Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000, 80-84

PARTICLE SIZE AND SHAPE ANALYSIS USING LASERSCATTERING AND IMAGE ANALYSIS.

R.XuParticle Characterization, Beckman Coulter

Miami, FL, USA

Abstraet

Characterization of particle size and shape has become an indispensable tool in many industrial processes includingin production and processing of granular materials. During the past decade, due to the evolution of other moderntechnologies, e.g., lasers, computers and automation, the methods used in particle characterization have changedconsiderably. In sizing granular materials centimeters or smaller, one of the most widely employed technology is laserdiffraction. Several conventional particle characterization methods, such as sieve analysis or sedimentation analysis,have gradually been replaced by laser diffraction and other methods based on light-matter interactions.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 twoyears, image analysis utilizing light-matter interaction combined with charge-coupled devices (CCD's) has advancedrapidly, providing another powerful tool for size and shape characterization of granular materials. New applications thatuse these non-invasive methods to characterize various particulate systems appear daily. Many new national andinternational standards related to these technologies have been and are still being established. In this article, a review ofcontemporary particle sizing technologies is provided with emphasis on using laser diffraction and CCD' s tocharacterize granular materials and the latest commercial development, and applications of these technologies aresummarized.

ResumenLa 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 laevolución de modernas tecnologías, como por ejemplo, lasers, computadoras y automatización, los métodos utilizadosen la caracterización de partículas han cambiado considerablemente. Para determinar el tamaño de materialesparticulados alrededor del centímetro o mas pequeños, una de las tecnologías más utilizada es la difracción laser. Variosmétodos de convencionales de caracterización de partículas, tales como tamizado o sedimentación, han sidogradualmente reemplazados por difarcción laser y otros métodos basados en la interacción de la luz con la materia. Enlos últimos dos años, análisis de imágenes utilizando interacción de la luz con la materia combinado con equipos decarga (CCD's) ha avanzado rápidamente, dando lugar a otra herramienta poderosa para la caracterización por tamaño yfoma de partícula de materiales granulados. Nuevas aplicaciones que usan estos métodos no invasivos aparecendiariamente para caracterizar sistemas particulados. Nuevos estandares nacionales e internacionales relacionados conestas tecnologías han sido y estan siendo establecidos todavía. En este artículo, se presenta un repaso de las técnologíascontemporáneas para determinación de tamaño de partícula con enfasis en el uso de la difracción laser y CCD's paracaracterizar 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 thathave been developed and successfully employed in sizingparticles from nanometers to millimeters. There weremore than 400 different methods applied in themeasurement of particle size, shape and surface area in1981 [1]. Prior to the adoption of light-based and othermodern technologies, most partic1e sizing methods reliedon either the physical separation of a sample, such as insieve analysis, or the analysis of a limited number ofpartic1es, as in the microscopic method. The results fromseparation methods consist of ensemble averages ofproperty of each fraetion, and the resuIts frommicroscopic methods pro vide only two-dimensional sizeinformation from the limited number of particlesexamined. During the last two decades, because of thecreation and commercialization of lasers andmicroelectronics (inc1uding. computers), the science andtechnology of particle sízing has evolved considerably,and technologies for particle shape analysis are emerging.Many new and sophistieated methods have beensuccessfully developed and applied and some previouslypopular characterization practices are now being phasedout in many fields. The following table lists commonsizing methods currently in use, arranged according to theapplieable sizing range of each technology.

Wire cloth

Electroformed sieving

Focused beam reflectanceOptical microscopy

Phase Doppler anemometry

Electrical sensing zoneGravitational sedimentation

Holography

Time-of-ílight analysis

Electroacouslic analysisOptical particle counling

Centrifugal sedimentation

Laser diffraclion

SEM

Acouslic spectroscopy

Image Analysis

pcsSubmicron aerosol

TEM

SEC

1,000 1,000,00d(nm)

Fig.1. Particle Sizing Technologies.

Among these technologies, laser diffraction has theseadvantages: ease of use and fast operation; highreproducibility; and an extremely broad dynamic sizerange, spanning almost five orders of magnitude, fromnanometers to millimeters. In the past two decades laser

81

diffraction has become a popular and important physicalmeans for sizing industrial particles. Laser diffraction hasto a large extent replaeed conventional methods such assieving and sedimentation in the sizing of partic1es smallerthan a few millimeters, and has taken the place of opticaland electron mieroseopy for particles larger than sometens of nanometers .

2. Laser Diffraction

In laser diffraction measurements one obtainsinformation about partic1e size distributions through themeasurement of scattering intensity as a function ofseattering angle and the wavelength of light, based onapplicable scattering models. This is an absolute methodin the sense that once the experimental arrangement orinstrument is correctly set up, ealibration or scaling is notnecessary [1,2]. For monodisperse particles there is a one-to-one relationship between the scattering intensity andthe scattering angle for any particular size. Forpolydisperse samples, each individual particle willcontribute its own unique seattering signal to thecomposite angular scattering pattem differentia11y. Theintensity detected at a specific scattering angle is anintegration of the seattering from a11partic1es:

dmax

ice) = f a(e,d)q(d)dd. (1)

The term a(9, d), calculated from either Mie theory orFraunhofer theory (as is the case in present practice), isthe unit volume scattering intensity from partic1es ofdiameter d, detected by a unit detector area at angle 9.This is the Fredholm integration of the first kind, witha(8, d) termed the kemel function; the exact numeriealsolution of which is an ill-posed prablem [3]. Particle sizedistribution is obtained using a feasible mathematicaltechnique to resolve q(d) from the measuredf(8) and thecomputed a(8,d). Fig. 2 shows a partic1e size distributionof com powder that was obtained from ' a 1 minmeasurement in Beckman-Coulter LS230 laser diffractioninstrument.

;!¿o>

0.1 1 10 100 1000d~m)

Fig.2. A Com Powder Size Distribution.Advancements in laser diffraction technology have

resulted in significant impravements in commercialinstruments, 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 foron-line measurement. The popularity of this technique hasled to several national and international collaborativestudies f~ctiSSini o~ -the accufacy ofthe technology bycomparison to other particle characterization methods. Forspherical particles the results are very promising, in thatthe mean size and cumulative distributions from differentmodels of laser diffraction instruments are close not onlyto each other but also to the values obtained by othermethods and demonstrate excellent reproducibility. Fornon-spherical particles, the results from the sameinstrument usually show only small standard deviationsbetween repeat measurements of the same preparations,and between different preparations of the same sample.For particles larger than 10 um, a modern commercialinstrument could easily achieve a relative standarddeviation of less than 3% for the median value in repeatmeasurements, and a relati ve standard deviation of lessthan 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., 430nm) is used as the main light source, which is moreadvantageous when measuring small particles. CCDdetectors are used in combination with silicon detectors tocapture the images of large particles; these images can beFourier transformed to generate diffraction pattems. Oncethe scattering pattern is computed, it can be combinedwith the scattering pattern of small particles acquiredusing silicon detectors at large scattering angles toproduce a particle size distribution over a broad sizerange.

All present commercial instruments employ anassumption that particles are spherical due to boththeoretical availability and practical feasibility. For non-spherical samples, bias and errors exist due to thespherical assumption. Since any bias is different for eachtechnology, if a spherical approximation is used for non-spherical particles, the variation in the size resultsobtained by laser diffraction compared with that fromsedimentation, for example, will be different than thoseobtained in a comparison with instruments employing theCoulter Principle. Practically, when comparing resultsfrom laser diffraction to results frorn another technology,some scaling or weighting factors for size and/or densitydistributions are often employed to artificially shift orreweigh one or the other of the instrumenr's results, Insome cases, a correlation study may be needed to find thefactors and their variations as a function of particle size.

Measurement of some finite number of particles,anywhere from one particle to less than some hundredparticles, leads to another way of shape determination forregularly shaped particles, because for any non-sphericalparticle there is a non-unifermity of scattered intensitywith azimuthal angle. The non-uniformity is clearlyshown in Fig.3 for a rectangular particle.

Fig.3. Scattering pattern (absolute electromagnetic fielddistribution) of an oriented cube for the scattering angle efrom 0° to 7°.

For a cylinder, scattering would be very strong in theplane orthogonal to its axis. Thus, by measuring theazimuthal distribution of scattering from one or a fewparticles, shape information can be obtained. In one suchapplication, a multi-element photodiode detector array isused in combination with a neural-network patternrecognition system to monitor shape variation [1]. In thiscase, airborne mineral fibers were distinguished from non-fibrous materials using the azimuthal scattering pattern [1,2]. In another instrument, eight azimuthal angles at aconstant side scattering angle of 55° were used to detectthe signals from a steadily flowing particle stream passingthrough the beam inside a spherical measuring chamber.For non-spherical particles, the signals detected at theeight detectors would be different. A "spherical index",calculated from the standard deviation of the eight signalsper particle was defined. The spherical index is unity forspheres and deviates from unity as the non-sphericity ofthe particle increases [1].

3. Image Analysis

All image analysis methods include an image captureprocedure and an image process and analysis procedure.The image capture procedure can be accomplished usingcontinuous illumination or pulsed light that issynchronized with the image capture mechanism. Theparticulate material flow falls between the light source anda CCD device. Dry, pourable bulk materials, granules andpowders, as well as suspended particles in liquid can bemeasured in real-time by using different sample handlingmodules. Using ever-faster computers, interfacing with aCCD device image frames (typically containing 5l2x512or 1024x1024 pixels and 256 gray levels) can be captured,transferred, stored, and analyzed at rates as fast as 30frames per second. Irnage analysis software has advancedso much during the past few years that, when connectedwith an image inquiring system, it can provide a particlesize distribution as well as shape information based onvarious algorithms at a speed of 20,000 particles persecando Particle dimensional measurement (size, area, andcord 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 ofagglomeration in situ [1]. The dynamic size range ofparticles that can be measured is determined by the opticsof the setup, dimension of the CCD, and paiticle selectioncriteria in image analysis, typically from a few microns toa few millimeters. The dynamic range can be increasedusing a double-optics arrangement, one part having a shortfocusing lens and the other having a long focusing lens.The combination of the two images from both lenses canprovide an overall dynamic range of 1:1000 (e.g., frorn 30um to 30 mm) in the same measurement. One has to becautious though when combining the two images, sincethey are not acquired from the same area and usually ha vedifferent resolution.

The major disadvantages of image analysis are 1) thatit only yields information from 2-D prajected areas ofparticles, which can vary depending on particleorientation, and 2) that only particles within the depth offield are measured. If particles in the sample module arenot homogeneous, bias in particle size distribution isunavoidable. The following picture shows an image thatcontains more than 500 particles ranging from 10 um to1700 um (a dynamic range of 1:170) obtained using theBeckman Coulter RapidVue instrument which employs asingle set of optics. Fig.5 and Fig.6 are the analysis resultsof 271 particles in FigA that are within the depth of field.In a sample analysis, the particle size distribution isobtained from analysis of a few hundred to a few thousandframes of images that can total up to more than 100,000particles.

, e. .-

."".! •

'..'. ;*'.

.'..~".

',' .olI" '" .'. .....

~ .... . .~.. .; . . "

. . ..' .',...FigA. An image from Beckman Coulter Rapid- Vueshowing particles ranging from 10 um to 1700 um.

83

50 r-----------------,40

2 30E~ 20

10

100 1000 10000

Dlametertum)

Fig.S. The Particle number distribution of equivalent circulardiameter of the particles in Fig. 4.

3 r----------------,2.5

~ 2E-;; 1.5E:::1"5 1>

0.5

o ~--------------------~---~10 100 1000

Diameter(/lm)10000

Fig.6. The particle volume distribution of equivalent circulardiameters of the particles in FigA.

4. References.

1. Kaye, B. H., in Particle Size Analysis 1981, Eds .Stanley-Wood, N., Allen, T., John Wiley and Sons,New York, pp3, 1982.

2. R. Xu, "Particle size analysis using laser scattering",Liquid and Surface Borne Particle MeasurementHandbook, Eds. 1. Knapp, T. Barber and A.Lieberman, Marcel Dekker, New York, Chpt18,pp745, 1996.

3. R. Xu, "Improvernents in particle size analysis usinglight scattering ", Particle and SurfaceCharacterisation Methods, Eds. R. H. Müller and W.Mehnert, Medpharrn Scientific Publishers, Stuttgart,Germany, Chpt 3, pp27, 1997,

4. A. N. Tikhonov and V. y. Arsenin, Solution of Ill-posed Problems, Winston, Washington D.C., 1977.

5. ISO 13320-1 Particle Size Analysis-Laser DiffractionMethods. Part 1: General Principle, IntemationalOrganization of Standardization, Genéve, 1999.

6. P. Kaye, E. Hirst and Z. Wang-Thomas, "Neural-network-based spatial light-scattering instrument forhazardous airbome fiber detection", App. Opt., 36,6149-6156, 1997.

84 Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000

7. H. Barthel, B. Sachweh and F. Ebert, "Measurementof airbome mineral fibres using a new differentiallight scattering device", Meas. Sci. Technol., 9, 206-216, 1998.

8. 1. List and R. Weichert, "Detection of fibers by lightdiffraction", in Preprints of Partec 98, th EuropeanSymp. Parto Charact., Nümberg, pp.705-714, 1998.

9. W. D. Dick, P. H. McMurry and B. Sachweh,"Distinguishing between spherical and non-sphericalparticles by measuring the variability in azimuthallight 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 thekinetics of agglomeration in suspension ", Parto PartoSys. Charact., 17, 16,2000.