Chocolate milk, paint and snow globes are all examples of solid-liquid suspensions found in daily life. In industry, colloids are also commonly used to process solids, as liquids are typically easier to handle than powders. Given the environmental concerns surrounding the usage of organic solvents, which are often evaporated and released during the process, water-based colloids are preferred. However, not all particles have a natural affinity with water. When particles have insufficient affinity for water, the solid and liquid phases segregate. To enable such particles to still form stable water-based suspensions, their surface properties can be modified....@en

Conventional fluidization of cohesive powders is challenging due to their strong interparticle forces, often requiring assistance methods. In this study, the hydrodynamics of pulsed and vibrated beds of cohesive Geldart C silica powder (Sauter mean diameter d₃₂=7.9μm) in a 19.2cm diameter column were investigated using X-ray imaging. The results show that low-frequency, moderate-amplitude gas pulsation improves fluidization by disrupting long, persistent gas channels. Higher-frequency pulsation is dampened throughout the bed, resulting in negligible improvement over unassisted systems. When coupled with mechanical vibration, gas pulsation slightly mitigates solid compaction at the bottom section, but the overall flow pattern remains largely unchanged compared to vibration alone. The findings highlight the potential of integrating gas pulsation with other assistance methods to enhance fluidization in practical applications.

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The fractal structure of aggregates consisting of primary nanoparticles naturally arises during their synthesis. While typically considered to be a fully stochastic process, we suspect long-range interactions, in particular van der Waals forces, to induce an active pull on particles, altering the clustering process. Using an off-grid 3D model, we show that an active pull decreases the density and fractal dimension of formed clusters. These findings could not be reproduced by 2D models, which underestimate screening effects. Additionally, we determined the range within which van der Waals forces dominate the aggregation process.@en

In this study, we investigated the wettability and agglomeration characteristics of polymer microspheres coated with low-temperature deposited SiO₂ in a fluidized bed atomic layer deposition (ALD) setup. Surface characterization revealed the presence of a significant amount of deposited Si-OH groups within the first cycles. A drastic decrease in agglomerate size, water contact angle (WCA), and droplet absorption time of the powder was observed when coating was applied. Furthermore, we observed an increase in the amount of Si-OH present on the particle surface with increasing coating cycles, while no significant improvement in water affinity was found after the first coating cycles. Our findings suggest that surface coverage is the primary factor in improving the colloid stability of particles, coated at low temperatures. The low temperature operation of our system introduced a chemical vapor deposition (CVD) component to our coating process, which allowed full surface coverage to be achieved within the first two coating cycles.

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Vibro-assisted fluidization of cohesive micro-silica has been studied by means of X-ray imaging, pressure drop measurements, and off-line determination of the agglomerate size. Pressure drop and bed height development could be explained by observable phenomena taking place in the bed; slugging, channeling, fluidization or densification. It was observed that channeling is the main cause of poor fluidization of the micro-silica, resulting in poor gas-solid contact and little internal mixing. Improvement in fluidization upon starting the mechanical vibration was achieved by disrupting the channels. Agglomerate sizes were found to not significantly change during experiments.

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Stirring has been recognized in the literature as a promising technique for facilitating fluidization of cohesive powders, via inputting additional energy to counteract interparticle forces. However, the influence of operating conditions and stirrer configurations on flow behavior remains largely unknown, which impedes the practical implementation of stirred fluidization. Utilizing X-ray imaging, this research demonstrates that stirring enhances fluidization in cohesive micron-silica powder (Sauter mean diameter [Formula presented]) by collapsing the powder packing structure, and transitioning channeling beds into bubbling states. Comb-like configurations featuring fewer stirrers and blades, placed in the bottom region, have shown to be highly effective. Excessive stirring can lead to air pockets and a compacted phase of particles on the column walls, undermining the interaction between particles and stirrers. Additionally, the experiments show that maximizing the sweeping coverage, employing complex asymmetrical configurations, and avoiding tortuous gas pathways are preferable.

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In this study, we investigate the correlation between fluidization behavior and flow properties of 10 commercially available cohesive powders through fluidization and rotating drum experiments. The rotating drum was operated at high speeds to aerate the powder, creating flow fields and stress conditions comparable to those in gas–solid fluidized beds. We introduce a fluidization quality index (FQI) computed from X-ray imaging, which addresses the limitations of conventional analyses for assessing fluidization quality of cohesive powders. By analyzing flow patterns, the FQI integrates gas holdup and its temporal variation, providing a reliable measure of fluidization quality. The findings establish a positive correlation between flowability and fluidization quality, demonstrating how flowability measurements can predict the effectiveness of mechanical vibration in enhancing fluidization characteristics. These results suggest that fast, user-friendly flowability assessments in a rotating drum can effectively predict fluidization potential, contributing to process optimization and advancing fluidization studies for cohesive powders.@en

In this study, the impact of different vibrational modes on the fluidization characteristics of cohesive micro- and nano-silica powder was examined. Fractional pressure drop, bed expansion measurements, and X-ray imaging were utilized to characterize the fluidization quality. The densities of the emulsion phase at the top and bottom of the column were quantified and compared, providing insights into the solid distribution within the fluidized bed. In the absence of vibration, neither powder could be fluidized within the considered range of superficial gas velocities. Vertical vibration was found to initiate fluidization for both powders. In contrast, elliptical vibration failed to overcome the channelling behavior when fluidizing the micro-powder. For nano-powder, combined channelling and powder compaction occurred when the bed was subjected to elliptical vibration. For the micro-powder, it was observed that bed homogeneity was independent of vertical vibration intensity but improved with increasing superficial gas velocity. For nano-powder, intensifying vertical vibration led to segregation, likely due to agglomerate densification. Furthermore, fractional pressure drop measurements proved to be a strong tool in assessing fluidization quality, providing insights that could not be attained by conventional indicators.

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The fluidization of cohesive powders has been extensively researched over the years. When looking at literature on the fluidization of cohesive particles, one will often find papers concerned with only micro- or only nano-sized powders. It is, however, unclear whether they should be treated differently at all. In this paper, we look at differences and similarities between cohesive powders across the size range of several nanometres to 10s of micrometres. Classification of fluidization behaviour based on particle size was found to be troublesome since cohesive powders form agglomerates and using the properties of these agglomerates introduces new problems. When looking at inter-particle forces, it is found that van der Waals forces dominate across the entire size range that is considered. Furthermore, when looking into agglomeration and modelling thereof, it was found that there is a fundamental difference between the size ranges in the way they agglomerate. Where the transition between the types of agglomeration is located is, however, unknown. Finally, how models are made and agglomerate sizes are measured is currently insufficient to accurately predict or measure their sizes consistently.

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