Powders, Volume 4, Issue 2 (June 2025) – 8 articles

The drying structures of droplets of colloidal aqueous dispersions exhibit a wide variety of patterns depending on experimental conditions. It has been established by previous researchers that capillary flows and Marangoni convection significantly influence the macroscopic pattern formation. To the best of our knowledge, this study is the first to focus on sessile droplets of aqueous dispersions containing hollow particles. These hollow particles have a lower density than water and thus float in the medium. The drying pattern of these droplets was markedly different from the well-known ring pattern. Instead, a bump-shaped structure—often referred to as a “coffee-eye”—was formed due to the accumulation of particles at the center of the dried film. While a ring pattern was still present, it was extremely narrow and barely noticeable. This behavior is attributed to the dominance of the buoyant motion of the hollow particles, which prevented their transport by capillary flow. The findings of this study provide fundamental and important insights into the drying structures of various types of colloidal droplets. Full article

The drying process of microcrystalline cellulose and adipic acid particles in a cylindrical fluidized bed was investigated using the Gaussian spectral technique to monitor fluid–dynamic regime transitions associated with surface moisture loss. Pressure fluctuation signals were recorded and analyzed to assess hydrodynamic behavior. Excess moisture significantly alters the bubbling characteristics of the bed, leading to instability in the fluidization regime. The results demonstrated that the Gaussian spectral technique effectively captured these hydrodynamic changes, particularly at the critical moisture content threshold, when compared with the drying rate curves of the materials. For microcrystalline cellulose and adipic acid particles, it is reasonable to conclude that a mean central frequency above 5.75–6.0 Hz and a standard deviation exceeding 3.7–3.8 Hz correspond to a bubbling regime, indicating that the critical drying point has been reached. This approach provides a non-intrusive and sensitive method for identifying transitions in the drying process, offering a valuable tool for real-time monitoring and control. The ability to track fluidization regime changes with high precision reinforces the potential of this technique for optimizing drying operations in the pharmaceutical, food, and chemical industries. Full article

Characterizing powder feedstock is crucial for ensuring the quality and reliability of parts produced through metal additive manufacturing (AM). The morphology of particles impacts the flowability, packing density, and spreadability of powders, affecting productivity and part quality. A new methodology has been developed to classify particle morphological features in AM powder feedstocks, such as spherical or elongated shapes, and the presence of satellites and facets. This approach uses multiple descriptors for quantitative evaluation. The results from shape descriptors can vary based on image resolution, gray/color thresholding, and software algorithms. There are various commercial systems available for characterizing particle shape, some of which use images taken of static particles, while others use images of particles in motion. This diversity can lead to differences in powder characterization across laboratories with different equipment and methods. This paper compares results from a particle classification approach using two software programs that work with metallographic images with those from an automated static particle analyzer. While traditional methods offer higher resolution and precision, this study shows that automated systems can achieve similar particle shape classification using different shape descriptors and thresholds. Full article

Plant by-products are undervalued as they are an important source of nutrients and bioactive compounds with potential health benefits, which also contribute to aroma and color. Therefore, their use in human food is a challenging field of study that deserves to be explored. This study proposes the conversion of fava bean pods into a powdered product as a high-quality, stable, and easy-to-handle food ingredient, thus contributing to the sustainability of the food industry within the framework of the circular economy. The powdered product was obtained by freeze-drying and grinding. As the particle size of powders is a determinant of their quality and functionality, some properties of two bean pod powder samples with mean particle sizes of 102.9 and 45.3 μm, obtained by sieving at 200 and 45 μm, respectively, are compared. The results obtained indicate good flowability of both powders. However, the sample with the largest size showed, in addition to a greener tone, lower interparticle porosity related to a better packing capacity, lower hygroscopicity, and much better wettability, along with its higher swelling capacity and water and oil retention capacity. Nevertheless, in this case, the extraction of proteins and phenols decreased by 18% and 25%, respectively, without compromising the total fiber content. Considering the use of fava bean pod powder as a versatile food ingredient, the largest size of those studied, 102.9 μm, is recommended. Only if the objective is to obtain a healthy food supplement would it be more desirable to grind it to a smaller particle size. Full article

Simulative and experimental studies were carried out to address multi-dimensional particle fractionation of non-biological particles according to size, shape, and density inside a high-throughput DLD array. Density sensitive separation was achieved for melamine and polystyrene particles at a diameter of 5 µm at a Reynolds number (Re) of 82, corresponding to an overall flow rate of 11.3 mL/min. This process is very sensitive, as no fractionation occurred for Re = 85 (11.7 mL/min). For the first time, the fractionation of elliptical polystyrene particles (5 × 10 µm) at Re > 1 was investigated up to Re = 80 (11 mL/min). A separation of elliptical particles from spherical melamine particles (5 µm) was observed in single experiments at all investigated Reynolds numbers. However, the separation is not reliably repeatable due to partial clogging of ellipsoidal particles along the posts. In addition, higher concentrations of polydisperse silica suspensions were experimentally investigated by using polydisperse silica particles at concentrations up to 0.4% (m/V) up to Re = 80 (20 mL/min). The separation size generally decreased with increasing Reynolds number and increased with increasing concentration. Separation efficiency decreased with increasing concentration, independent of the Reynolds number. In order to investigate the material-dependent separation in a contactless dielectrophoresis system (cDEP), the resolved CFD-DEM software was extended to calculate dielectrophoretic forces on particles. With this, the second stage of a serial-combined DLD-DEP system was simulated, showing good separation at lower flow rates. For these systems, different fabrication methods to minimize the distance between the electrodes and the fluid as well as the requirement to withstand high-throughput applications, were investigated. Full article

This collection of studies, conducted within the framework of the DFG-funded Priority Program SPP 2045, explores the role of X-ray tomography in advancing the multidimensional characterization of particulate systems, with a strong focus on enhancing 3D particle-discrete data quality. It critically assesses the limitations of traditional particle characterization methods, particularly those reliant on imaging techniques, and demonstrates how advanced methodologies can overcome these constraints by providing highly detailed and accurate geometric and structural 3D data. The research further introduces innovative sample preparation techniques for particle collectives, aiming to reduce post-processing efforts in image analysis. Additionally, the development of a particle database, aligned with FAIR data principles (Findable, Accessible, Interoperable, and Reusable), supports data sharing and collaborative research. Ultimately, this collection underscores the transformative potential of 3D particle-discrete datasets acquired through X-ray tomography in advancing particle technology and improving particle system analyses across diverse scientific and industrial fields. Full article

To obtain a more comprehensive understanding of the specific properties of complex-shaped technical aerosols—such as partially sintered aggregates formed in combustion processes or structured particles resulting from complex synthesis processes—it is essential to measure more than a single equivalent size. This study examines a novel method for determining a two-dimensional distribution of two distinct particle properties within the size range from $50\mathrm{nm}$ to $1000\mathrm{nm}$: the Centrifugal Differential Mobility Analyzer (CDMA). The CDMA enables the simultaneous measurement of both mobility and Stokes equivalent diameters, providing a detailed two-dimensional particle property distribution. This, in turn, allows for the extraction of shape-related information, which is essential for characterizing particles in terms of their chemical composition, reactivity, and other physicochemical properties. This paper presents a detailed evaluation of a first CDMA prototype. First, CFD simulations of the flow field within the classifier are presented in order to assess and understand non-idealities arising from the exact geometry. Subsequently, the transfer function is evaluated by particle trajectory calculations based on the simulated flow field. It can be demonstrated that the simulated transfer functions agree quite well with transfer functions derived from streamlines of an ideal flow field, indicating that the non-idealities in the classifying region are almost negligible in their effect on the classification result. An experimental determination of the transfer function shows additional effects not covered by the previous simulations, like broadening by diffusion and losses due to diffusion and precipitation within the in- and outlet of the classifier. Finally, the determined transfer functions are used to determine the full two-dimensional distribution with regard to the mobility and Stokes equivalent diameter of real aerosols, like spherical particles and aggregates at different sintering stages, respectively. Full article

This study aims to investigate the suitability of earthen housing for refugees and establish a more efficient system for selecting and adjusting materials by quantitatively analyzing the influence of various factors affecting the mechanical performance of earthen housing. This paper examined the impact of dry unit weight, particle size distribution of soil solidification, clay minerals, and pH on the mechanical performance of soil solidification through compressive strength testing. Additionally, the tensile strength resulting from capillary forces between particles was estimated using a prediction equation, and the effects of capillary forces and cohesive forces due to clay minerals were quantitatively assessed by comparing them with the results of the compressive strength experiments. In an alkaline environment, conflicting results were obtained regarding the effect of the dispersion of particles on compressive strength, depending on the presence or absence of clay minerals in the soil. Based on these results, the implementation of earthen houses constructed in the target refugee camp was evaluated, and strategies for building houses more efficiently under given conditions were discussed. Full article