PDF Link - Revista Latinoamericana de Metalurgia y Materiales

Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000, 80-84
PARTICLE SIZE AND SHAPE ANALYSIS USING LASER
SCATTERING AND IMAGE ANALYSIS.
R.Xu
Particle Characterization, Beckman Coulter
Miami, FL, USA
Abstraet
Characterization of particle size and shape has become an indispensable tool in many industrial processes including
in production and processing of granular materials. During the past decade, due to the evolution of other modern
technologies, e.g., lasers, computers and automation, the methods used in particle characterization have changed
considerably. In sizing granular materials centimeters or smaller, one of the most widely employed technology is laser
diffraction. Several conventional particle characterization methods, such as sieve analysis or sedimentation analysis,
have gradually been replaced by laser diffraction and other methods based on light-matter interactions. In the past two
years, image analysis utilizing light-matter interaction combined with charge-coupled devices (CCD's) has advanced
rapidly, providing another powerful tool for size and shape characterization of granular materials. New applications that
use these non-invasive methods to characterize various particulate systems appear daily. Many new national and
international standards related to these technologies have been and are still being established. In this article, a review of
contemporary particle sizing technologies is provided with emphasis on using laser diffraction and CCD' s to
characterize granular materials and the latest commercial development, and applications of these technologies are
summarized.
Resumen
La caracterización del tamaño y forma de partícula se ha hecho una herramienta indispensable en muchos procesos
industriales incluyendo la producción y procesamiento de materiales granulados. Durante la última década, debido a la
evolución de modernas tecnologías, como por ejemplo, lasers, computadoras y automatización, los métodos utilizados
en la caracterización de partículas han cambiado considerablemente.
Para determinar el tamaño de materiales
particulados alrededor del centímetro o mas pequeños, una de las tecnologías más utilizada es la difracción laser. Varios
métodos de convencionales de caracterización de partículas, tales como
tamizado o sedimentación, han sido
gradualmente reemplazados por difarcción laser y otros métodos basados en la interacción de la luz con la materia. En
los últimos dos años, análisis de imágenes utilizando interacción de la luz con la materia combinado con equipos de
carga (CCD's) ha avanzado rápidamente, dando lugar a otra herramienta poderosa para la caracterización por tamaño y
foma de partícula de materiales granulados. Nuevas aplicaciones que usan estos métodos no invasivos aparecen
diariamente para caracterizar sistemas particulados. Nuevos estandares nacionales e internacionales relacionados con
estas tecnologías han sido y estan siendo establecidos todavía. En este artículo, se presenta un repaso de las técnologías
contemporáneas para determinación de tamaño de partícula con enfasis en el uso de la difracción laser y CCD's para
caracterizar materiales granulados y los últimos desarrollos comerciales y aplicaciones de estas tecnologías.
R. Xu. /Revista Latinoamericana de Metalurgia y Materiales
1. Overview of Particle Size Analysís Technologíes,
Out of necessity, there are many technologies that
have been developed and successfully employed in sizing
particles from nanometers to millimeters. There were
more than 400 different methods applied in the
measurement of particle size, shape and surface area in
1981 [1]. Prior to the adoption of light-based and other
modern technologies, most partic1e sizing methods relied
on either the physical separation of a sample, such as in
sieve analysis, or the analysis of a limited number of
partic1es, as in the microscopic method. The results from
separation methods consist of ensemble averages of
property of each fraetion, and the resuIts from
microscopic methods pro vide only two-dimensional size
information
from the limited number of particles
examined. During the last two decades, because of the
creation
and
commercialization
of
lasers
and
microelectronics (inc1uding. computers), the science and
technology of particle sízing has evolved considerably,
and technologies for particle shape analysis are emerging.
Many new and sophistieated
methods have been
successfully developed and applied and some previously
popular characterization practices are now being phased
out in many fields. The following table lists common
sizing methods currently in use, arranged according to the
applieable sizing range of each technology.
Electroformed
sensing zone
Gravitational sedimentation
Holography
Time-of-ílight
Electroacouslic
Laser Diffraction
In laser diffraction
measurements
one obtains
information about partic1e size distributions through the
measurement of scattering intensity as a function of
seattering angle and the wavelength of light, based on
applicable scattering models. This is an absolute method
in the sense that once the experimental arrangement or
instrument is correctly set up, ealibration or scaling is not
necessary [1,2]. For monodisperse particles there is a oneto-one relationship between the scattering intensity and
the scattering angle for any particular
size. For
polydisperse
samples, each individual particle will
contribute its own unique seattering signal to the
composite angular scattering pattem differentia11y. The
intensity detected at a specific scattering angle is an
integration of the seattering from a11partic1es:
dmax
=
f a(e,d)q(d)dd.
(1)
The term a(9, d), calculated from either Mie theory or
Fraunhofer theory (as is the case in present practice), is
the unit volume scattering intensity from partic1es of
diameter d, detected by a unit detector area at angle 9.
This is the Fredholm integration of the first kind, with
a(8, d) termed the kemel function; the exact numerieal
solution of which is an ill-posed prablem [3]. Particle size
distribution is obtained using a feasible mathematical
technique to resolve q(d) from the measuredf(8) and the
computed a(8,d). Fig. 2 shows a partic1e size distribution
of com powder that was obtained from ' a 1 min
measurement in Beckman-Coulter LS230 laser diffraction
instrument.
microscopy
Phase Doppler anemometry
Electrical
2.
sieving
Focused beam reflectance
Optical
diffraction has become a popular and important physical
means for sizing industrial particles. Laser diffraction has
to a large extent replaeed conventional methods such as
sieving and sedimentation in the sizing of partic1es smaller
than a few millimeters, and has taken the place of optical
and electron mieroseopy for particles larger than some
tens of nanometers .
ice)
Wire cloth
81
analysis
analysis
Optical particle counling
Centrifugal sedimentation
Laser diffraclion
SEM
Acouslic spectroscopy
Image Analysis
pcs
Submicron aerosol
TEM
SEC
;!¿
o
1,000
>
1,000,00
d(nm)
Fig.1. Particle Sizing Technologies.
Among these technologies, laser diffraction has
advantages:
ease of use and fast operation;
reproducibility; and an extremely broad dynamic
range, spanning almost five orders of magnitude,
nanometers to millimeters. In the past two decades
these
high
size
from
laser
0.1
1
10
100
1000
d~m)
Fig.2. A Com Powder Size Distribution.
Advancements in laser diffraction technology have
resulted in significant impravements
in commercial
instruments, such as greater precision, increased ease of
82
Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2,2000
use, more versatile sample handling, and the capability for
on-line measurement. The popularity of this technique has
led to several national and international collaborative
studies f~ctiSSini o~ -the accufacy ofthe technology by
comparison to other particle characterization methods. For
spherical particles the results are very promising, in that
the mean size and cumulative distributions from different
models of laser diffraction instruments are close not only
to each other but also to the values obtained by other
methods and demonstrate excellent reproducibility. For
non-spherical
particles, the results from the same
instrument usually show only small standard deviations
between repeat measurements of the same preparations,
and between different preparations of the same sample.
For particles larger than 10 um, a modern commercial
instrument could easily achieve a relative standard
deviation of less than 3% for the median value in repeat
measurements, and a relati ve standard deviation of less
than 5% at the extremes of the distribution (the dIOand d90
points) [1].
In the latest development
of laser diffiaction
instrument, a laser diode of short wavelength (e.g., 430
nm) is used as the main light source, which is more
advantageous when measuring small particles. CCD
detectors are used in combination with silicon detectors to
capture the images of large particles; these images can be
Fourier transformed to generate diffraction pattems. Once
the scattering pattern is computed, it can be combined
with the scattering pattern of small particles acquired
using silicon detectors at large scattering angles to
produce a particle size distribution over a broad size
range.
All present commercial
instruments
employ an
assumption that particles are spherical due to both
theoretical availability and practical feasibility. For nonspherical samples, bias and errors exist due to the
spherical assumption. Since any bias is different for each
technology, if a spherical approximation is used for nonspherical particles, the variation in the size results
obtained by laser diffraction compared with that from
sedimentation, for example, will be different than those
obtained in a comparison with instruments employing the
Coulter Principle. Practically, when comparing results
from laser diffraction to results frorn another technology,
some scaling or weighting factors for size and/or density
distributions are often employed to artificially shift or
reweigh one or the other of the instrumenr's results, In
some cases, a correlation study may be needed to find the
factors and their variations as a function of particle size.
Measurement of some finite number of particles,
anywhere from one particle to less than some hundred
particles, leads to another way of shape determination for
regularly shaped particles, because for any non-spherical
particle there is a non-unifermity of scattered intensity
with azimuthal angle. The non-uniformity is clearly
shown in Fig.3 for a rectangular particle.
Fig.3. Scattering pattern (absolute electromagnetic field
distribution) of an oriented cube for the scattering angle e
from 0° to 7°.
For a cylinder, scattering would be very strong in the
plane orthogonal to its axis. Thus, by measuring the
azimuthal distribution of scattering from one or a few
particles, shape information can be obtained. In one such
application, a multi-element photodiode detector array is
used in combination with a neural-network
pattern
recognition system to monitor shape variation [1]. In this
case, airborne mineral fibers were distinguished from nonfibrous materials using the azimuthal scattering pattern [1,
2]. In another instrument, eight azimuthal angles at a
constant side scattering angle of 55° were used to detect
the signals from a steadily flowing particle stream passing
through the beam inside a spherical measuring chamber.
For non-spherical particles, the signals detected at the
eight detectors would be different. A "spherical index",
calculated from the standard deviation of the eight signals
per particle was defined. The spherical index is unity for
spheres and deviates from unity as the non-sphericity of
the particle increases [1].
3.
Image Analysis
All image analysis methods include an image capture
procedure and an image process and analysis procedure.
The image capture procedure can be accomplished using
continuous
illumination
or pulsed
light that is
synchronized with the image capture mechanism. The
particulate material flow falls between the light source and
a CCD device. Dry, pourable bulk materials, granules and
powders, as well as suspended particles in liquid can be
measured in real-time by using different sample handling
modules. Using ever-faster computers, interfacing with a
CCD device image frames (typically containing 5l2x512
or 1024x1024 pixels and 256 gray levels) can be captured,
transferred, stored, and analyzed at rates as fast as 30
frames per second. Irnage analysis software has advanced
so much during the past few years that, when connected
with an image inquiring system, it can provide a particle
size distribution as well as shape information based on
various algorithms at a speed of 20,000 particles per
secando Particle dimensional measurement (size, area, and
cord length), particle count, shape analysis, and even
R. Xu. /Revista Latinoamericana de Metalurgia y Materiales
fractal analysis can be accomplished by image analysis.
Image analysis can also be used to study the kinetics of
agglomeration in situ [1]. The dynamic size range of
particles that can be measured is determined by the optics
of the setup, dimension of the CCD, and paiticle selection
criteria in image analysis, typically from a few microns to
a few millimeters. The dynamic range can be increased
using a double-optics arrangement, one part having a short
focusing lens and the other having a long focusing lens.
The combination of the two images from both lenses can
provide an overall dynamic range of 1:1000 (e.g., frorn 30
um to 30 mm) in the same measurement. One has to be
cautious though when combining the two images, since
they are not acquired from the same area and usually ha ve
different resolution.
The major disadvantages of image analysis are 1) that
it only yields information from 2-D prajected areas of
particles,
which can vary depending
on particle
orientation, and 2) that only particles within the depth of
field are measured. If particles in the sample module are
not homogeneous, bias in particle size distribution is
unavoidable. The following picture shows an image that
contains more than 500 particles ranging from 10 um to
1700 um (a dynamic range of 1:170) obtained using the
Beckman Coulter RapidVue instrument which employs a
single set of optics. Fig.5 and Fig.6 are the analysis results
of 271 particles in FigA that are within the depth of field.
In a sample analysis, the particle size distribution is
obtained from analysis of a few hundred to a few thousand
frames of images that can total up to more than 100,000
particles.
,
"
'.
."
.
.
.'..
','
olI"
.
~ .... .
..
..
;
40
2
30
E
~ 20
10
100
1000
10000
Dlametertum)
The Particle number distribution
diameter of the particles in Fig. 4.
Fig.S.
3
of equivalent
circular
r----------------,
2.5
~
2
E
-;; 1.5
E
:::1
"5
>
1
0.5
o ~--------------------~---~
10
100
1000
10000
Diameter(/lm)
Fig.6. The particle volume distribution
diameters of the particles in FigA.
of equivalent
circular
4. References.
1.
! •
'.
~".
r-----------------,
e. .-
.'.
.
50
83
'"
.
.'.
;*
2.
.... .
.'
3.
.~
."
.
..'
,.
FigA. An image from Beckman Coulter Rapid- Vue
showing particles ranging from 10 um to 1700 um.
4.
5.
6.
Kaye, B. H., in Particle Size Analysis 1981, Eds .
Stanley-Wood, N., Allen, T., John Wiley and Sons,
New York, pp3, 1982.
R. Xu, "Particle size analysis using laser scattering",
Liquid and Surface Borne Particle Measurement
Handbook,
Eds. 1. Knapp, T. Barber and A.
Lieberman, Marcel Dekker, New York, Chpt18,
pp745, 1996.
R. Xu, "Improvernents in particle size analysis using
light
scattering ",
Particle
and
Surface
Characterisation Methods, Eds. R. H. Müller and W.
Mehnert, Medpharrn Scientific Publishers, Stuttgart,
Germany, Chpt 3, pp27, 1997,
A. N. Tikhonov and V. y. Arsenin, Solution of Illposed Problems, Winston, Washington D.C., 1977.
ISO 13320-1 Particle Size Analysis-Laser Diffraction
Methods. Part 1: General Principle, Intemational
Organization of Standardization, Genéve, 1999.
P. Kaye, E. Hirst and Z. Wang-Thomas, "Neuralnetwork-based spatial light-scattering instrument for
hazardous airbome fiber detection", App. Opt., 36,
6149-6156, 1997.
84
7.
Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000
H. Barthel, B. Sachweh and F. Ebert, "Measurement
of airbome mineral fibres using a new differential
light scattering device", Meas. Sci. Technol., 9, 206216, 1998.
8. 1. List and R. Weichert, "Detection of fibers by light
diffraction", in Preprints of Partec 98, th European
Symp. Parto Charact., Nümberg, pp.705-714, 1998.
9. W. D. Dick, P. H. McMurry and B. Sachweh,
"Distinguishing between spherical and non-spherical
particles by measuring the variability in azimuthal
light scattering", Aerosol Sci. Tech., 23, 373-391,
1995.
10. A. Blandin, A. Rivoire, D. Mangin, J. Klein, J.
Bossoutrot, "Using in situ image analysis to study the
kinetics of agglomeration in suspension ", Parto Parto
Sys. Charact., 17, 16,2000.