Executive Summary
Non-invasivness is an ideal trait for any measurement technique. Unfortunately, very few materials characterization methods are truly non-invasive. For the purpose of this review, a non- invasive method is one which does not change the sample under study in any way. Techniques which require the insertion of a physical probe have been excluded and, while some methods which are widely used for on-line measurement have been included, on-line applications have not been emphasized. In the context of particle characterization, non-invasive measurements take on added significance because some of the most important and interesting samples occur in environments or at concentrations which are difficult to study without sample removal, dilution, drying, coating or other conditioning, all of which can alter critical inter-particle relationships.
This review examines a group of techniques which meet the criteria for non-invasiveness and have experienced significant development and/or commercialization within the last 5-10 years. Therefore, a few well-known techniques, such as classical static light scattering, which is quite non- invasive and very widely used in dilute suspensions, have been left out. The goal, rather, has been to examine newer techniques which are only recently being applied to particle characterization. These include several optical methods including some conventional and holographic imaging techniques, confocal microscopy and dynamic light scattering (with and without fiber optics). Tomographic techniques have been limited to optical and optical coherence tomography and x-ray tomography. Also included are environmental scanning electron microscopy, stray-field nuclear magnetic resonance imaging and ultrasonic spectroscopy.
These methods are bringing a whole new level of information about particulate systems, especially those that provide complete, microscopic 3-dimensional imaging. The ability to follow the detailed physical behavior of an ensemble of particles with single particle resolution opens up remarkable opportunities for understanding inter-particle behavior and the dynamics of heterogeneous particle systems. Where possible, examples of recent applications and instrumental developments are given.