Studies of the Effects of Inter-particle Forces on the Properties of
Granular Matter Using Iron Particles in a Magnetic Field
Sean Hutton
(Formerly of Monash University, Melbourne Australia - Currently at Center for the Physics of Geological Processes University of Oslo)
A novel method of altering inter-particle force (Fip) is used to examine the effect on the properties and behavior of granular materials as inter-particle
force is altered. The method involves using iron particles in a magnetic field. Using this technique, the effect of altering inter-particle force on
segregation, fluidization, pattern formation, packing and angle of repose is investigated. The results of these experiments may have important
implications for the behavior of cohesive powders.
Granular materials include materials such as sand, salt,
rice, gravel and flour and display a wide range of
interesting properties. They can flow like a liquid, support
weight like a solid and form clouds like a gas (for example dust storms). Under certain circumstances they can also
form complex patterns, such as sand ripples and dunes.
The properties and behavior of granular matter can be
influenced by the presence of inter-particle forces. These
include friction, liquid bridging forces and electrostatic and
van der Waals type forces. The differences in inter-particle
forces can explain such things as the differences in the flow
behavior of dry sand and flour.
Using this technique, the effect of altering inter-particle force
on several different aspects of granular matter were
investigated.
Key Results
Angle of Repose: The poured static angle of repose was
found to increase linearly with inter-particle force. This was
accompanied by an alteration in the geometry of the
granular profile. The fractal dimension of the profile was
found to increase with applied field. Experiments on the
dynamic angle of repose also reveal a linear increase with
increasing inter-particle force.
Packing: Experiments were also conducted to investigate
how the void fraction is affected by inter-particle force It
was found that the void fraction increased with Fip and that it
depended only on the ratio of inter-particle force to particle
weight Fip/mg.
Comparisons with data from other
experiments indicate that this is a universal phenomenon
that also applies to other non-magnetic systems.
Fluidization: Experiments using beds of fluidized iron
particles in the magnetic field demonstrated that transitions
from bubbling, spouting and other behaviors could be
induced by changing the inter-particle force.
Figure 1: Method for determining the inter-particle force to
particle weight ratio or Fip/mg.
Method
A pair of Helmholtz coils supplied the magnetic field for this
experiment. The magnetic field axis was in the vertical
direction.
To directly measure the inter-particle force between two
particles as a function of field strength, the following
method was used. An iron sphere of the appropriate size
was glued to a plastic spatula and positioned on the axis of
symmetry at the point midway between the coils. With the
field at full strength (approximately 6400 A/m ) a second
particle of the same size was brought into contact with the
fixed sphere (as shown in Figure 1). Due to the strong
magnetic force between the two particles, the second
particle became suspended from the first. The current in
the coils, and hence the magnetic field strength, was slowly
decreased until the suspended particle fell from the fixed
particle. At this point, the inter-particle force is equal to the
weight of the particle. The weight of the particle was
measured with a sensitive laboratory balance.
To obtain further points for the calibration graph, small (500
µm) bronze spheres were carefully glued to the "bottom'' of
the particle to be suspended. This served to increase the
weight of the particle without significantly altering the interparticle force. This new “particle” was then weighed. By
dividing through by the weight of each particle, the ratio of
inter-particle force to weight, Fip/mg, was obtained.
The method of investigating the effects of inter-particle
force on particulate materials introduced in this research
allows the effect of inter-particle force to be investigated in
isolation from other factors.
Figure 2: Segregation in a rotating drum. Axial segregation in a
mixture of magnetized and non- magnetized material was found to
be suppressed by increasing Fip/mg. Radial segregation was
found to be reduced and then reversed as Fip/mg was increased.
The angle the material makes with the horizontal is know as the
dynamic angle of repose and was found to increase with Fip/mg.
Pattern Formation: Experiments were carried out on pattern
formation in vertically vibrated granular layers. Inter-particle
forces were found to suppress pattern formation over a wide
range of frequencies. For vibration frequencies of between
approximately 23 Hz and 30 Hz however, it was found that
increasing the inter-particle force had a similar effect to
altering the vibration amplitude, which is to alter the type
patterns produced.
Mixing and Segregation: The effect of inter-particle force on
mixing and segregation was also examined. This was done
by mixing the iron particles with non-magnetically influenced
particles and observing the mixing behavior as Fip was
altered. Experiments using this method demonstrated that,
under certain circumstances, inter-particle force can
increase both mixing and segregation. It was found that
avalanche segregation and stratification can be reduced
and even reversed by changes in inter-particle force.
Radial segregation in rotating drums could be reduced
(mixing increased) and even reversed by altering interparticle forces. Axial segregation in the drums could be
suppressed by increasing Fip.
Figure 3: (Left) Reversal of segregation with increasing Fip/mg
(top) low field (bottom) high field. (Right) Reversal of stratification
with increasing Fip/mg (top) low field (bottom) high field.
Figure 5: (Above) Reversal of stratification of materials mixed
with iron particles. d represents the difference in angle of repose..
Figure 6: (Above) Apparatus for producing vibrated
granular layers. When material is placed in evacuated
container, vibrated and viewed from above, the patterns
shown in Figure 7 are observed.
References
S. Hutton, A.J Forsyth, M.J. Rhodes and C.F.Osborne, “The Role of Interparticle Force in Segregation of Particulate Materials”, presented at AIChE
Annual Meeting, Los Angeles, California, 2000, 12-17 November, paper 12g
S. Hutton, A.J Forsyth, M.J. Rhodes and C.F.Osborne, “Segregation of
Particulate Materials: Inter-particle Force Effects”, First Asian Particle
Technology Symposium, 2000, paper 0084, Bangkok, Thailand, September,
session S-I(2)-5
S. Hutton, A.J Forsyth, M.J. Rhodes and C.F.Osborne, “Effects of Inter-particle
Force on Segregation in a Rotating Drum”, World Engineering Congress, 2001.
Figure 4: (Above) Reversal of segregation of materials
mixed with iron particles. ? represents the difference in
particle diameter.
Figure 7: (Right) Patterns observed in vibrated granular
layers. (Image Paul Umbanhowar from “Wave Patterns in
Vibrated Granular Layers”, Ph.D. thesis, University of
Texas Austin, 1996. All other images by the Author)
A.J Forsyth, S. Hutton, M.J. Rhodes and C.F.Osborne, “Effect of applied interparticle force on the static and dynamic angles of repose of spherical granular
material”, Physical Review E, 2001, v63,3, March
A.J Forsyth, S. Hutton, M.J. Rhodes and C.F.Osborne, “Effect of inter-particle
force on the packing of spherical granular material”, Physical Review Letters,
2001,24, December
A.J Forsyth, S. Hutton, M.J. Rhodes and C.F.Osborne, “Effect of Applied Interparticle Force on packing density and the angles of repose in mono-sized
granular material”, presented at CHEMICA, 2000, 9-12 July, Perth,