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Invited review paper

Aerodynamic dispersion of cohesive powders: A review of understanding

and technology

G. Calvert, M. Ghadiri *, R. Tweedie

Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT, UK

a r t i c l e i n f o

Article history:

Received 19 August 2008Accepted 10 September 2008

Keywords:

Dispersion

Aerodynamic

Cohesive powders

Particle characterisation

Dry powder inhalers

a b s t r a c t

Dispersion is the desired disintegration of particle clusters down to their primary constituents through

the application of external forces, which overcome the interparticle attraction forces. This method is ben-

eficial for many processes but especially for the characterisation of particulate systems and therapeutic

drug delivery via the lungs from dry powder inhalers (DPIs). Dry powder dispersion is becoming increas-

ingly popular as a method of sample preparation for a range of instruments such as a laser diffraction

measurement device. There are many advantages for dry dispersion compared to wet methods. However,

complete dispersion of fine cohesive powders is difficult due to the relatively large interparticle attraction

forces compared to separating forces arising from fluid energy. This review identifies the current state of

theoretical and experimental understanding of powder dispersing in a gaseous medium. The approaches

to relate bulk powder properties to dispersion, the stresses produced on a particulate structure due to

aerodynamic forces and possible approaches for linking the two are discussed. Furthermore, the available

dispersion technology is reviewed with a discussion of individual dispersers and commercial devices

used for dispersing bulk powder. Also, the review highlights the research needed in this field to gain a

better understanding of how bulk powders interact with a dispersing fluid.

2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

The efficient dispersion of particulate solids is of great impor-

tance in a number of industries such as pharmaceutical, bulk

chemical and food. The majority of the products in these industries

are primarily in powder form; therefore, particle size analysis is

essential when producing powders as even small differences in size

and/or shape as well as surface properties affect process ability or

performance and end product attributes. Consequently, powder

dispersion is widely used to break up loose aggregate clusters as

a means to determine particle size distributions; this can be

achieved using laser diffraction [1], time-of-flight techniques

[2,3] and microscopy methods [4]. Dispersion of powders can be

done in a gas or a liquid phase and a variety of theoretical and

experimental researches are available which investigate liquid sys-

tems[514]. In contrast, the understanding of the interactions be-

tween bulk powder and gaseous medium and the transformation

from an aggregate state to a dispersed state are lacking. Despite

this, aerodynamic dispersion remains a popular method of sample

preparation [15,16]. There are several reasons, including most

notably, ease of use, speed of measurement, absence of liquids

and a high optical property contrast between the particle and the

gas phase.

A further area where aerodynamic dispersion is highly impor-

tant is in therapeutic drug delivery via the lungs using dry pow-

der inhalers (DPIs). There are many devices on the market

including Rotahaler, Aerohaler, and Diskhaler. Replacement

of the chlorofluorocarbon (CFC)-driven pressurised metered dose

inhalers (pMDIs) was necessitated by the concerns of CFC damage

to the earths ozone layer. This resulted in the reformulation of

pMDIs with the chlorine-free propellant hydrofluoroalkane gas

(HFA-134a and HFA-227). However, HFAs are also greenhouse

gases, and although their potency is less than CFCs, their effect

is still 2000 times greater than that of carbon dioxide. Therefore,

it is inevitable that these substances are going to be subjected to

future controls [17]. A major alternative to the pMDI is the

breath-actuated DPI which has obvious environmental benefits.

In order to effectively deliver a drug to the desired region of

the lung, the turbulent air stream created by any DPI must pro-

vide adequate power to disperse the powder that produces a

cloud of respirable fine particles. This involves a balance between

DPI design, drug formulation and the patient-generated inspired

flow rate [18,19].

The ability to control dispersion of a wide range of powders,

from friable to robust and free-flowing to extremely cohesive

material, is an area of great interest and importance. However, it

0921-8831/$ - see front matter 2008, The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved.doi:10.1016/j.apt.2008.09.001

* Corresponding author.

E-mail address: [email protected](M. Ghadiri).

Advanced Powder Technology 20 (2009) 416

Contents lists available at ScienceDirect

Advanced Powder Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p t
mailto:[email protected]://www.sciencedirect.com/science/journal/09218831http://www.elsevier.com/locate/apthttp://www.elsevier.com/locate/apthttp://www.sciencedirect.com/science/journal/09218831mailto:[email protected]


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is recognised that the complete dispersion of fine cohesive

powders, especially in the size range below 20 lm[20], is difficult

due to the relatively large interparticle attraction forces, namely

van der Waals, electrostatic and liquid bridge, compared to sepa-

rating forces[21]. In general, the relative strength of these forces

increases with decreasing particle size. The van der Waals force,

the main interparticle attraction force as particle size decreases,

is approximately 100 times greater than gravity for 10lm particles

[22]. As a consequence, a great amount of energy is required to de-

form and disintegrate these clusters completely to their primary

constituents. Unfortunately, the application of such large disper-sive energies may lead to particle breakage, an obvious disadvan-

tage when measuring particle size. Additionally, particle

morphological features such as shape and surface asperities also

affect particle interactions and hence dispersion performance

[23]. As reported by Kousaka et al. [24], irregular-shaped particles

disperse more easily compared to spherical particles.

A wide range of disperser types are available for academic and/

or commercial applications. Typical dispersion units include educ-

tor, nozzle, capillary tube, Venturi, mixer type, and fluidised bed.

These different configurations are discussed in the technology

section.

In this paper, the current understanding and the technology

available for dry powder dispersion are critically reviewed and

summarised. Furthermore, the available research and commercial

devices used for dispersing bulk dry powders are reviewed. The re-

search needed in this field to gain a more comprehensive under-

standing of how bulk powders interact with a dispersing fluid is

also highlighted.

2. Interparticle forces affecting bulk flow properties

Insufficient powder dispersion in gas flow arises due to inter-

particle forces. Simons [25] classified these forces according to

their relative magnitudes, from strongest to weakest: (a) solid

bridge forces; (b) liquid bridge forces; (c) van der Waals; (d) elec-

trostatic; and (e) magnetic. Extensive reviews of theories andexperimental studies are available in the literature [21,22,2631].

Aerodynamic dispersion of bulk powders is the successful appli-

cation of separating forces to overcome bulk powder attraction

forces producing a cloud of primary particles. Consequently it is

of interest to investigate the factors which influence these forces

that give rise to aggregate strength. Obvious factors include the

primary particle size and shape, density, packing structure, poros-

ity and contact area, and the interparticle bond strength.

When considering powder dispersibility, there are two intrinsic

bulk powder properties of interest: the bulk tensile strength and

the shear strength. In the literature, a number of models are avail-

able, which relate the microscopic interparticle interactions to the

tensile strength of a particle cluster. Rumpf[32]introduced a mod-

el which, described the mechanical stability of aggregates; he con-sidered the pull-off force necessary to separate the structure along

Nomenclature

Latin lettersA Hamaker constant (Number of atoms/m3)CF drag coefficient ()c ratio of diameter of the circular contact area to the par-

ticle diameter ()

Dp particle diameter (m)DpW particle diameter corresponding to the inertia parame-

ter, Wc(m)Dp50 median particle diameter (m)Dp50s mass median primary particle diameter by sedimenta-

tion method (m)DS dispersibility parameter ()Dv volume equivalent diameter (m)Dp frequency distribution curve intersection between f0

andfd (m)d capillary tube diameter (m)dpag particle cluster diameter (m)d0 orifice disperser diameter (mm)ds impact obstacle diameter (m)E elastic modulus (Pa)

Fd separating force (N)Fad interparticle attraction force (N)Fp characteristic value of re-entrainment (Pa)F0p characteristic value of interparticle cohesion force (Pa)fd/0 particle size frequency distribution curves (%/lm)H interparticle separation distance (m)k proportionality constant ()kn particle coordination number (-)mA/B particle mass (kg)n impeller rotational speed (radians/s)DP pressure drop (Pa)Q0 air flow rate (l/min)Re Reynolds number ()R particle radius (m)RfA/B fluid resistance forces acting on a particle (N)r surface asperity radius (m)

t time (s)Dt duration of impact (s)u air velocity (m/s)ur particlefluid relative velocity (m/s)up particle velocity (m/s)

u average air velocity (m)vi impact velocity (m/s)We Weber number ()W powder flow rate (kg/s)Z particle diameter ratio ()

Greek symbolsC interface energy (J/m2)D agglomerate breakage dimensionless group ()/ packing fraction ()Wc inertia parameter ()b dispersion efficiency ()bi correction factor to account for second particles pres-

ence ()c velocity gradient acting across the sphere (1/s)cA/B surface energy (J/m2)e porosity ()ed specific dissipated power (J/m

3s)gT target efficiency ()j dynamic shape factor ()l fluid viscosity (Pa s)qa air density (kg/m

3)qf density of the fluid (kg/m

3)qp particle density (kg/m

3)r tensile strength/stress (Pa)rdisp dispersion strength (Pa)rc bending moment (Pa)s shear stress (Pa)sc critical shear stress for re-entrainment (Pa)

G. Calvert et al. / Advanced Powder Technology 20 (2009) 416 5


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its cross section into two halves and calculated tensile strength, r,

of a particle cluster as given by [33]:

r /knFad

pD2p1

where / is the packing fraction, Fad is the attraction force between

particles of diameter Dpand knis the particle coordination number.

If the particles are dry and assuming that van der Waals interactionsare the only attraction forces, Fad is given by

Fad A

6H2R1R2

R1 R2

2

where A is the Hamaker constant, His the interparticle separation

distance and R1 and R2 are the radii of the contacting spheres,

respectively. However, this approach is limited when evaluating

real situations as structures present heterogeneous properties. As

a result, particle clusters break along the line of least mechanical

resistance.

Several attempts have been made to improve this model by

adding various terms. For example, it is possible to include surface

features such as particle roughness[34]:

Fad A

6H2rR

r R

R

1 rH

2" #

3

whereris the radius of the asperity on the surface. A common ap-

proach for improving the flow properties of cohesive powders is the

use of spacer particles, which are discussed later. It has been shown

that the above model was able to correlate the tensile strength

when spacer particles, several nanometres in size, were added to

cohesive powders[35]. In this situation, the surface asperities with

radius rwere substituted by the spacer aggregate size. However, the

tensile strength is reduced only as a result of size and not by consid-

ering the different chemical compositions of the spacer aggregates.

Dispersion performance is affected by particle interactions, andmethods are available for measuring bulk powder tensile strength

[3641]or shear strength[42]. Iinoya and Masuda[43]carried out

an experimental study on the performance of three different types

of disperser: a mixer type; a fluidised bed and an eductor. They

determined the cohesion force per unit area by using a shear tester.

It is seen that the finer more cohesive powder, metallic silicon, dis-

perses less well compared with calcium carbonate, which has a lar-

ger mass median diameter, and is less cohesive.

In these standard approaches, the forces acting on the bulk pow-

der are mechanical forces. It is of great interest to elucidate if such

methods also apply to the highly dynamic forces which powders

experience during dispersion. Masuda et al. [44]developed a parti-

cle re-entrainment test, which can measure the rupture phenome-

non of aggregated particles caused by the flow of a gas over a

powder bed. The apparatus used in the particle re-entrainment test

is shown inFig. 1. The test cell is packed with powder and dry air is

fed into the test cell. The air flow rate is controlled so that the cross

sectional average velocity increases by 1 m/s every minute. Once

the cross-sectional average velocityreaches a critical value, the flow

begins to entrain aggregated particles. The process is observed by a

microscope with a video system, and is detected based on the con-

tact electrification of particles.

At the onset of entrainment the critical shear stress, sc, is calcu-

lated using the following equations:

sc CFqau

2

2

CF 16Re1 Re< 2300

CF 0:0791Re14 ReP 2300

4

where qa is the air density, CFis the drag coefficient, u is the air

velocity and Reynolds number (Re) is based on the equivalent diam-

eter of the rectangular channel. Previously, Masuda et al. [44]found

that for fly ash no. 10 (JIS Z8901), the critical shear stress defined in

Eq.(4) was proportional to the tensile strength, r, of the powdersc kr k 0:38 for the fly-ash no: 10 5

Eq.(1) introduced by Rumpf[33]has been used by Masuda and Got-

oh [45] for evaluating the capability of using the re-entrainment

test to estimate the dispersion performance of a given powder. They

applied this approach to a mixer type and an eductor disperser (see

Table 1). In order to estimate the coordination number necessary to

calculate the tensile strength in Eq. (1), the RidgwayTarbuck

empirical equation was used[46]

kn 13:8 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

175 232/p

6

Substituting Eq.(1) into Eq.(5) results in the following:

kFad

pD2p sc

/kn F

p 7

The right-hand side of Eq.(7), defined as the characteristic value of

re-entrainment,Fp, can be determined by the re-entrainment test. It

is seen that for most powders, Fpincreases with the packing fraction

due to the increase in the critical shear stress and tend to a maxi-

mum value asymptotically. This maximum value is defined as the

characteristic value of interparticle cohesive force, F0p. Furthermore,

Masuda and Gotoh[45]introduced the following approach for eval-

uating the dispersion efficiency, b:

b

Z Dp0

fd dDp

Z 1Dp

f0 dDp 8

wheref0 is the particle size distribution of fully dispersed primaryparticles measured by a centrifugal sedimentation method, fd is

the particle size distribution of the aerodynamically dispersed pow-

der and Dp represents the intersection of the two frequency distri-

butions. The above method relates the size distribution of the

dispersed powder to the size distribution if the powder was fully

dispersed. The re-entrainment test has been shown to correlate well

with the dispersion efficiency defined in Eq. (8)[45]. However, this

approach is based upon the powder critical shear strength being lin-

early proportional to the tensile strength. This may not be the case

for all powders.

Recently Weiler et al. [47] introduced a theoretical model for

describing the complete disintegration of dry powder clusters. A

number of assumptions were introduced to simplify powder dis-

persion, including the clusters disintegrated entirely and instantly,so that the forces affected the complete surface area of all particles;

MonitorVideo

TV Camera

Microscope

Buffer volume withdiffusion layer

Test section

PumpValve

Flow meter

Test cell

Testedpowder

Detector

Fig. 1. Apparatus for particle re-entrainment test[45].

6 G. Calvert et al. / Advanced Powder Technology 20 (2009) 416


4/13


5/13

the clusters and primary particles were spherical; and the only

cohesive forces were in the direct surroundings of the particles.

This model considers the breakage of every contact inside the

agglomerate to estimate dispersion strength, rdisp

rdispFad1 ekn

2pDpag

D3p9

where dpagis the particle cluster diameter and e is porosity. Eq.(9) islimited to agglomerates, where DpagDp. In the case where the

particle size ratio Dpag/Dp< 50, Eq.(9) is modified to the following:

rdispFad1ekn

2pD2pag

D3pag

D3p 4f

b2

where

b Dp

DpagDp f 1 arccosb

180

10

Unfortunately, at present there is no empirical evidence to validate

this approach. However, it is expected that it will underestimate the

difficulty of powder dispersion. It is unrealistic for all contacts to be

broken at the same time due to forces from the dispersing fluid act-

ing only upon the aggregate surface exposed to the fluid forces. It is

reasonable to assume the fluid within the porous structure remains

essentially quiescent[48]. Therefore, the fluid force acting on a par-

ticle cluster will affect particles exposed to the fluid flow, which

may disperse the exposed particles or the force might propagate

to particles within the cluster. This will result in the deformation

and eventual break up of loose aggregates [12].

In the literature, attempts have been made to analyse the effect

of bond strength on agglomerate breakage using the distinct ele-

ment method (DEM)[4954]. Kafui and Thornton[49] investigated

the effect of surface energy on the strength of regularly packed

agglomerates and related the breakage of interparticle contacts

to the Weber number,We, as defined by

We qpDpu

2p

C 11

whereqpis the particle density,upis the particle velocity and C is

the interface energy, which is defined by the Dupr equation[28]as

C cA cB cAB 12

where cA and cB are the surface energies of two particles made of

different materials, A and B , in contact with each other and cAB is

the interaction energy between them. For materials which are the

same, cAB is zero and therefore C = 2c. However, in later work

Thornton et al. [50] modified Eq. (11) by including a lower limit

of impact velocity, below which no contact is broken. It was shown

that this approach was better for relating the breakage of interpar-

ticle contacts. Subero et al. [52]used DEM to analyse the effect of

interface energy in the range 0.55.0 J/m2 in the randomly packed

agglomerates. They found that the results were in good agreementwith Thornton et al.[50]. However, if the interface energy was in-

creased well beyond one order of magnitude, as investigated above,

then the standard and modified Weber numbers no longer provided

an adequate description of the trend[53].

MorenoAtanasio and Ghadiri [54] introduced a mechanistic

model, relating the number of broken contacts in a particle assem-

bly due to impact velocity, interparticle adhesion energy and the

properties of the particles which form the cluster. They assumed

that the energy used to break contacts during impact was propor-

tional to the incident kinetic energy of the particle assembly. The

number of broken contacts was shown to depend on the following

dimensionless group, D:

D qpD

5=3p E

2=3u2p

C5=3 13

where Eis the elastic modulus. Their simulation results showed that

the effect of surface energy on agglomerate breakage was better de-

scribed by this mechanistic model than by the Weber number

alone, as previously used to characterise the impact strength of

agglomerates.

3. Aerodynamic dispersion mechanisms

The interaction between loose aggregates and dispersing fluid

has been investigated both from a theoretical and experimental

point of view. Loosely aggregated particles suspended in fluid

flows experience several kinds of forces caused by rapid accelera-

tion, deceleration, turbulent eddies, etc. When considering powder

fluid interactions, the following mechanisms are used to describe

the dominating dispersion process[55]:

(1) dispersion by rapid acceleration or deceleration and/or shear

flow;

(2) dispersion of particle clusters by impact onto a stationary or

moving target;

(3) dispersion by other mechanical forces (e.g. fluidisation, mix-

ing, vibration and scraping).

The dispersion of dry powders using one or a combination of the

above mechanisms is most likely divided into the subsequent three

zones[56]:

(1) Delivery the powder is introduced to the dispersion unit,

for example, by a vibratory tray.

(2) Dispersion and/or entrainment the powder is dispersed by

forces acting upon the loose aggregates without breaking

primary particles.

(3) Transport and presentation the powder is subsequently

conveyed from the dispersing device and presented to a

region for a specific purpose downstream.

3.1. Powder delivery

The powder delivery stage of dispersion suffers fromall theprob-

lems associated with powder flow, and will impact upon dispersion

performance. If the dominant dispersion mechanism is powder

acceleration, an important parameter is the relative velocity differ-

ence between the powder and dispersing fluid [24]. Hence, it is

desirable that the particles have comparatively negligible velocity

when enteringthe dispersionregion. The nature of interparticle col-

lisions andcollisions with devicewalls may aiddispersion butcould

result in tribocharging which might hinder the process. Further-

more, environmental conditions such as relative humidity will im-

pact upon dispersion performance and reproducibility.

3.2. Powder dispersion

To effectively disperse bulk powders, the efficient application of

a separating force is paramount. In addition, it is well practiced in

industry to reduce the interparticle attraction force, hence reduc-

ing the separating force necessary for efficient dispersion, a tech-

nique commonly adopted in the DPI industry. A reduction in

interparticle attraction can be achieved through the use of spacer

particles, by coating particle surfaces or using carrier particles.

These approaches are discussed later in this section.

3.2.1. Dispersion by acceleration in a uniform flow field

The greatest stress acting upon loose aggregates occurs at max-imum acceleration; this is as soon as the cluster enters the fluid



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flow. The stresses on a sphere in simple flow fields when the

sphere is initially at rest have been analysed by Bagster and Tomi

[48]. It has been shown that the maximum stresses, tensile, rmax,

and shear,smax, in a uniform flow field may be approximated by

rmax andsmax3lur

Dp14

where l is the fluid viscosity and uris the particlefluid relativevelocity. This particular approach adopts a major simplifying

assumption, whereby the aggregates are homogeneous spheres

undergoing bulk motion as single entities and possess characteristic

structural properties. Furthermore, the fluid within the porous

structure remains stationary. However, this is a preliminary at-

tempt in relating the variables, which govern the break up of aggre-

gates due to aerodynamic forces. When analysing dispersion

mechanisms, it is helpful to treat a loose aggregate as a cluster con-

sisting of a number of primary particles, which are bonded by inter-

particle attraction. The separation of two particles forming a

doublet[24,58]and three particles forming a triplet [59]has been

considered in a uniform flow field. Kousaka et al. [24] analysed

the separating force between the two particles of a doublet using

Newtons second law of motion (Fig. 2)

mAdupdt

RfA Fd

mBdupdt

RfB Fd15

whereRfAand RfB are the fluid resistance forces acting on each par-

ticle, Fd is the force acting on each particle, mA and mB are the

masses of the respective particles, which can be found if the parti-

cles diameters and densities are known.

Kousaka et al. [24] assumed that the densities of the particles

are equal and operating under Stokes law with the following rela-

tions for the respective resistance forces, RfA and RfB:

RfA 3plurDpARfB 3plurDpB

16

where DpA and DpB are the diameters of particles A and B, respec-tively. Substituting Eq. (16) into Eq. (15) the product can reduce

to give the force acting on the contacting area between the two

particles

Fd 3plurDpADpBDpB DpA

D2pA DpADpB D2pB

17

Thus, if the attraction force is known, Eq. (17)will estimate whether

the doublet particles will theoretically detach from one another.

Furthermore, if the doublet consists of equal-sized spheres then

based on the above approach dispersion will not occur, as the sep-

arating force is zero.

A similar approach to that outlined above was adopted by Yuu

and Oda [57], when investigating the dispersion process due to

acceleration through an orifice-type disperser. In addition, thismodel allows for the dispersion of clusters consisting of more than

two particles. For example, this is achieved by approximating a

cluster to a doublet that disperses into two smaller doublets which

eventually disperse into primary particles. Therefore the clusters of

more than two particles are assumed to be doublets which break

up to form doublets. This research introduced a correction factor,

bi, similar to Reed and Morrison [60] to account for the effect of

the presence of a second sphere

mA dupdt 3pbAurDpA Fd

mBdupdt

3pbBurDpB Fd18

Assuming that van der Waals interaction is the only attractive force

acting between particles, Yuu and Oda [57] derived a population

balance equation using Eq.(18)for predicting the change of the par-

ticle size distribution due to acceleration (or deceleration). In a

number of studies, the importance of the diameter ratio between

two particles in contact, Z, has been highlighted[24,5759].

ZDpADpB

19

Using the approach of Yuu and Oda, it is possible to calculate the

possible dispersion range as a function ofZ, i.e. for what values of

Zunder given flow conditions that will result in particle dispersion.As expected, with an increase in inertia the dispersion range in-

creases. Unfortunately, this model has unrealistic assumptions: a

particle cluster consisting of more than two particles is dispersed

to two clusters, each consisting of two particles, which are then dis-

persed to two single particles. This is over simplifying the aerody-

namic dispersion phenomenon, which will involve complex loose

aggregates, dispersion through particle erosion and collisions, etc.

Furthermore, the correction factor, bi, is not yet fully characterised

and the model only considers the effect of van der Waals forces.

Kousaka et al. [58]extended the separating force well beyond

the Stokes regime, Reynolds numbers up to 104, whereby the dis-

persion force acting on a doublet can be calculated using

Fd 0:119qa

u2r

D2

pBZ2jA ZjB 2:07lq

au3

rD6

pBZ3

1=2

jA Z3=2jB 9:05lurDpBZjA Z

2jBZ3 1

1 20

where j is the dynamic shape factor (=1 for a spherical particle).

When jA= jB= 1 andZ= 0.5, then the separating force is maximum

for the dispersing conditions. As previously mentioned, a doublet of

two equal sized spheres cannot be dispersed because the separating

force acting at the contact is zero. This has been experimentally ob-

served by Endo et al.[59], when the percentage of doublets seemed

not to vary significantly with pressure. If particles A and/or B are

themselves loose aggregates, the dispersion force is also calculated

from Eq.(20), taking into account the volume equivalent diameter,

Dv, instead ofDpand the dynamic shape factor for each particle.

Particle B

Particle Az

0

Fig. 3. Illustration of a model doublet in a simple shear flow field.

a b

Fig. 2. Illustration of a model doublet in a uniform flow field.



7/13

3.2.2. Dispersion in a shear flow field

In addition to dispersion in a simple uniform flow field, there

have been attempts to understand the behaviour of particles,

which are placed in a simple shear flow field[24,48,61,62], as illus-

trated inFig. 3.

Bagster and Tomi[48]analysed the stress induced on a spheri-

cal particle in a shear flow field and reported that the shear stress,

s, acting on a sphere is at its maximum in the central plane, the va-lue of which is expressed as follows:

s 8:5lc 21

where c is the velocity gradient acting across the sphere. This ap-

proach is obviously limited to simple systems, and a more extensive

analysis is necessary for dense phase powder dispersion.

With respect to two particles forming a doublet, Kousaka et al.

[24]discussed theoretically the dispersion in a shear flow field. It is

found that when assuming particleBis sufficiently larger than par-

ticleA, a bending moment is induced, rc, which is approximated

using the following equation:

rc93lcpc3

22

where cis the ratio of the diameter of the circular contact area tothe diameter of the particle, and is dependent on the contact config-

uration. In general, it is seen thatchas a value much less than unity

and as a result the bending moment induced is greater than the

shear stress, suggesting that the bending moment is dominant in

a shear flow field. Furthermore, it is suggested that the stress on

small particles in a shear flow field is less important than that of

acceleration in a uniform flow.

3.2.3. Dispersion by impaction

The breakage of agglomerates due to impaction has been exten-

sively investigated due to agglomerate strength being important in

a vast number of manufactured products in the chemical, pharma-

ceutical and food industries [4954,6365]. However, little atten-

tion has been made to aerodynamic dispersion involvingimpaction.

Particles in a fluid flow will collide with each other and walls,

depending upon their inertia. As expected, if the stress induced

by an impact is greater than the strength of the loose aggregates

they will deform, break or disintegrate aiding dispersion. Kousaka

et al. [24] investigated the stress induced and the probability of

impaction in a fluid flow field. Assuming the particle is spherical,

the compressive stress,ri, at the plane through the particle centre

is expressed as follows:

ri2

3qpDpag

vi

Dt

23

where viis the particle impact velocity and Dtis the duration of the

impact. If the stress given by Eq. (23)is assumed to be much largerthan the strength of the aggregates because of a sufficiently large

value of vi/Dt, the breakage of the particle cluster depends upon

the probability of impact with the obstacle, which is given as, gT

gT fWc where WcqpurD

2pag

18lds24

where Wcis the inertia parameter anddsis the diameter of the im-

pact obstacle. Suppose a certain value ofWcwhen the target colli-

sion efficiency is 50%, i.e. half of the clusters impacting upon the

obstacle; the particle diameter, DpW, is given as follows:

DpW

18ldsWcqpur

!12

25

If the particles, half of which impact during dispersion, are observed

experimentally, the diameter of the dispersed particles is repre-

sented by Eq.(25). It is shown that the impaction force, the com-

pression stress, and the inertia parameter increase with the

diameter of the loosely aggregated particles. Therefore, indepen-

dent of the structure of the cluster, dispersion by impaction is effec-

tive for larger particle clusters.

3.2.4. Reduction of interparticle attraction force

The reduction of interparticle attraction is an approach which

can be beneficial for the dispersion of dry powders. Techniques in-

clude powder coating[45,66,67], spacer or glidant particles[35,68]

or the addition of secondary material [23,6971].

Particle coatings can be used to reduce the Hamaker constant

and consequently reduce van der Waals attraction between parti-

cles. Spacer or glidant particles are also used within industry to im-

prove the flow properties of powders. Tedeschi et al. [68]reported

improved dispersion performance of 1545 nm thin aluminium

flakes when mixed with fumed silica. The addition of the fumed sil-

ica increased the interparticle spacing, thus reducing the van der

Waals forces between the flakes. The silica was shown to have

maximum effectiveness at 4 wt%.DPI formulations usually contain an exipient with a common

powder being fine lactose. It has been shown that an increase in

dispersion is possible when lactose is added to a DPI formulation,

due to lactose having a lower interparticle attraction[69]. The abil-

ity to improve powder dispersibility without additional material

but by increasing surface roughness has also been investigated

[23]. The asperities reduce particle contact area therefore reducing

cohesion/adhesion. The advantage of these methods is that the

choice of inhaler and required entrainment flowrate are less criti-

cal. Chew et al. [70] quantified the degree of surface corrugation

with powder dispersion by the surface fractal dimension (Ds) ob-

tained by light scattering and showed that increasing Ds slightly

from 2.06 to 2.18 enhanced the FPF from 27% to 41%. This is ex-

plained through an increase in particle separation and a decrease

in contact area. However, a further increase in Ds did not improve

the FPF.

The addition of ternary components within dry powder aerosols

has been analysed [71]. The fine particle fraction (FPF) from ternary

mixtures was dependent on carrier type, ternary concentration,

and ternary component type. Ternary mixtures produced higher

FPF than binary mixtures, except those containing superfine pow-

der. They suggested that the carrier particles have sites of varying

degrees of adhesion.

Masuda and Gotoh[45]evaluated the effect of treating powders

with ethanol vapour and used the previously discussed re-entrain-

ment test. By treating powders with ethanol vapour for 12 h, a sig-

nificant decrease in cohesion was observed, accompanied with a

small increase in dispersion performance. However, treating pow-

ders with ethanol vapour is time dependent with powders revers-ing back to their original state when exposed to air.

Further powder engineering approaches are available in the lit-

erature, which are designed to lower powder attraction, including

needle crystals[72], AIRTM particles[73], aerogel powders[74]and

spray-freeze dried-particles[75].

4. Technology overview

Dry powder dispersion is widely used for many technological

applications, and in this section the available devices are intro-

duced and discussed. The focus is upon devices used within re-

search and commercial applications for particle characterisation.

A comprehensive review of the DPI market is already available[17,76]. Ashurst et al. [76] concluded that the DPI market is full



8/13

of creative designs yet no design seems to fulfil all the require-

ments of an ideal DPI. Recently, Chan[77]discussed the critical is-

sues associated with the development of a dry powder inhaler

delivery system. It is suggested that a greater understanding of sur-

face phenomena with regards to powder production methods, for-

mulation properties, and powder dispersion is needed to improve

DPI performance.

4.1. Typical dispersion devices

Many different types of dispersers are available [78] and can

be classified according to the main dispersion mechanisms which

have been previously discussed. Typical devices are listed in

Table 1. The dominant dispersion mechanisms of each are also

given.

4.1.1. Eductor disperser

A typical eductor disperser, shown inTable 1(a), has two feed

inlets, one provides pressurised air and the second contains the

powder to be dispersed. The powder may enter under the influence

of various forces such as gravity or an existing pressure difference

between the two inlets forcing the powder towards the dispersionregion. Dispersion is intended to occur at the throat of the device,

where the pressurised air and the powder feed meet with the dom-

inant dispersion force being rapid acceleration and/or shear flow.

Additionally, particleparticle and particlewall collisions are

likely to aid the dispersion process as shown in Fig. 4. In contrast

to the eductor depicted inTable 1,the pressurised air and powder

feed may be reversed.

In the literature, a number of studies have investigated the

eductor disperser and its ability to disperse fine powders. Several

of these investigations [45,58,59,79] have used the eductor de-

signed by Nisshin Flour Milling Co. [80]. Kousaka et al. [58]used

this particular device to disperse aggregate polystyrene latex par-

ticles with diameters of 2 and 5.2 lm. The experimental results

suggested that particles consisting of 5 lm primary particles were

almost entirely dispersed by acceleration. Furthermore, Masuda

and Gotoh[45]have demonstrated that the Nisshin Flour Milling

Co. eductor[80]performs approximately the same as a mixer-type

disperser; seeTable 1(f) and Section4.1.8, so long as the mass flow

ratio is kept constant.

Endo et al. [59] investigated the dispersion of fine powders

(around 1 lm) using an eductor disperser [80] and attempted to

relate dispersion performance to powder cohesion. The aggregated

particles were sucked into the eductor through a stainless steel

tube. At the eductor throat, the powder is dispersed by using high

pressure (0.310 MPa) nitrogen gas. The dispersed particles were

collected by gravitational settling and in each experiment 500

1000 particles were examined using optical or electron micros-

copy. A dimensionless dispersibility parameter (DS) defined as

the ratio of the generated dispersion force (Eq. (20)) to the inter-particle attraction force (Eq. (2)) was used to predict powder

dispersion

DS Fd2Fad

26

Using Eqs. (2) and (20), numerical simulations of aggregates of poly-

disperse spheres have been calculated, and are compared with

experimental observations[59]. For some powders including mono-

disperse and polydisperse latex particles, SiC and TiO2, the experi-

mental results were in good agreement with the simulated data.

This suggests that the theoretical dispersion and relative cohesionforce are adequate tools for characterising the dispersion state of

these powders. On the contrary, for Al2O3and CaCO3particles there

was little correlation. Endo et al. [59] suggest that the cohesion

force is greater for Al2O3 and CaCO3than the value calculated on

the assumption of spherical particle shape. This is supported with

scanning electron micrographs showing an increase in contact area

between adjacent particles[59].

Recently, Tang et al. [81] investigated a simple, cost effective

eductor as a device for dispersing powders. The eductor was con-

nected to the Malvern Instruments Mastersizer 2000 laser diffrac-

tion system for particle size measurement. A comparison was

made between the performance of this device and the small-scale

powder disperser (SSPD) model 3433 (TSI, Shoreview, USA) and the

Scirocco dry powder disperser (Malvern Instruments, Malvern,UK). The eductor introduced by Tang et al. [81] worked as effi-

ciently as the two commercial devices, and furthermore introduced

the possibility of using swirl flow as an approach for aiding powder

dispersion.

4.1.2. Venturi disperser

A standard Venturi disperser uses converging and expanding

flows, and the powder may already be entrained within the air flow

or enter at the secondary inlet (Table 1(b)). Kousaka et al.[24]had

previously shown that the dominant dispersion mechanism in this

type of disperser is acceleration or deceleration, which can be fairly

effective as long as the relative velocity between particle and fluid

is large. However, Kousaka et al.[24]report that with a Venturi de-

vice it is difficult to obtain a large relative velocity, which may ac-count for the lack of further investigation.

4.1.3. Nozzle disperser

A typical nozzle disperser contains a particle laden pipe with

pressurised air as shown in Table 1(c). As a result, the particles

are already at a relatively high velocity before the pipe diameter

is reduced to accelerate the air stream and to disperse aggregates

even further. This device is referred to as a typical disperser device

[55]; however, there appears to be limited information confirming

or debating its dispersion performance ability. As illustrated in

Table 1(g), it is possible for nozzle dispersers to be used in conjunc-

tion with a secondary dispersion step such as an impaction plate.

4.1.4. Capillary tube disperser

A capillary tube disperser, illustrated inTable 1(d), is generally

regarded as using a velocity gradient acting across the cross section

of the device to shear powders promoting dispersion. Initially, one

Fig. 4. Particleparticle and particlewall collisions.



9/13

might think that the capillary tube disperser would be dominated

by shear forces. However, it is suggested that dispersion occurs due

to impaction as the powder enters the capillary tube. This was

investigated by changing the length of the capillary tube disperser

from 0.06 to 0.5 m, which had no effect on particle size[24]. This

implies that the surviving particle clusters do not disperse further

along the tube length by the prevailing shear. A possible explana-

tion could be that the particles tend towards the centre of the pipewhere shear is minimal.

Yamamoto et al. [83,84] investigated capillary tube dispersers of

various diameters. In the range of 3080% relative humidity they

showed that the environmental conditions can have a large impact

upon certain, but not all, materials. This is an area of dispersion

which needs a greater understanding. In agreement with that ob-

served by Kousaka et al.[24], the dispersed particles reach an equi-

librium size distribution over a short distance, approximately 5 mm

in a capillarytube which has a total length of 500 mm. Additionally,

an empirical relationship was found between the median particle

diameter and the specific dissipated power of the air stream at

thevena contracta,ed, which is related to the pressure drop, DP

Dp50 15e0:2d

ed DP

u2:5d

27

whereDp50is the median particle diameter, d is the diameter of the

capillary tube, and u is the average air velocity. Eq.(27)can be re-

lated to the dispersion performance of different materials in a cap-

illary disperser, i.e. the required power necessary for effective

dispersion.

A new dispersing approach from those shown in Table 1 has

been developed by Kousaka et al.[85,86], which was termed boil-

ing method. After suspending a powder consisting of submicron

particles in pure water or Fluorinert, the suspension is continu-

ously heated and boiled in a capillary tube to generate aerosol par-

ticles dispersed into primary particles. The vapourised liquid is

separated from the particles by a cooling condenser thereby pro-

ducing dry particles dispersed in air.

4.1.5. Orifice disperser

An orifice disperser is a device which adopts a sudden reduction

followed by a sudden increase in cross sectional diameter produc-

ing rapid acceleration and deceleration; furthermore, impaction

may occur upon the constricting walls of the orifice, see Table

1(e). Kousaka et al. [24] categorised the dominating dispersion

mechanism as impaction in this device.

Yamamoto and Suganuma [87] applied the dissipated power

approach developed for the capillary tube disperser to an orifice

disperser. For all materials investigated, a relationship between

the degree of dispersion and the intensity of the air stream through

the orifice was found

Dp50

Dp50s 31:3e0:2d

ed 0:4DPu

d0

28

where Dp50s is the mass median diameter of primary particles and

d0 is the diameter of the orifice. The dissipated power is related to

the air flow rate, Q0, using the following:

ed 2:7 106Q

30

d70

29

Eqs.(28) and (29)are used to determine the dissipated power nec-

essary to fully disperse the powder (i.e. Dp50/Dp50s= 1), based on

which the required air flow rate can be calculated. However, this

may not be the case for all types of powder due to differences in

their individual material properties. Hence, such an approach needsto include the influence of cohesion, shape, etc.

4.1.6. Mixer-type disperser

As shown inTable 1(f), a mixer-type disperser can be used to

generate different dispersion mechanisms. Kousaka et al. [24]ob-

served that the dominant dispersion mechanism is dependent

upon how the powder is introduced to the system. If the powder

is already reasonably well dispersed as a particle cloud, then accel-

eration is deemed as the dominating mechanism; however, if the

powder enters as aggregated clusters then inertial impaction onthe rotating blades is likely to be dominant. Usually, the dispersed

powder exits via an outlet tube attached tangentially at the top of

the mixer vessel. It has been shown that a great advantage of the

mixer disperser is concentration control of the exiting particles

[43]. As a result of this, the mixer-type disperser has been studied

in detail[8992].

Masuda and Gotoh [93] evaluated different mixer-type dis-

perser sizes to understand the effects of disperser scale on the dis-

charge air flow rate and the stable operation range, which is related

to the deposition of particles. The discharged air flow rate is di-

rectly proportional to the impeller rotational speed and the dis-

charge coefficient is nearly proportional to the 5/3 power of the

impeller length. A semi-empirical equation was derived for deter-

mining the mass median diameter from a mixer-type disperser

Dp50 7:0 102 1:65 107W2:5n2 Dp50s 30

wheren is the impeller rotational speed andWis the powder flow

rate. Eq.(30)provides a useful tool for defining the optimal rota-

tional speed, where the minimum mass median diameter is ob-

tained at a constant concentration and a good correlation

between experimental data and Eq.(30)has been observed[93].

4.1.7. Impact disperser

Table 1(g) shows a typical impact disperser having particles

which are entrained within an air flow and pass through a nozzle

subsequently colliding with an obstacle that is perpendicular to

the flow. As the particles are accelerated through a nozzle, some

dispersion may occur but the main dispersion mechanism is

impaction upon the plate. It has been suggested that this particulardispersion arrangement is the most aggressive, even being de-

scribed as a crusher [55].Additionally, a typical impact disperser

may provide some dispersion in the wake region due to particle

particle collisions as observed by Gomes and Vincent [82].

Kousaka et al.[24]investigated a novel approach for impact dis-

persion using eight wire screens, and concluded that dispersion

dominated by impaction was the most efficient mechanism. This

device has been tested in the relative humidity range 40100%.

There is very little difference in the dispersed particle size over this

range of environmental conditions. It is demonstrated that the size

reduction by impact can be estimated from the existing theory of

the target efficiency of a particle in an air stream (Eq. (25)) [24].

Pressurised

air inlet

Powder inlet

Dispersed powder

Fig. 5. Malvern Scirocco dry powder dispersion device[95].



10/13

On the other hand, impaction may be regarded as an aggressive

dispersion approach that could result in primary particle breakage

if the material is fragile.

4.1.8. Fluidised bed disperser

Fluidised beds are used in many applications such as drying of a

powder bed or gas cleaning [88], and for dispersing particles. Astandard fluidised bed disperser, as shown inTable 1(h), generates

forces through impaction and, less desirably, attrition by neigh-

bouring fluidising particles. Cohesive powders can be fluidised by

using large fluidising particles, dispersion aids, which mix the

powder and collide with cohered material freeing the primary par-

ticles to be subsequently entrained in the fluid flow.

Iinoya and Masuda [43] have shown that effective dispersion

with a fluidised bed is achievable through the use of dispersion

aids. An important parameter is the quantity of dispersion aids

needed to optimise performance. It is seen that the mass median

particle diameter is typically smaller when the dispersion aid bed

height is between 0.01 and 0.02 m. When the bed height is optimal,

the performance of the fluidised bed disperser is comparable to a

mixer-type disperser investigated in this study [43]. However,

the upper limit of gas velocity is the point when the dispersion aids

are entrained. If this is the case, denser dispersion aids may be

used, but the effect of various dispersion aids has not been

documented.

4.2. Commercial devices

A wide variety of commercial dispersion devices are available

for the process of particle characterisation. Most systems that areemployed for dispersion consist of a constant rate feeder and an

entrainment apparatus. A typical example would be the Wright

dust feeder[94]. In this device, the powder is placed within a rotat-

ing dust cylinder which allows for the powder bed to be scraped

and entrained into a carrier gas with any large particle clusters

being dispersed by impaction. Usually, the details of commercial

devices are not always fully disclosed, and there is little work in

the literature comparing the performance of different designs.

However, it does seem that commercial devices incorporate differ-

ent dispersion mechanisms in order to disperse a wide range of

powders more efficiently.

Dispersion in the dry feeder, known as the Scirocco, coupled

with the Malvern Instruments Mastersizer 2000 ( Malvern, Worcs,

UK) laser diffraction instrument is achieved by accelerating pow-

ders in a vertical eductor arrangement (Fig. 5). Dispersion occurs

due to great particle acceleration along with shear forces and par-

Powder feeding

region

Powder dispersion

region

Dispersedpowder

Fig. 6. Sympatec Rodos dry powder dispersion device.

Dispersed

powder

Powder bed

Pressurisedair

Shear

region

Fig. 7. The Aero-Disperser used in the AerosizerTM [100].



11/13

ticleparticle and particlewall collisions, especially at the right

angle region downstream.

The air inlet pressure can be varied from 0.1 to 4.0 bar to deter-

mine the optimum pressure that can disperse the powder without

damaging individual particles. It is possible to calibrate the ideal

dispersing pressure through a comparison with wet dispersion

particle size analysis.

Sympatec GmbH (Zellerfeld, Germany) produces a dry powderdisperser called Rodos[96](Fig. 6). This device is specified as being

capable of dispersing particles down to 0.1 lm[97]. Depending on

the degree of powder cohesion, it is possible to change the dosing

method to the device. For relatively free-flowing materials, a vibra-

tory feeder and an inlet funnel connected to the dosing line are

used; however, for extremely fine or cohesive materials a rotating

table having a grove with a compacting roller is recommended

(Fig. 6). The powder is dispersed by using pressurised air in order

to accelerate the particles as soon as they arrive in the dispersing

line [1]. The pressure of the air feed can be varied from 0.1 to

6.0 bar to achieve a fully dispersed state. Furthermore, there are

different dispersing lines available (4, 6, and 10 mm) to enhance

the dispersion performance of more cohesive materials.

The AerosizerTM LD (Amherst Process Instruments, Hadley, MA) is

a time-of-flight measurement device capable of measuring the size

of particles in the range of 0.2700 lm. The Aero-DisperserTM used

in conjunction with the AerosizerTM analyser uses pulsed jet tech-

nology to lift the sample from the sample holder and then applies

high shear flow to complete dispersion down to the primary parti-

cles(Fig.7) [3]. There are four controllable variablesavailable to im-

prove dispersion performance of various materials, including shear

force, feed rate, transport velocity from the fluidised bed to the dis-

perser pin, and pin vibration[98]. This device has been shown to

disperse cohesive material without evidence of attrition [99].

Beckman Coulter GmbH (Krefeld, Germany) produces a dry

powder disperser called the TornadoTM [101], which breaks up a

powder by using a swirling air flow, sucked through a ring gap

above the sample via rotation of the sample container support,

leading to strong impaction with controllable intensities [102].Again pressure optimisation (0.14.0 bar) is recommended to find

the ideal pressure (Fig. 8).

The dispersing unit manufactured by Fritsch GmbH (Oberstein,

Germany) uses mechanical and pneumatic forces to disperse pow-

ders [103]. An amplitude-controlled vibratory feeder is used for

material dosing and the dispersion occurs in a two-phase annular

gap nozzle, where powders are accelerated at a high flow rate. The

default pressure is set at 3 bar with a maximum of 4 bar; however,

it can be reduced to 1 bar for fragile materials to avoid attrition.

The dispersing device manufactured by Horiba Instruments(Japan), PowderJet II, is a coaxial vertical eductor [104]designed

Pressurised

air inlet

Powder inlet

riAriA

Laser light

Vacuum suction

Fig. 9. The Horiba Instruments PowderJet II device[105].

Non-pressu

rised ambientair enters the space

between the suction probeand the sample holder and

forms a downward flow

High shear force generated

by a change in air velocity

direction

Dry powder bed

Dispersed dry powder

delivered to a particle

analyser via vacuum

Vortices created by tapered

lower end of the suction probecreating low energy particle to

wall and particle to particle

collisions

Fig. 8. The basic principle of the Tornado dry powder system [101].

Nominal isokinetic

sample removal

Powder sampleRotating table

Venturi throat

Exhaust to filter

Capillary tube

Dispersed powder output

Fig. 10. The TSI small-scale powder disperser.



12/13

to provide a straight particle flow path to minimise physical impact

of the particles (Fig. 9). The disperser has the option of three differ-

ent nozzles, which can be used for dispersing various materials: a

larger nozzle for less cohesive larger materials; an intermediate

nozzle; and a small nozzle to be used with fine cohesive materials.

It is suggested that the smaller nozzle can disperse fine titanium

dioxide down to 0.1 lm[105].

The small-scale powder disperser (SSPD) manufactured by TSI(Shoreview, USA) is depicted inFig. 10.The powder to be dispersed

is placed over the surface of one of the three annular rings of abra-

sive paper, which in turn is glued to the top of a rotating table.

Powder is removed from the rotating table by suction through a

Venturi aspirator and capillary delivery tube. A region of low pres-

sure is created by the increased velocity of air through the Venturi

throat, which draws the fluid up through the capillary tube. The

device has been found to work well under the following operating

conditions: flowrate through the Venturi throat 16.5 L/min;

velocity through the Venturi throat 310 m/s; flowrate through

the capillary delivery tube 2.0 L/min; and velocity through the

capillary delivery tube 32.5 m/s[106]. An evaluation of the SSPD

can be found in the literature[107].

5. Conclusions and recommendations

The ability to completely disperse a wide range of powders from

cohered clustersto their primaryconstituentsis of great importance

for many reasons, be it to fully characterise a powder or delivery

therapeutic drugs to the lung. There is still a need to further under-

stand how bulk powders interact with a dispersing fluid. There are

modelsfor therelatively simple systems, butlittle is known regard-

ingcomplex fluid flows with bulk solid.In addition,there appears to

be little understanding of the limit of interparticle cohesion, for

which acceleration and/or shear flow is no longer effective.

A number of methods are available for characterising bulk pow-

der cohesion such as tensile test, shear test, or the re-entrainment

test. However, there is still little evidence confirming, whetherthese approaches are reliable tools for determining dispersion per-

formance. In addition, there is little understanding that links bulk

powder properties and aerodynamic dispersion performance.

A large number of dispersion devices are available and a num-

ber of investigations have been conducted evaluating their perfor-

mance. It is seen that dispersion by impaction is the most efficient

mechanism; however, this may not be the case with all powders,

particularly if weak or friable. Furthermore, a great deal of atten-

tion has been given to the impeller mixer-type disperser due to

ability to control the particle concentration leaving the disperser.

The performance of an eductor disperser is similar to the mixer

type when the mass flow ratio is the same.

A relatively untouched area is the use of computational ap-

proaches to further understand dispersion behaviour. New devel-opments in coupling the distinct element method (DEM) with

computational fluid dynamics (CFD) enables a more fundamental

study to be carried out.

Acknowledgement

The authors gratefully acknowledge the financial support from

the Engineering and Physical Sciences Research Council (EPSRC)

and Malvern Instruments.

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