Drop Test: A New Method to Measure the Adhesion Force of
Particles
Umair Zafar, Colin Hare, Ali Hassanpour, Mojtaba Ghadiri
1
Introduction
Interactions between particles, and between particles and surfaces,influence many processes such as powder flow, powder dispersion ingases and liquids, filtration and cleaning.
Approximately 60% of chemical plants world wide handle powders.
Adhesion can generate desirable or undesirable effects across different industries.
It becomes very important to determine the magnitude of adhesion forces between a surface and particles with different properties.
Arching & Blockage
Storage Transport Pharmaceutical
Segregation Tableting 2
Introduction
Control particle size and interparticle forces (van der Waals, capillary) in order
to form good mixtures.
Formulation Detachment of API from carriers
3
Mechanisms of Adhesion
a) Surface and Field Forces at direct contact-Van der Waals forces
-Electrostatic forces
-Magnetic forces
b) Material bridge between contacts-Organic macromolecules
-Recrystallisation of liquid bridges
-Contact fusion by sintering
-Chemical solid-solid reaction
H
H
H
c) Interlocking by hook-link bonds
4
Adhesion Force
Single particle detachment method
Multi-particle detachment method
AFM Pendulum Micro-balance Centrifuge Electrical field
Vibration Tensile Strength
Adhesion Force Measurement
5
Why New Technique
Cost Effective
Easy to setup
Quick estimation for bulk particles
Method Disadvantages
AFM Time consuming, expensive, expertise/skills and prone to noise.
Centrifuge Expensive, difficult to balance weights with high rpm.
Electrical field
Dependent on particle resistivity
Requirements
6
Experimental Setup
7
velocity Impactv =
time Contactt =
force DetachmentF =
Drop Test: Analysis
tvm
F
=
Force of detachment can be determined by using Newtons second law of motion:
8
Drop Test: Analysis
After testBefore test
Particle 3 and 4 has not been detached
The largest particle which has not been detached is particle 4 and smallest particle which has been detached is particle 1
Critical Diameter = Diameter particle 1 + Diameter particle 42
9
Drop Test: Analysis
JKR theory of adhesion:
force AdhesionFad =
radius ParticleR =
energy Interface =
R23Fad = L
R23
tvm
FF
ad
d =
=
detaches Particle 1L If >
detachment No 1L If
10
Size and Shape Analysis
Morphologi G3 digital image analyzer was used to analyse the size and shape of the particles.
Measures materials from 0.5m to 3000m.
Fast and easy to use.
For the measurement of emulsions, suspensions and dry powders
Control of dispersion pressure, injection time and settling time
11
G3 Morphology analysis of particles
Before test
After test
Dispersion unit of G3
Malvern G3 Morphology
12
Change in Critical diameter of glass beads with Impact Velocity
Silanised Glass beads (25-125micron) adhered to silanised glass slides
At higher impact velocities percentage of particles detached goes on increasing.
Salazar-Banda, R.G. et al. 2007 used the Centrifuge method had same observations13
sizeAn overall view on the interface energy for all the impact velocities used so far
There is no change in the interface energy values (Zafar et al., 2010)
The seven different impact velocities were created as a result of the dropping the samples
from various tube heights
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7
Inte
rface
ene
rgy(
mJ/
m)
Impact velocity(m/sec)
14
Materials for experiment
Figure: (a) Salinised glass beads (b) Starch (c) Avivel (d) Lactose 15
sizeComparison of interface energy for regular and irregular particles
The particles had similar size distribution of 45-125micron and were dispersed on glass slides
Mizes, H. et al 2000 concluded from their experiments using centrifugal detachment
method that the increase in the sharpness at the contact points between the particle and
the substrate the reduces force of adhesion significantly. 16
0
5
10
15
20
25
30
35
Glass Beads Starch Avicel Lactose
Surf
ace
ener
gy (m
J/m
2)
size
Among the three rough particles Avicel shows the lowest interface energy and Lactose the
highest for all the three surfaces.
Comparison in interface energy of irregular particles on different surfaces
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Effect of functional Group
05
101520253035404550
Hexane NH2 CH3 CF3
Inte
rfac
e En
ergy
(mJ/
m2)
Functional Group
The interface energy calculated using drop test technique is different for different saline coatings.
However, the trends of the order was similar to measurements done by IGC.
Conclusions
Study shows the drop test methodology can be used to estimate the effective interface energy between fine particles to a surface.
According to the results, the same interface energy has been obtained for each sample at different impact velocities.
Drop test Technique can detect variation of size , shape and particle-surface roughness
Improvement plan
Use of piezoelectric device for measurement of contact time.
19
Acknowledgements
20
Thank You
Any questions
21
Piezoelectric Ceramics
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
-0.3 0.2 0.7 1.2 1.7 2.2 2.7
Volt
age
(V)
Time (s)
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Adhesion Force
23
Atomic Force Microscopy
laserdiode
split photodiode
transparentelectrode
particle
piezoelectric:moves upand down
cantilever20 m
Probes adhesion and attraction of single particles to surfaces with high precision
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As deposited After 8.9 kRPM spin After 20 kRPM spin
Probes adhesion of heterogenous particles to a surface and any spatial effects.
Centrifugal Detachment
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VDonor ReceiverV
Uses evanescent waves to monitor the particle removed from a plate by an applied electric field.
Electric Field Detachment
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Three different granules (A, B, C) with size range 2-3mm are mixed with a
white powder with the sieve size range 45-325 m, using shaking device at
20Hz frequency for 2 minutes
A CB
Sample Materials
==tmv
Fd 0.14 mN
12
34
1
Drop Test: Analysis
D= 350 m
3D6
v
= mass
P= 976 kg/m3
Image Pro Plus
V= 4.26 m/s t = 0.66 ms
200 m
Drop Test: Analysis
JKR theory of adhesion:
force AdhesionFad =
radius ParticleR =energy Interface =
R23
Fad = LR
23
tmv
FF
ad
d ==
detaches Particle 1LIf >
detachment No 1LIf
12
3 4
1
(Particle 1)
(Particle 2)
Before the test After the test200 m
Sample Height 1 Height 2
Particle A 10.3 1.6 (mJ/m-2) 10.5 1.4 (mJ/m-2)
Particle B 57.1 1.4 (mJ/m-2) 55.1 0. 9 (mJ/m-2)
Particle C 34.3 1.3 (mJ/m-2) 35.4 0. 8 (mJ/m-2)
Effective Interface Energy
sizeChange in interface energy due to aging of the silane coat
Silanised Glass beads on Silanised glass slides dried for 16hrs at 30C
There is no significant change in the interface energy.31
DMT Mechanics Derjaguin, Muller, Toporov 1975 Applies to high modulus, low adhesion, small radii of curvature systems Sphere/plate geometry remains at initial loading, from surface forces) At equilibrium attractive surface forces balanced by repulsive. DMT result coincides with a non-deformable sphere
equilibrium pull-off
Theories of Adhesion
RP OffPull = 2
32
radius ParticleR =
energy Interface =
JKR Mechanics Johnson, Kendall, Roberts, 1971 Applies to low modulus, high adhesion, large radius of curvature systems Upon pull-off a neck forms between adhering surfaces resulting in adhesion
hysteresis Note: Predicted force of adhesion is 75% of that predicted by DMT model
equilibrium pull-off
Theories of Adhesion
RP OffPull = 23
33
