granule formation mechanisms and morphology from single drop impact on powder beds 2011 powder...

8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology

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Granule formation mechanisms and morphology from single drop impact onpowder beds

Heather N Emady a Defne Kayrak-Talay a William C Schwerin c James D Litster ab

a School of Chemical Engineering Purdue University West Lafayette IN 47907 USAb School of Industrial and Physical Pharmacy Purdue University West Lafayette IN 47907 USAc Honeywell Des Plaines IL 60017 USA

a b s t r a c ta r t i c l e i n f o

Article historyReceived 13 January 2011

Received in revised form 20 April 2011

Accepted 26 April 2011

Available online 3 May 2011

Keywords

Nucleation

Wet granulation

Granule morphology

Tunneling

Spreading

Crater formation

Single drops impacting static powder beds were studied to explain the different resulting granule structuresThree chemically similar powders with different physical properties formed static beds with porosities of

066ndash069 054 and030ndash035 respectively Three different binder1047298uids (distilled water andsilicone oils with

viscosities of 93 and 96 mPa s) were released onto these powder beds from two different heights (05 and

30 cm) The initial drop impact as well as complete penetration of the dropletinto the bedwas recordedwith

a high speed camera The high speed camera videos were analyzed and three different granule formation

mechanisms were identi1047297ed Tunneling Spreading and Crater Formation Tunneling occurred for loose

cohesive powder beds Powder aggregates were sucked into the drop which then tunneled into the beds For

coarser powders granules were formed by a Spreading mechanism at a low impact velocity At a high impact

velocity the drop formed a crater in the bed surface and deformed elastically in the crater coating the drop in

a layer of powder before penetrating into the bed by capillary action Using all three dimensions of the

granule a new shape factor the vertical aspect ratio (the ratio of the granules projected area diameter to its

maximum vertical height) was proposed as a more accurate descriptor of granule shape than currently used

descriptors such as the horizontal aspect ratio The different granule shapes observed were explained by the

granule formation mechanisms The Tunneling mechanism always produced round granules the Spreading

mechanism always produced 1047298

at disks and the Crater Formation mechanism produced granules of varyingshapes that were dependant on liquid binder properties The results of this study have important implications

for being able to predict granule structure from granule formation mechanisms and to be able to choose the

desired granule properties by operating in the appropriate regime

copy 2011 Elsevier BV All rights reserved

1 Introduction

The interaction between liquid drops and powders is an important

process in many applications such as powder coating and spray

drying through to the interaction of rain drops with the soil with wet

granulation as an application of particular interest

Wet granulation is carried out in a diverse range of processing

equipmentincludinghigh shear mixers drums pans and1047298uid beds All of

these granulators involve a binder spray along with varying levels of

powderagitation Thethree distinct rateprocessesthatoccur in industrial

granulators are wetting and nucleation consolidation and growth and

breakage and attrition [1] Due to the combination of many of these rate

processes occurring simultaneously it is dif 1047297cult to produce narrow

distributions of product properties such as size shape and density

Recently the wetting and nucleation regime has received signi1047297cant

attention in an attempt to quantify it and separate it from the other rate

processes [2ndash4] Hapgood et al recommended operation in the drop

controlled nucleation regime where one drop forms one granule[3] This

is the ideal nucleation regime in which to operate for the best control of

granule properties The nucleation regime map proposed by Hapgood et

al quanti1047297es the boundaries of drop controlled nucleation based on

formulation properties and process parameters [3] For drop controlled

nucleation to occur both the dimensionless spray 1047298ux and dimensionless

drop penetration time must be less than 01 A low drop penetration time

indicates good wetting of the powder (controlled by formulation

properties) [2] while a low dimensionless spray 1047298ux indicates little

dropoverlap (controlledby process conditions)[5] Thenucleation regime

map is a good tool for control of the wetting and nucleation regime

Granulation by nucleation alone has the potential to give a narrow

granule size distribution [4] However it does not provide a means of

controlling the granule density Therefore wetting and nucleation

followed by a consolidation and growth regime has the potential to give

control of the desired 1047297nal granule properties Recently Wildeboer et al

successfully separated nucleation from other rate processes [4] They

Powder Technology 212 (2011) 69ndash79

Corresponding author at Purdue University Forney Hall of Chemical Engineering

Room 1053 480 Stadium Mall Drive West Lafayette IN 47907 USA Tel +1 765 496

2836 fax +1 765 494 0805

E-mail address jlitsterpurdueedu (JD Litster)

0032-5910$ ndash see front matter copy 2011 Elsevier BV All rights reserved

doi101016jpowtec201104030

Contents lists available at ScienceDirect

Powder Technology

j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e p ow t e c



constructed a nucleation apparatus that involves spraying liquid drops

onto a conveyor belt of powder Through the use of a monosized droplet

generator with multiple nozzles monosized nuclei granules were

produced Similar approaches have been tested for 1047298uidized systems

[67] These processes demonstrate that very narrow granule size

distributions can be achieved with nucleation alone In the type of

granulator proposed by Wildeboer et al the drop size will be quite large

when compared to current industrial practice (02 to 2 mm) as the drops

will be of the same order as the desired size of the granular productAlthough the nucleation regimemap is a usefulguide to determineif

drop controlled nucleation will occur it does not predict the structure

and morphology of granules that are formed or details of the

mechanisms by which they are formed For operation in the drop

controlled regime and especially in regime separated granulators

knowledge of these mechanisms and how they affect nuclei properties

will be particularly important for predicting and controlling 1047297nal

granule properties

The granule properties that have been given the most attention in

the literature are granule size and size distribution since these are the

easiest properties to measure [8] However other granule properties

such as shape porosity and internal structure are equally as important

in dictating granule end use performance Hapgood made some

qualitative observations about the shape of the granules produced

from her drop penetration time experiments [9] She observed a wide

range of shapes for different powders and liquid binders most of which

were either hemispherical or mushroom-shaped Some work has also

been done to quantitatively describe granule shape Bouwman et al

tested many granule shape descriptors and concluded that circularity

and a newly proposed projection shape factor with a roughness factor

best portray granule shape [10] However these shape descriptors only

consider a two-dimensional projection of the granule Although many

researchers have observed changes in granule morphology and shape

with different granulation processes no work has been done to

quantitatively relate these granule properties to formulation properties

of the powder and liquid binder as well as process conditions Many

applications require round granules for good product performance or

simply visual appeal of the product In these cases ability to predict the

shape of the nuclei granules and the 1047297nal product granules will be veryimportant

This paper will investigate drop impacts on powderbedswith widely

varying properties using high speed video to provide an understanding

of the possible granule formation mechanisms than can occur by drop

controlled nucleation in regime separated granulation In addition to

investigating the granule formation mechanisms granule morphology

resulting from these different mechanisms will be examined in detail

and the morphology will be linked to the formulation and process

conditions The results of this study will be useful in the design and

operation of regime separated granulation systems as well as other

processes in which drops impact powder beds

2 Background

The conventional way of describing nucleus formation is by capillary

penetration of theliquidthrough thepowderporesmodeling thepowder

bed as if it were a porous non-deformable solid [2] However this

mechanism has not been probed in detail A better explanation of the

nucleation mechanism requires the study of impacting drops There is a

large body of work that investigates drop impact on liquids solids and

even porous solids [11ndash15] Some of these concepts apply to drops

impacting on powders although the powder system is much more

complex The authors who studied drop impacts agree that the governing

dimensionless groups are the Weber and Reynolds numbers

We = ddU 2ρ

γ

eth1THORN

Re = ddU ρ

μ eth2THORN

where dd is the drop diameter U is the drop impact velocity ρ is the

drop density γ is the drop surface tension and μ is the drop viscosity

The Weber number is the ratio of inertial to surface tension forces

while the Reynolds number is the ratio of inertial to viscous forces

These dimensionless groups only involve liquid properties so they

canonly partially describe thephenomena of drops impactingpowdersystems Particle properties and powder bed packing will play a

signi1047297cant role in the mechanism of drop impact with these systems

For nucleation and drops impacting powder beds there is some

disagreement in the literature on the exact mechanisms occurring

The major reported mechanisms for drop impact into powder beds

include capillary penetration spreading crater formation and solid

spreading although no model or set of conditions exist for predicting

which mechanism will occur

Werner et al investigated drop impacts on anhydrous milk fat

powders for air-suspension coating applications in the food industry

[16ndash18] They recognized the importance of drop impact behavior in

addition to the typically studied wettability on the 1047297nal granule

attributes The two drop impact mechanisms reported include

in1047297ltration (capillary penetration) and spreading [16] Popovich et

al also observed the simultaneous spreadingand penetration of drops

on carbon black compacts [19]

Although the majority of their experiments were performed on

hard powder surfaces Werner et al observed cratering upon impact

when their powder surface became soft after a long period of time

[17] Since cratering prevented spreading and was therefore not

desirable for this application no further detail was given on this

impact phenomenon Ghardiri investigated crater formation in soil

sand and pastes from the impact of rain drops [20] He measured

crater diameter and depth and related the crater volume to the

surface shear strength and drop impact impulse

Many researchers have performed single drop nucleation experi-

mentswhere singledropsare released ontoa loose powderbed often in

a Petri dish [221ndash30] Only a few of these workers have reported

granule formation mechanisms Agland and Iveson conducted singledrop experiments on large glass beads where they varied impact

velocity and liquid binder [28] They observed a variety of impact

mechanisms and concluded that drops penetrate the powder bed

through capillary forces at low impact velocities while they spread on

the powderbed surface at high impact velocities Charles-Williamset al

recognized the competitive spreading versus capillary penetration

mechanisms in the formation of granules [29] They proposed empirical

scaling relationships for the spreading velocity and in1047297ltration rate of

the drop which were dependent on both powder and liquid properties

Hapgood and colleagues have investigated the granule formation

mechanism for hydrophobic powders and hydrophobichydrophilic

powder bed combinations [23ndash27] They propose a solid spreading

mechanism where the hydrophobic particles spread over the surface of

thedrop upon impact to form liquid marbles with a powder shell Dropimpact had a prominent effect on this mechanism as the surface

coverage of the drop increased with increasing drop height [2426]

However thedriving force behindthe solid spreadingmechanism is still

not well understood [31]

Recently Lee and Sojka [21] studied drop impact on beds of large

ballotini using a high speed camera They showed that elastic

deformation of the drop and crater formation occur over the same

short timescale and have a strong in1047298uence on the drop footprint The

elastically deforming drop picks up particlesfrom the cratersurface as

it retracts However they did not study the morphology of the

granules that were formed Marston et al also looked at drop impacts

onto glass ballotini but they focused more on the drop dynamics than

the actual granule formation [30] However the importance of

powder bed porosity as well as Weber number was realized Both

70 HN Emady et al Powder Technology 212 (2011) 69ndash79



spreading and crater formation were observed and empirical 1047297ts were

developed for both the maximum spread diameter and crater diameter

as functions of impact velocity or Weber number A few different

granule shapes were observed but not quanti1047297ed The granule

diameters were all 23ndash29 mm which shows that granule size was

insensitive to all experimental conditions tested on the glass ballotini

In summary while a variety of interesting mechanisms have been

identi1047297ed for drop impact and interaction with powder beds these

mechanisms are not incorporated into nucleation models for

granulation The effect of powder bed properties on these mecha-

nisms is not quanti1047297ed and few studies have used 1047297ne cohesive

powders which are the staple of granulation processes In addition

there are no studies which report details of the granule structure and

shape and relate these important properties to the nucleation

mechanism

3 Experimental

31 Materials characterization

Two refractory inorganic powders supplied by Honeywell were used

as model materials The powders were chemically similar but with

different size distributions porosities and bulk properties Powder B (thecoarser powder) milled to give a similar particle size distribution to

Powder A was used as a third model powder (Powder C) Particle size

characterization was performed by wet dispersed laser diffraction

(Malvern Mastersizer 2000) True particle density was measured by

Heliumpycnometry (Micromeritics AccupycII 1340)Tapped density and

bulkdensity were measured ina 100 mLgraduated cylinderwith a Varian

Tapped Density Tester The powder characterization summary with 95

con1047297dence intervals is given in Table 1 The volume frequency

distribution of particle size visually shows the differences in size

distributions (see Fig 1)

Three different binders were used including distilled water and

two different viscosity silicone oils to see the effects of viscosity and

surface tension Surface tension was measured by the Wilhemy plate

technique (Kruumlss Processor Tensiometer K100) The liquid binderproperties with 95 con1047297dence intervals are given in Table 2

32 Experimental methods

Single drop granule nucleation experiments were conducted to

investigate liquid drop impact with powder beds The powder was

lightly sievedthrough a 200 mm sieve into a Petri dish andthen leveled

with a plastic ruler to get a smooth surface The powder bed density

ρbed was calculated by dividing the mass of powder in the Petri dish by

the volume of the Petri dish The bed porosity was then calculated as

εbed = 1minusρbed = ρ p eth3THORN

where ρ p is the apparent density of the primary particles

A 100 μ L syringe was 1047297lled with binder and held in place at either

05 or 30 cm above the powder surface with a clamp Two different

drop heights were used to examine the effect of drop impact velocity

Single drops were released from the syringe manually and the

powder was covered with binder droplets far enough apart to avoid

coalescence of drops The granules were subsequently excavated by

either lightly pouring the powder out into a 200 mm sieve with the

non-granulated powder falling through the sieve or scooping the

weak granules out individually with a spatula

A high speed camera (Photron Fastcam-X 1024 PCI) was used to

capture the nucleus formation mechanisms Two important time

scales were observed during the nucleation process Drop impact

drop deformation and crater formationoccurred over therange of 1 to

20 ms Drop spreading penetration and tunneling took up to 5 min

depending on the properties of the drop and the powder bed The

initial drop impact was recorded at 1000 framesper second while the

complete drop penetration was recorded at 60 frames per second

The drop size was captured with the high speed camera

immediately after the drop was released from a 100 μ L syringe The

drop diameter was calculated by taking an average of its vertical and

horizontal diameters measured manually with UTHSCSA ImageTool

300 For each liquid binder 11ndash12 images were taken to calculate the

drop size Differentsyringe needlegauges were used forwaterand thesilicone oils to keep drop size similar for the three different model

1047298uids

A picture of the single drop apparatus and high speed camera set-

up is shown in Fig 2

33 Granule characterization

A Nikon SMZ-1500 Stereoscopic Zoom Microscope was used to

capture images of the granules Each granule was placed next to a

prism to capture its third dimension the side view (see Fig 3) Each

resultingimagecontained theprojected area view on the left side and

Table 1

Physical properties of model powders

Powder A Powder C Powder B

Surface mean d 32 (μ m) 297 plusmn 001 36 plusmn 02 15 plusmn1

Volume mean d43 (μ m) 380 plusmn 006 71 plusmn 02 53 plusmn3

d10 (μ m) 176 plusmn 002 152 plusmn 005 9 plusmn2



True particle density ρs (gcm3) 2495 plusmn 0004 25431 plusmn 00007 2479 plusmn0002

Pore volumea

V p (cm3

g) 083 plusmn 001 045 plusmn 001 045 plusmn001Apparent particle density ρ p= ρs (1 +V p ρs) (gcm3) 0812 plusmn 0007 119 plusmn 001 117 plusmn001

Bulk Density ρB (gcm3) 030 plusmn 003 051 plusmn 003 078 plusmn003

Tapped Density ρT (gcm3) 0523 plusmn 0003 087 plusmn 002 100 plusmn0008

Loose packed bed porosity 1minus ρB ρ p 068 plusmn 001 054 033 plusmn002

a Data from nitrogen adsorption performed by Honeywell (Des Plaines IL USA)

Powder A

Powder B

Powder C

Powder A

Powder B

Powder C

0

1

2

3

4

5

6

7

8

9

10

11

01 1 10 100 1000

V o l u m e

Particle Size [microm]

Powder A

Powder B

Powder C

Fig 1 Volume frequency distribution of powders

71HN Emady et al Powder Technology 212 (2011) 69ndash79



the side viewon the right side(see Figs 4ndash6) If thegranulesproduced

were too large to 1047297t within the microscope view images of both the

top and side views were taken with a digital camera For each

experiment 8ndash20 granules were captured depending on how well the

granules survived handling

Adobe Photoshop CS4 with the Fovea Pro 40 plug-in was used to

analyzethe images Themeasurements taken from the software include

projectedareaequivalentdiameter (da) circularity(4π AreaPerimeter 2)

horizontal aspect ratio (dmax dmin) and vertical aspect ratio (da hmax)

The maximum granule height hmax wasmeasuredmanuallywithimage

analysis software accompanying the microscope (Nikon NIS-Elements

D 300) The circularity horizontal aspect ratio and vertical aspect ratio

values should all be close to one to indicate round granules The most

sensitive measure of granule shape is vertical aspect ratio since it

incorporates the third dimension of the granule

4 Granule size and morphology

For Powder A and Powder B experiments were performed using all

three liquid binders andat two drop heights 05 and 30 cm For Powder

C only water was used as the liquid binder The granule microscope

images and their corresponding characterization results with 95

con1047297dence intervals are given in Figs 4ndash6 Results were compared

statistically using ANOVA withTukeys tests at the 95 con1047297dence limit

At each set of experimental conditions the granules have very narrow

size and shape distributions For example all Powder A and C granule

samples have coef 1047297cients of variance of projected area diameter (da)

and maximum height (hmax) less than 10 indicating that the drop

controlled granules are effectively monosized There was slightly morevariation with the Powder B granules with coef 1047297cients of variance up to

27 for the size measurements

The granule sizes vary for the three different powders and drop

heightand liquid bindertype haddifferent effects on thegranule size for

each powderPowder A granules have projectedarea diameters of 298ndash

476 mm and maximum heights of 281ndash443 mm Drop height did not

signi1047297cantlyaffect granule size (da and hmax)atthe95con1047297dencelevel

However liquid binder had a substantial effect on granule size with

signi1047297cant differences between granules formed at each liquid pair at

both drop heights The granule size increased when the silicone oils

were used instead of water Powder C granules have projected area

diameters of 390ndash424 mm and maximum heights of 341ndash362 mm

Drop height had a signi1047297cant effect on granule size with an increase in

size as height increased Powder B granules have projected area


The effects of drop height and liquid binder were different for da and

hmax Drop height had a signi1047297cant effect on granule size for water with

da decreasing and hmax increasing with drop height There was also a

signi1047297cant effect on da for 93 mPamiddots silicone oil with thesametrend as

with waterAt a drop heightof 30 cm there was a signi1047297cant difference

between water and each of the silicone oils for both da and hmax with daincreasingand hmaxdecreasingwhenswitching fromwaterto 93 mPamiddots

silicone oil as the liquid binder The same results occurred at a dropheight of 05 cm for da For da with a 30 cm drop height there was a

difference between the two silicone oils with size increasing with

increasing liquid viscosity

Fig 2 Single drop experimental set-up

Fig 3 The morphology imaging set-up where a granule is placed next to a prism to

capture its re1047298ected height under a microscope (a) Side view of the granule and prism

under a microscope (b) Top view of the granule and prism where the granules

re1047298ected height can be seen in the prism

Table 2

Liquid binder properties

Viscosity (mPa s) De nsity (gmL) Surface tension (mN m) Syri nge ne edle gaug e D rop diameter (mm)

Distilled water 1 1 720 plusmn 03 22 271 plusmn 003

Silicone oil 93 a 093 a 202 plusmn 02 14 258 plusmn 001


a Data from Sigma-Aldrich




There is a dramatic difference in morphology between the

granules formed by the three different powder types In general

Powder A granules are round (see Fig 4) and Powder C granules are

mushroom-shaped (see Fig 5) The morphology of Powder B granules

varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)

Powder A granules formed from the loose packed bed have vertical

aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835

and horizontal aspect ratio (HAR) values of 113ndash118 The granules

formed are approximately spherical and their morphology is quite

insensitive to binder properties and process conditions Neither binder

type nor drop height has a signi1047297cant effect on the granule shape

descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally

forms a high porosity packed bed (ε =066ndash069) When the bed is

compacted to a much lower porosity (ε =033) moderate changes in

granule morphology occur The granules formed are more hemispherical

with the 1047298at side corresponding to the compacted bed surface The VAR

values re1047298ect this change increasing from values in the range 105 to 110

for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values

of 0760ndash0918 and HAR values of 105ndash109 For the Powder B

granules the VAR values are very different from the HAR values

Powder B has HAR values close to one indicating that the drop

footprint on the powder surface is approximately circular but all of the

VAR values are much larger (see Fig 6) Since most of the Powder B

granules are 1047298at disks the HAR values from 105ndash109 are misleading

in indicating that the granules are round Therefore the VAR values

will be used for roundness comparisons in the discussion

For this powder different combinations of binder type and drop

height had a signi1047297cant effect on granule shape Granules produced at

the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash

273) There was a signi1047297cant difference between VAR values of

granules produced with water and 93 mPa s silicone oil at this drop

Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image

Fig 5 Microscope images of Powder C granules with size and shape characterization

values The projected area view is on the left and the side view is on the right of each

image




height with the VAR value increasing when switching from water to

93 mPas silicone oilas theliquid binder Granules produced from a large

drop height (30 cm) were more mushroom shaped and signi1047297cantly

rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between

VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were

used instead of water No height effects existed with the 96 mPa s

silicone oil binder Granules formed using water as the liquid binder

were rounder than those formed with either of the silicone oil binders

A granule shape comparison for Powder A and PowderB is given in

Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch

more sensitive to drop height and liquid binder than Powder A For

Powder B the improvement in VAR with increasing drop height is

obvious and the major improvement can be seen with water as the

liquid binder In contrast Powder A VAR values are consistently near

10 independent of binder type and drop height

Powder C granules were formed with water as binder from two

different drop heights Their morphology was intermediate between

Powder A and Powder B Drop height did not signi1047297cantly affect the

granule shape at the 95 con1047297dence level

5 Visualization of granule formation mechanisms

Theresults above showeda wide range of granulemorphologies The

type of powder and the powder bed packing were very important in

determining granule shape The binder properties and the binder drop

height primarily affected the granule properties for Powder B To help

gain a better understanding of the granule formation process high

speed camera videos of drop impact with the different powder bed

surfaces were produced Two different types of granule formation

mechanisms were observed A Tunneling mechanism was observed for

the cohesive powder beds of Powder A and Powder C producing fairly

round granules A SpreadingCrater Formation mechanism was observed

for the free1047298owing powder bed of Powder B The Spreading mechanism

occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-

ing rounder granules Details of these mechanisms are describedbelow

51 Tunneling mechanism

Fig 8 illustrates the Tunneling mechanism Theloose powderbed is

not homogeneous but is composed of a 1047297ne cohesive powder that

forms larger loose aggregates with large pores or cavities (see Fig 8a)

When the droplet hits the powder bed it bounces and rolls then

comes to an equilibrium position (see Fig 8b and c) The liquid

penetration is driven by capillary forces Therefore the liquid prefers

to penetrateinto the small pores of thedry aggregates ratherthan the

large pores in between the aggregates [2] The capillary force is

greater than the adhesive force between the dry aggregates causing

Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image

Powder A 05 cmPowder A 30 cm

Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325

Water 93 mPas Silicone Oil 96 mPas Silicone Oil

V A R

Binder Type


Powder B 05 cm

Powder B 30 cm

Fig 7 VAR comparison for Powder A and Powder B granules




the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates

enter the droplet from all sides and migrate inside the droplet The

particle currents can be seen inside the droplets This migration of

aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles

and aggregates from the new surface (see Fig 8e) Thus this nucleation

mechanism is somewhat similar to the engul1047297ng mechanism observed

in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original

shape during nucleation Thus the nucleus has a strong spherical core

with some protrusions on the surface (see Fig 8f) The protrusions are

caused by dry agglomerates going into the droplet but without enough

liquid available to fully penetrate into the droplet

All of the Powder A and Powder C granules are formed via the

Tunneling mechanism although their morphologies are slightly

different The Tunneling mechanism with loose powder beds explains

why granules formed with Powder A are consistently round The

mushroom-shaped granules occurring with Powder C could indicate

that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater

Formation mechanism discussed in the next section

Overall neither binder type nor drop height has a signi1047297cant effect

on the morphology of Tunneling formed granules over the range of

conditions tested in this study The difference in shape between the

Powder A granules and Powder C granules can be explained by the

different powder bed porosities The VAR values improve with

increasing powder bed porosity

52 SpreadingCrater Formation mechanism

521 Spreading mechanism

The mechanism of drop penetration into Powder B from low drop

heights can be seen in Fig 9 The uniformly packed powder bed is

composed of a coarse powder with a large particle size distribution that

forms a smooth surface (see Fig 9a) When the droplet hits the powder

surface it elastically deforms splashing a small amount of powder and

making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the

concentration of the gathered particles is low they do not form an

immobile layer on theliquidsurface allowing thedroplet to spreadon the

powder surfaceover a longer time scale(08 s to 1 min depending on the

liquid viscosity) The liquid spreads over the surface while it is simulta-

neously penetrating into the powder bed by capillary forces (see Fig 9d)

As the rate of penetration is slow compared to the rate of spreading the

resultant granules are 1047298at with a slightly higher rim (see Fig 9e)

522 Crater Formation mechanism

The mechanism of drop penetration into Powder B from high drop

heights canbe seen in Fig 10 The homogeneously packed powder bed

is composed of a coarse powder with a large particle size distribution

that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a

larger splash diameter (see Fig 10b) The droplet deforms elastically

along the crater surface up to the rim picking up particles from the

powder surface and these particles form a thick layer on the droplet

surface (see Fig 10c) The particle layer combined with the steep

surface of the crater reduces the mobility of the droplet surface and

decreases the extent of liquid spreading over the powder surface The

liquid then penetrates into the powder bed by capillary forces (see

Fig 10d) Towards the end of the penetration time the remaining

liquid sinks down into the center of the granule causing a concave

surface to format the top of the granule (see Fig 10e) Thediameter of

theconcavity increaseswhen going from water to thetwo silicone oils

as binders and it is related to the diameter that is not occupied with

the particles gathered during the initial impact

a

g

b c d e f

Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)




For Powder B granules the project area diameter is always larger

than the maximum vertical height (see Fig 6) This is due to the

spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of

Powder B granules improves when the drop height is increased from

05 cm to 30 cm for all liquid binders used This can be explained by

the different mechanisms observed at the different drop heights

At a drop height of 05 cm the Spreading mechanism occurs Since

the drop spreads along the powder bed surface and only penetrates

slightly1047298at disks are produced These1047298at disks are formed regardless

of liquid binder as indicated by the high VAR values (see Fig 6)

At a drop height of 30 cm the Crater Formation mechanism occurs

producinga range of granule morphologies that depends on the liquid

binder (see Fig 6) Within this mechanism the VAR value improves

with decreasing viscosity and increasing surface tension Releasing

the liquid binder drops from a high drop height reduces the resting

drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface

tension water than the low surface tension silicone oils signi1047297cantly

impeding spreading to form rounder granules

The best VAR value for PowderB granules is observed with water

as the liquid binder at a drop height of 30 cm The combination of a

low viscosity high surface tension binder and a high drop height are

the most favorable conditions for producing round granules from

uniformly packed powder beds

6 Discussion

Three different mechanisms for the development of granules by

drop interaction with the powder bed have been identi1047297ed in this

study While Spreading and Crater Formation have previously been

reported in the literature Tunneling is formally identi1047297ed asa separate

mechanism for the 1047297rst time A possible reason for this oversight is

that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the

mechanism in action for nucleation experiments with 1047297ne powders

reported in the literature [229] However as these studies focused on

penetration time rather than granule structure the distinction in

formation mechanisms was not identi1047297ed Since the shape and

structure of the granule formed is strongly dependent on the

formation mechanism identifying conditions that control the gran-

ulation mechanism is important

This study shows that the distinction between Tunneling and

SpreadingCrater Formation is largely driven by the structure of the

powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of

cohesive1047297ne powders Here the structure of the bed is complex with

dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the

mechanism on powder bed structure the bed porosity should be a

good indicator of whether the Tunneling mechanism will occur Here

Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling

behavior while Powder B (ε =030ndash035) showed either Spreading or

Crater Formation

For low bed porosity (large particle size) powders the distinction

between Spreading and Crater Formation as the granule formation

mechanism depends on the impact and elastic deformation of the

drop and therefore on the Re and We Both of these dimensionless

groups take into account only 1047298uid properties We hypothesize that

the boundary between Tunneling and SpreadingCrater Formation is

primarilydictated by thestructure of thepowder bed which is related

to the bed cohesivity (represented by Bond number Bo at the particle

a

f

b c d e

Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)




scale or Hausner ratio at the bulk powder scale) and bed porosity ε

With more data covering a wider range of 1047298uid and especially powder

properties intermediate between Powder A and Powder B it should

be possible to test these hypotheses and construct a series of regime

maps of the granule formation mechanisms Development and

validation of such maps is a topic for further study

It is important to emphasize that the granule shape is primarily

determined by which mechanism is controlling the granule forma-

tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater

Formation regime does the granule shape change substantially with

process conditions

Note that this study has used inert powders and simple 1047298uids to

avoid properties which change with time due to binder-powder

interactions or apparent viscosities that vary with strainrate In many

real systems such effects cannot be neglected For example with the

use of a non-Newtonian 1047298uid of which the properties change with

operatingconditionsthe granule shape andsize maybe differentthan

expected in the Crater Formation regime When a shear thinning 1047298uid

is used to form granules the shear rates are high duringinitial impact

therefore the instantaneous viscosity would be low and the extent of

spreading would increase After the drop retracts back and comes to

the equilibrium position the viscosity would be higher During liquid

penetration into the capillaries the shear rates are not expected be

high therefore the viscosity should not be affected by the shear

thinning The changes in the viscosity of the shear thinning 1047298uid will

have an effect on the amount of particles picked up during initial

impact but not during 1047297nal penetration More particles would be

picked up if the viscosity is lowered during initial impact thus the

roundness of the granule would be higher than expected

The wide variety of granule shapes and structures that can be

produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the

granulator design proposed by Wildeboer [4] coalescence and

breakage is avoided Therefore the size and shape of the granule is

largely set by the drop controlled nucleation stage In most cases

nearly spherical granulated products are preferred The process is

likely to be robust for producing spherical granules when operated in

the Tunneling regime but sensitive to formulation properties and

process conditions in the SpreadingCrater Formation regime Densi1047297ca-

tion of granules will also affect their shape with weak granules likely to

become less spherical or even break while strong granules will be

further rounded [33] In the Tunneling regime granules are likely to be

strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round

granules (low viscosity) will also lead to weak granules which may be

a

f

b c d e

Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)




problematicOn theother hand controlling the nucleation regimecould

be seen as an opportunity for tailor made control of granule shape mdash a

new concept for wet granulated materials

Although this work is directly applicable to regime separated

granulation systems the 1047297ndings may also be useful when operating

in the drop controlled regime in traditional granulators When one

liquid drop forms one granule nucleus the formation mechanism will

determine the initial nuclei characteristics but the existence of other

granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation

mechanism alone Evaluation of the nucleus formation mechanism

regime approach for traditional granulation may be an area of future

research interest

Future work incorporating the mechanisms into regime maps will

enhance the ability to predict the granule formation mechanisms over

a wider range of powder and liquid properties Once the mechanisms

are better quanti1047297ed there will be an opportunity to implement the

behavior into nucleation kernels for population balance models in a

similar manner to a previous study relating primary particle

morphology to aggregation kernels [34] A deeper understanding of

the formation mechanisms may improve current nucleation kernels

that are based on drop penetration time [35] Also this work will lead

towards the ability to predict the shape and structure of nuclei

granules as well as their size which is valuable for thedevelopment of

multidimensional population balance models [35] Ingeneral the new

1047297ndings on granule formation mechanisms have the potential to

completely transform the way in which nucleation in wet granulation

is approached

7 Conclusions

From this study three different granule formation mechanisms

were identi1047297ed

bull Tunneling in which powder aggregates are sucked into the drop

which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and

then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a

crater in the bed surface During elastic spreading and retraction of

the drop a layer of powder is formed on the drop surface The drop

then penetrates into the bed from the bottom of the crater with

limited spreading

The controlling mechanism was dependent on the properties of

the powder as well as the structure of the powder bed Each

mechanism produced granules with dramatically different morphol-

ogies Fine cohesive powders (Powder A) formed spherical granules

via the Tunneling mechanism Coarser powders (Powder B) formed

granules that were 1047298at disks at a low drop height via the Spreading

mechanism while rounder granules were formed at a high drop

height with the Crater Formation mechanism Powder C while still

cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to

SpreadingCrater Formation and were mushroom-shaped The bed

porosity is a good predictor of whether tunneling behavior will occur

The granule shape is primarily determined by which mechanism is

controlling the granule formation Granules formed via Tunneling are

always nearly round while granules formed by Spreading are always

disks independent of the liquid properties and process conditions

Liquid binder properties did have a signi1047297cant effect on granules

formed by the Crater Formation mechanism with water giving

rounder granules than the two silicone oils

A new method was developed to characterize granule shape using

a prism and microscope set-up to view a granules third dimension

From this set-up a new dimensionless number was calculated by

taking the ratio of the granules projected area diameter to its

maximum vertical height This vertical aspect ratio was found to be a

more discriminatory granule shape descriptor than the convention-

ally used horizontal aspect ratio

This was the 1047297rst study to relate granule morphology to an in

depth examination of granule formation mechanisms based on

formulation properties and process conditions The results have

signi1047297cant impact on the design of regime separated granulators

emphasizing that operation in the drop controlled regime is not

suf 1047297

cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule

formation mechanism

Acknowledgments

This project was funded by Honeywell Within Honeywell the

authors would like to thank Nan Greenlay for her help in developing

the prism set-up used to capture all dimensions of the granule along

with the subsequent image analysis using Adobe Photoshop CS4 with

the Fovea Pro 40 plug-in

References

[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes

Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366

[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361

[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105

[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39

[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187

[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785

[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)

184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The

University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape

factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu

Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an

impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608

[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30

[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214

[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120

[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45

[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345

[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530

[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74

[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89

[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009

[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190

[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder

beds Chemical Engineering Science 64 (24) (2009) 5210ndash

5221




[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262

[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95

[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132

[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254

[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999

[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman

Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid

drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236

[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310

[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665

[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101

[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and

accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash

384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329




constructed a nucleation apparatus that involves spraying liquid drops

onto a conveyor belt of powder Through the use of a monosized droplet

generator with multiple nozzles monosized nuclei granules were

produced Similar approaches have been tested for 1047298uidized systems

[67] These processes demonstrate that very narrow granule size

distributions can be achieved with nucleation alone In the type of

granulator proposed by Wildeboer et al the drop size will be quite large

when compared to current industrial practice (02 to 2 mm) as the drops

will be of the same order as the desired size of the granular productAlthough the nucleation regimemap is a usefulguide to determineif

drop controlled nucleation will occur it does not predict the structure

and morphology of granules that are formed or details of the

mechanisms by which they are formed For operation in the drop

controlled regime and especially in regime separated granulators

knowledge of these mechanisms and how they affect nuclei properties

will be particularly important for predicting and controlling 1047297nal

granule properties

The granule properties that have been given the most attention in

the literature are granule size and size distribution since these are the

easiest properties to measure [8] However other granule properties

such as shape porosity and internal structure are equally as important

in dictating granule end use performance Hapgood made some

qualitative observations about the shape of the granules produced

from her drop penetration time experiments [9] She observed a wide

range of shapes for different powders and liquid binders most of which

were either hemispherical or mushroom-shaped Some work has also

been done to quantitatively describe granule shape Bouwman et al

tested many granule shape descriptors and concluded that circularity

and a newly proposed projection shape factor with a roughness factor

best portray granule shape [10] However these shape descriptors only

consider a two-dimensional projection of the granule Although many

researchers have observed changes in granule morphology and shape

with different granulation processes no work has been done to

quantitatively relate these granule properties to formulation properties

of the powder and liquid binder as well as process conditions Many

applications require round granules for good product performance or

simply visual appeal of the product In these cases ability to predict the

shape of the nuclei granules and the 1047297nal product granules will be veryimportant

This paper will investigate drop impacts on powderbedswith widely

varying properties using high speed video to provide an understanding

of the possible granule formation mechanisms than can occur by drop

controlled nucleation in regime separated granulation In addition to

investigating the granule formation mechanisms granule morphology

resulting from these different mechanisms will be examined in detail

and the morphology will be linked to the formulation and process

conditions The results of this study will be useful in the design and

operation of regime separated granulation systems as well as other

processes in which drops impact powder beds

2 Background

The conventional way of describing nucleus formation is by capillary

penetration of theliquidthrough thepowderporesmodeling thepowder

bed as if it were a porous non-deformable solid [2] However this

mechanism has not been probed in detail A better explanation of the

nucleation mechanism requires the study of impacting drops There is a

large body of work that investigates drop impact on liquids solids and

even porous solids [11ndash15] Some of these concepts apply to drops

impacting on powders although the powder system is much more

complex The authors who studied drop impacts agree that the governing

dimensionless groups are the Weber and Reynolds numbers

We = ddU 2ρ

γ

eth1THORN

Re = ddU ρ

μ eth2THORN

where dd is the drop diameter U is the drop impact velocity ρ is the

drop density γ is the drop surface tension and μ is the drop viscosity

The Weber number is the ratio of inertial to surface tension forces

while the Reynolds number is the ratio of inertial to viscous forces

These dimensionless groups only involve liquid properties so they

canonly partially describe thephenomena of drops impactingpowdersystems Particle properties and powder bed packing will play a

signi1047297cant role in the mechanism of drop impact with these systems

For nucleation and drops impacting powder beds there is some

disagreement in the literature on the exact mechanisms occurring

The major reported mechanisms for drop impact into powder beds

include capillary penetration spreading crater formation and solid

spreading although no model or set of conditions exist for predicting

which mechanism will occur

Werner et al investigated drop impacts on anhydrous milk fat

powders for air-suspension coating applications in the food industry

[16ndash18] They recognized the importance of drop impact behavior in

addition to the typically studied wettability on the 1047297nal granule

attributes The two drop impact mechanisms reported include

in1047297ltration (capillary penetration) and spreading [16] Popovich et

al also observed the simultaneous spreadingand penetration of drops

on carbon black compacts [19]

Although the majority of their experiments were performed on

hard powder surfaces Werner et al observed cratering upon impact

when their powder surface became soft after a long period of time

[17] Since cratering prevented spreading and was therefore not

desirable for this application no further detail was given on this

impact phenomenon Ghardiri investigated crater formation in soil

sand and pastes from the impact of rain drops [20] He measured

crater diameter and depth and related the crater volume to the

surface shear strength and drop impact impulse

Many researchers have performed single drop nucleation experi-

mentswhere singledropsare released ontoa loose powderbed often in

a Petri dish [221ndash30] Only a few of these workers have reported

granule formation mechanisms Agland and Iveson conducted singledrop experiments on large glass beads where they varied impact

velocity and liquid binder [28] They observed a variety of impact

mechanisms and concluded that drops penetrate the powder bed

through capillary forces at low impact velocities while they spread on

the powderbed surface at high impact velocities Charles-Williamset al

recognized the competitive spreading versus capillary penetration

mechanisms in the formation of granules [29] They proposed empirical

scaling relationships for the spreading velocity and in1047297ltration rate of

the drop which were dependent on both powder and liquid properties

Hapgood and colleagues have investigated the granule formation

mechanism for hydrophobic powders and hydrophobichydrophilic

powder bed combinations [23ndash27] They propose a solid spreading

mechanism where the hydrophobic particles spread over the surface of

thedrop upon impact to form liquid marbles with a powder shell Dropimpact had a prominent effect on this mechanism as the surface

coverage of the drop increased with increasing drop height [2426]

However thedriving force behindthe solid spreadingmechanism is still

not well understood [31]

Recently Lee and Sojka [21] studied drop impact on beds of large

ballotini using a high speed camera They showed that elastic

deformation of the drop and crater formation occur over the same

short timescale and have a strong in1047298uence on the drop footprint The

elastically deforming drop picks up particlesfrom the cratersurface as

it retracts However they did not study the morphology of the

granules that were formed Marston et al also looked at drop impacts

onto glass ballotini but they focused more on the drop dynamics than

the actual granule formation [30] However the importance of

powder bed porosity as well as Weber number was realized Both


















mechanism

3 Experimental


















































1047298uids








Table 1









Pore volumea

V p (cm3






Powder A

Powder B

Powder C

Powder A

Powder B

Powder C

0

1

2

3

4

5

6

7

8

9

10

11

01 1 10 100 1000

V o l u m e


Powder A

Powder B

Powder C




























































Table 2












































image

















































Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325


V A R

Binder Type


Powder B 05 cm

Powder B 30 cm






























































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221



































mechanism

3 Experimental


















































1047298uids








Table 1









Pore volumea

V p (cm3






Powder A

Powder B

Powder C

Powder A

Powder B

Powder C

0

1

2

3

4

5

6

7

8

9

10

11

01 1 10 100 1000

V o l u m e


Powder A

Powder B

Powder C




























































Table 2












































image

















































Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325


V A R

Binder Type


Powder B 05 cm

Powder B 30 cm






























































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221












































































Table 2












































image

















































Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325


V A R

Binder Type


Powder B 05 cm

Powder B 30 cm






























































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221























































image

















































Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325


V A R

Binder Type


Powder B 05 cm

Powder B 30 cm






























































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221


































































Powder B 05 cm

Powder B 30 cm


Powder B 05 cm

Powder B 30 cm

1

125

15

175

2

25

275

225

3

325


V A R

Binder Type


Powder B 05 cm

Powder B 30 cm






























































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221














































































a

g

b c d e f




























6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221












































6 Discussion























Crater Formation









a

f

b c d e















process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221































process conditions

































a

f

b c d e
















research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221
































research interest















is approached

7 Conclusions









limited spreading































suf 1047297


formation mechanism

Acknowledgments






References


























5221



















granule formation mechanisms and morphology from single drop impact on powder beds 2011 powder...

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