Homogeneity and dynamics of particle packing structures

The particle packing structure is the arrangement of particles within a material. It plays a crucial role in determining the mechanical properties of a material, including its elastic modulus. The elastic modulus is a measure of a material’s stiffness, and it is an important parameter in predicting the material’s behavior under mechanical stress. In the case of wet powder compacts, the particle packing structure is particularly important because it affects the density and porosity of the material. The density of the material is related to the packing fraction, which is the ratio of the volume of the particles to the total volume of the material. The porosity, on the other hand, is related to the void fraction, which is the ratio of the volume of the voids to the total volume of the material. A compact with a high packing fraction and low void fraction will have a high density and a low porosity, resulting in a high elastic modulus. Conversely, a compact with a low packing fraction and high void fraction will have a low density and a high porosity, resulting in a low elastic modulus. The particle packing structure can be controlled through various methods such as changing the particle size, particle shape, particle size distribution, and the amount of binder used in the compaction process. Understanding the relationship between particle packing structure and elastic modulus can help in designing and optimizing wet powder compacts for various applications, including pharmaceuticals, ceramics, and metallurgy. However, analyzing the properties of such powder systems is intricate and challenging as they are not solely influenced by the physical attributes of the particles but also by their composition and organization, including the distribution of gas and liquid phases. The elastic modulus is one of the most fundamental mechanical characteristics of powder compacts in this context, as well as a significant indicator of goods produced by granulation and compaction procedures. It is well known that internal stresses and particle friction, both of which are significantly influenced by the presence of water, have an impact on the elastic modulus of powder bodies.

Conventional methods, such as coordination number and packing density, are not sufficient to fully capture the connection and characteristics of the structural information because they extract only localized or averaged information. Furthermore, as saturation decreases to funicular and pendular regions, the packing structure of wet powders is not homogeneous and water distribution is uneven. Due to the increased variety of local particle-particle interaction forces, it is challenging to extrapolate the microscopic interparticle forces to the entire packing structure. Therefore, it is still unclear how the three-dimensional packing structure affects the mechanical characteristics of wet powder compacts, and conventional methods cannot infer these characteristics from powder properties.

In a recent study published in the peer-reviewed Journal of Advanced Powder Technology, Assistant Professor Shingo Ishihara and Professor Junya Kano from Tohoku University in Japan together with Professor George Franks from the University of Melbourne in Australia addressed the problem of the need for a more accurate and efficient method for analyzing the structural properties of wet powder compacts and predicting their elastic modulus. The authors proposed the use of persistent homology, a new topological data analysis method, to analyze the packing structure of wet powders and develop an empirical equation for predicting their elastic modulus based on this structure.

The research team prepared and tested an aqueous suspension of high purity alpha-alumina powder with different particle sizes. The compacts were saturated, and a uniaxial compression test was used to determine the elastic modulus. They found that for each volume ratio, the elastic modulus decreased as saturation increased. Additionally, there was a non-linear correlation between the elastic modulus and the volume ratio. The bulk density of the compacts, however, did not correlate with the elastic modulus. The size and shape of the particles in the packing structures could be explained by the cavity creation and distribution in the packing structures at various volume ratios, which were revealed by the PD analysis.

The researchers developed an equation to accurately predict the elastic modulus of a wet powder compact from its packing structure information. The equation was based on the concept of structural homogeneity, specific surface area, surface tension, and void fraction. The equation assumed that the saturated elastic modulus (Es) was constant, and the elastic modulus was a linear function of saturation (S) with the influence of saturation on the slope of the equation determined by surface tension, porosity, equivalent specific surface diameter, and index H. The authors validated their equation by comparing the predicted elastic modulus with experimental results and found that the predicted values agreed well with the experimental values. The authors also investigated the effect of saturation on the intercept and slope of the elastic modulus and found that the intercept decreased with increasing bulk density, while the slope decreased with increasing saturation. The authors reported the coefficients of the linear approximation of the relationship between elastic modulus and saturation at each volume ratio.

The new findings of Shingo Ishihara and colleagues could have significant implications in various industries that use wet powders, such as pharmaceuticals and ceramics. It had the potential to result in enhanced product design and production procedures by offering a more precise and effective way for analyzing the structural properties of these materials. Additionally, the new empirical equation could be used to tailor the mechanical characteristics of wet powders for particular uses, creating more useful and effective products across a variety of industries.