The properties of powders and disperse systems containing solid particles depend greatly on the geometry of the solid phase. There is as yet no universal method to describe the shape of powder particles. This, therefore, results in problems in associating the function of powders during processing and handling and their shape.
Microscopy has become very efficient with respect to the characterisation of particle size and shape since the provision of modern image analysis techniques. The shape factors available are based on the dimensions of two-dimensional particle outlines i.e. the shadows projected from the microscope onto the screen. Four characteristic features should be distinguished here: particle size, shape, roundness and “roughness”. All four features show a distribution function in the powder bulk. Hence they should be assessed as distributions rather than single values.
The simplest two-dimensional shape factors are derived from particle dimensions such as length and breadth only. Here, care has to be taken, because the individual image analysis systems define these dimensions differently, and hence, shape factors based on these dimensions are not always comparable. The most popular of these shape factors are Aspect Ratio and Elongation. These shape factors are unable to distinguish between different geometric figures such as circles and squares, or rectangles and ellipses. This is also valid for shape factors involving the projected area or perimeter of a particle outline, for example, Roundness and Circularity Shape Factor. These shape factors should only be used to evaluate the deviation from being round, and it is an ongoing misuse that these shape factors are employed to assess general particle shape. Shape factors based on inscribing and circumscribing circles are also not suitable for general particle shape analysis. However, there are a few shape factors, which combine particle dimensions, projected area and perimeter in a more sophisticated way (shape factor NS, surface and volume shape factors, One Plane Critical Stability). These appear useful to define the geometric shape of powder particles, and they can detect small variability in particle shape. Fractal dimensions define the degree of irregularity of the particle perimeter, but they do not describe surface roughness in a physical sense. Also, they are extremely insensitive to changes in particle shape, and hence their general use in particle shape analysis is questionable. Fourier analysis can provide a particle or a powder signature, but to process such data requires more mathematical effort, and the comparison between sets of data can only be performed using multivariate statistical methods such as Principal Component or Cluster Analysis, or the use of Neural Networks.
Other methods to detect and describe particle shape are based on laser light scattering. In many industries, particle size analysis is performed with laser light scattering techniques, and hence, it would be very useful and time saving, if the assessment of particle shape and shape distribution could be performed simultaneous with the particle size analysis. However, here more fundamental research is required, and the collaboration with equipment manufacturers is needed.
The choice of a suitable shape factor is crucial, when the influence of particle shape on powder behaviour, processing and handling has to be described. For example, the use of pellet shaped particles relies on their sphericity, and a shape factor characterising the roundness of the pellets is required. In composites, fibrous particles are used to enhance the mechanical strength and a shape factor describing roundness would be misplaced. Instead, the shape factor should quantify the elongation of the particles. When predicting the re-suspension properties of powder inhalations, a distinction between spherical, regular shaped or fibrous particles is required. Particles in a powder bulk or a powder mixture may comprise various particle shapes, and the distribution of shape influences, for example, powder flow, packing and compaction. Hence, the shape factor should be able to distinguish between geometric figures, and it should quantify small variability in particle shape. Particles conveyed on chutes should be characterised with ‘One Plane Critical Stability’ to identify conveyer problems related to particle shape.
There is controversy in the literature data relating powder handling properties to particle shape. However, here dubious results often arise due to other uncontrolled influence factors, first of all particle size and size distributions, but also other particulate properties such as true surface roughness, surface free energy, hardness or elasticity. Also, process parameters are often randomly varied in one study, and there is generally no consistency in test criteria and process parameters between individual studies.
The main areas, in which relationships between particle shape and powder properties should be sought, are fracture mechanics, powder compaction and compact properties, particle re-suspension and powder mixing. Packing and powder flow have been studied rather extensively in the past. There are inconsistencies in the relationships reported, but it seems unlikely that a more concise concept can be developed in the near future. This is mainly due to a lack of good, reproducible and controlled methods to study powder flow, and the knowledge about the role of adhesion and friction forces during powder packing appears to have got lost, leading to general use of inappropriate test equipment.
One major problem in testing the influence of particle shape on powder properties is the separation of different particle shapes in one powder bulk. While shape sorting machines are readily available for larger particles, there is no suitable shape sorting method known to date, that can sort particle shapes in powders of particle size below about 60 μm.