveness - Very fine particles may reveal cohesiveness owing to the electrostatic effect which increases the angle of repose.
d) The method by which the angle of repose is measured.
e) Coefficient of friction between particles.


3. What other methods that can be used to determine the angle of repose of the materials?

(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 atleast 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

         Angle of repose is one of the methods used to characterize the flow of a material. In this experiment, angle of repose for different sizes of sand and with the presence of a glidant is determined. The factors that affect the angle of repose are also studied.

         From the result obtained, in the absence of glidant, the angle of repose decreases as the size of particles increases. This is because smaller particles have dominant cohesive and adhesive forces as compared to particle weight, whereas in bigger particles gravity plays a dominant role thus giving a smaller angle of repose. Studies have shown that angle of repose is also gravity-dependant. Cohesiveness of finer particles due to electrostatic forces causes difficulties in flowing thus forming a steeper pile which shown by the 355 micron sand having the highest angle (36.3°) compared to 500 and 850 micron sand. On the other hand we can see that mixture of various sizes of sand give a high angle of repose also (39.2°). When particles of different sizes and irregular shape is mixed together, the mechanical interlocking of particles increases and thereby increase the rolling friction. As a result a steeper pile with bigger angle of repose is formed. Generally, a value of θ <30° indicates ‘excellent’ flow whereas θ >56° indicates ‘very poor’ flow. The intermediate scale indicates ‘good’ (θ between 31–35°), ‘fair’ (θ  between 36–40°), ‘passable which may hang up’ (θ  between 41–45°), and ‘poor which must be agitated or vibrated’ (θ  between 46–55°). In this experiment, sand of 355 micron size have a fair flow, 500 and 850 micron size have a good flow, and various sizes of sand have a fair flow.

          When 3% concentration of magnesium stearate is added, the angle of repose increases. Magnesium stearate is used as a glidant. It may also act as a lubricant in reducing friction. Glidant functions in improving the flow of a material. From the result, 500,850 micron sand and various sizes sand have a ‘passable which may hang up’ flow. On the other hand, 335 micron sand have a poor flow. The addition of a glidant should decreases the angle of repose and enhances the flow of the materials. But however the addition of 3% magnesium stearate in this experiment produces a product of viceversa. This is because the rate of flow is improved by the addition of magnesium stearate up to a limiting concentration of glidant. Above a certain concentration (in this case 3%), the glidant will in fact function to inhibit flowability. Thus, a glidant will only work at a certain range of concentrations. From the result of other groups, it is shown that addition of magnesium stearate is only effective at concentration of 0.5% and 1% where the angle of repose decreasing with the presence of a glidant. This proved it function in improving the flow of the material.

           For a better result, it is adviced to use a protractor instead of ruler as it reduces parallax error and it measures an accurate angle. The glidant and the sand should be mixed until an even distribution of mixture is obtained. This is to ensure that the glidant function effectively. Besides, the lifting velocity of the cylinder should just be moderate to avoid distraction to the material flow. For a clean and tidy working environment, a paper should be placed before starting the experiment so that the sand did not cluttered elsewhere. Experimenter must also wear goggle, mask and lab coat all the time to protect the eyes and nose from coming in contact with the sand.



CONCLUSION

From the experiment, the angle of repose for different materials is measured to describe the flow of each material. Smaller angle indicates a good flow property compared to bigger angle. Several factors that influence the angle of repose is also determined which are the particle size, particle shape, cohesiveness and the method by which the angle is measured. Smaller particles have a bigger angle of repose due to the cohesiveness. This cohesivity causes a poor flow. Mixture of particles with various sizes also gives a bigger angle of repose owing to the friction. Besides the angle of repose is also gravity-dependant. The flow of the materials is improved with the addition of a glidant at low concentration. The glidant only work at a certain range of concentration.


REFERENCES

1. Liu, Zhichao (2011) Measuring The Angle Of Repose Of Granular Systems Using Hollow Cylinders ; http://d-scholarship.pitt.edu/6401/
2. Rakhi B. Shah, Mobin A. Tawakkul, Mansoor A. Khan (2008 February 15) Comparative Evaluation of Flow for Pharmaceutical Powders and Granules ; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2976911/
3. T. M. JONES (1969) The effect of glidant addition on the flowability of bulk particulate solids ; http://journal.scconline.org/pdf/cc1970/cc021n07/p00483-p00500.pdf
4. Aug 05, 2011 Angle of Repose ; http://www.slideshare.net/visualbeeNetwork/angle-of-repose