(258a) Particle Characterization - from Bulldog, to Eagle, to Dinosaur

Since Sherlock Holmes first plucked a hair from the coat of the suspect and showed it had come from the head of the victim, particle characterization has been widely performed. Even the general public has been made aware of the powers of characterization and has responded enthusiastically to such prime-time television shows as CSI, CSI Miami, and CSI New York.

Particle characterization began for me in 1953 when employed by the National Coal Board in England. There, I was introduced to coal particle sampling underground and on the mine surface; to sampling from screens on washeries that graded coal into size classes; to qualify control after milling; and even to the measurement of airborne dust in black lung investigations.

It was expanded widely during a six-year research contract beginning in 1967 at IIT Research Institute in Chicago. The work scope was to evaluate all known instrumentation on the market for the measurement of particle size distribution, shape distribution, density, surface area, and pore size distribution, and to advise the 32 industrial sponsors on what they should use for quality control. Visits were paid to each company four times in the six-year program and so the contract took me to 42 states. Consulting in particle characterization became my full-time job in 1975 and led to 23 years of employment with E. I. DuPont de Nemours and Company.

I have witnessed over 50 years of development and application of particle characterization techniques. This invited talk is the first of two I will give in 2006 and will focus on size, shape, density, surface, and pore size measurements. The second on bulk particle characterization will be presented at the 5th International Conference for Conveying and Handling of Particulate Solids in Sorrento, Italy, on August 27-31, 2006.

In England in the 1950's, my bulldog years, particle sizing was slow. Microscopy, sieving, and gravity sedimentation predominated. Data analysis was done via simple basic equations and specialty graph papers were necessary to compare distributions and to convert from population to mass. T. Allen used these to analyze powder mixtures and to show the modification of distributions by classification or other means. Some of this appears to have been lost by modern computing systems; though at first sight, data manipulation is simpler. What has been gained and what has been lost? This period saw the development of the Coulter Counter Model A, which began to transform size analysis.

Shape measurement in the bulldog days was archaic. Shape was only described by verbal descriptions such as sphere, cube, needle, and flake. 2-D microscopy was used and simple chord measurements were used to form elongation ratios and flakiness ratios, or area measurements such as in Hausner ratios. Three-dimensional shape measurements were prevalent in geological studies on very large particles but not with powders. Length, breadth, and thickness parameters were combined and displayed in triangular diagrams- some numerically, some pictorially. They found use in assessing stress and strain histories of rocks and pebbles.

Surface area was calculated using the BET equation from adsorption measurements on glass apparatus. Pore-size distributions were laboriously measured on cross-sections of large samples.

The 70's, 80's and 90's in the U.S.A.-the Eagle years-saw the emergence of centrifugal and optical techniques. In particular, optical devices dominated aerosol characterization entirely until the emergence of electrical charge methods. Optical systems dominated contamination control and early environmental measurements in the laboratory and in the atmosphere. Of most significance was the development of the computer and data processor. Calculations could be made instantly and made specialty graph papers and simple equations obsolete. In size analysis perhaps the biggest impact was made by laser diffraction techniques. These provide rapid and easy size-analysis data. However, I believe that to the uninitiated, they have conveyed that particle sizing is simple whereas it is not. Also, the mathematics are hidden in confidential algorithms so the basis of the generated numbers is not obvious.

Shape measurement saw the development of automatic microscopes and hard-wired shape measurement modules. It saw the emergence of Fourier descriptors and fractal dimensions. It later saw the complete transformation to software-driven shape analyzers; and again, this conveyed to the uninitiated that shape measurement is easy, whereas it is not. 2-dimensional and ?dimensional shapes have been recently depicted in complex mathematical models, which are used to suggest improvements in packing and shipping. However, despite these amazing developments, how often is shape incorporated into chemical and material processing design? So how useful have these developments really been? How much is hype and how much is practically useful?

Surface-area and pore-size measurement benefited from the development of better instrumentation that permitted the use of multiple gases and allowed low surface areas to be measured. However, little has changed in the basic mathematics; and here, unlike in sizing, the data can be correlated with know and visible equations.

However, despite all of the innovations, the fundamental questions still remain. At first glance, simple to answer; but the more you know the more unanswerable they become. What is a particle? What is size? What is shape? What is surface area? Bond-strength distributions dictate the state of the particle at all times, but methods are not available to completely measure them. The state of dispersion is critical to size and shape measurement, and knowledge is difficult to find that confidently predicts the particle state and its representation at the moment of measurement. What is known and what isn't?

Today, I am a dinosaur. Nanoscience and bioscience have further complicated particle characterization. What happens to measurement techniques when the size is only a few molecules? What does dispersion and stabilization mean at these levels? How does one characterize particles in the body, or bacteria, or viruses? After seven years of retirement, I have lost my technical edge. I am becoming extinct. It remains for others younger and with careers to build to focus on the problems at hand.

In summary, this talk gives my perspective on the development of characterization practices over the last half-century. It suggests what is needed and gives a road map for the future.