Predictive modelling of agglomeration in spray drying and particle capture in aerosol scavenging requires a fundamental understanding of droplet-particle collisions. The study complements prior work by investigating mid-air collisions between free micron-sized spherical droplets and particles with a size ratio of three. Particle wettability and density are varied to elucidate the mechanisms governing collision outcomes and the role of collision offset. Results show that particle density determines whether a particle is engulfed by the droplet or remains at the droplet interface during capture, while high wettability suppresses particle separation even in glancing collisions. A modified effective Weber number incorporating particle density and wettability is proposed to map collision outcomes. To assess its robustness, the present data are combined with literature results in a unified regime map. The regime boundaries separating collision outcomes collapse when the size ratio and Ohnesorge number are held constant. However, at a given collision offset, variations in size ratio and Ohnesorge number alter the critical effective Weber number for particle separation through changes in collision geometry and viscous resistance.

Abstract The morphology of spray-dried granules critically influences their performance, with applications requiring either hollow shells or dense particles. We investigated shell formation and deformation in spray drying using suspensions of silica particles and polymers. In monodisperse silica systems, buckling was driven by capillary stresses, with the critical buckling size determined by particle radius and shell modulus. Adding smaller particles increased packing density and shell stiffness, but the accompanying rise in capillary pressure still promoted buckling. Replacing small particles with polymers fundamentally altered the deformation mechanism. Poly(vinylpyrrolidone) lowered the effective shell modulus, producing hollow granules with multiple buckling sites, whereas maltodextrin formed a dense surface layer, resulting in dented and blow-holed granules. The results demonstrate a transition from capillary-driven shell buckling to polymer shrinkage and viscoelastic deformation, providing design principles for tailoring granule porosity, mechanical integrity, and internal architecture.

A detailed droplet drying model based on a diffusive transport equation, embedded in an Eulerian-Lagrangian CFD model, is used to simulate drying behavior of droplets of lactose-water solution, a skin-forming material, and resulting particles in a laboratory-scale co-current spray dryer (Buchi B-290). Amorphous to crystalline transformation within the particles is studied using a glass transition temperature based approach. The process yield is predicted considering deposition of particles on the dryer wall using a deposition criterion based on the particle surface glass transition temperature. More than 50% of the total tracked particles of size less than 2 & micro;m get deposited on the wall, while the deposition rate for larger particles is lower. A large gradient of solute concentration exists within larger particles (> 3 & micro;m) during drying, leading to more crystalline transformation in the central region of the particles compared to the surface and a higher overall crystallinity of larger particles compared to the smaller ones. The predicted dried powder glass transition temperature and the outlet air temperature are compared with measurements available in the literature with a difference of 5.1 degrees C for the glass transition temperature and 9 degrees C for the outlet air temperature. This study shows that CFD-based modeling can be used as an effective tool for optimizing process conditions for the production of amorphous or crystalline particles and to maximize the process yield.

Flocculation–thickening is widely used in mineral processing and various chemical engineering fields. The flocculation in the thickener feedwell plays a key role in the tailings slurry thickening process. Hydrodynamic conditions directly affect particle flocculation kinetics and subsequent settling rates, thus determining the overall performance of the thickener. This study employs a multiscale modeling approach to investigate how feed solid concentration affects flow characteristics and flocculation–settling performance in a pilot‐scale deep cone thickener, in which Computational Fluid Dynamics‐Population Balance Model (CFD‐PBM) and a Two‐Fluid Model with Kinetic Theory of Granular Flow (TFM‐KTGF) were applied to simulate flocculation and settling behavior, respectively. Results show that medium solid concentration promotes particle aggregation via optimal turbulence dissipation. Increasing concentration reduces both the initial settling rate ratio of flocs and the settling differential between particle sizes. These findings enhance the understanding of flocculation–thickening mechanisms and support process optimization in solid–liquid separation fields.

Drying of organic solvates remains hard to scale down and fully understand, as (a) residual solvent is typically hard to remove due to high solid-phase transport resistances, and (b) precise control over crystal properties is challenging. These issues are especially relevant to the pharmaceutical sector and its stringent quality criteria. We have identified a Schiff base forming a methanol solvate (derived from o-vanillin and para-aminobenzoic acid) with chemical complexity representative of pharmaceutical active ingredients that is cost-effective and a straightforward process for its manufacture has been developed. Initial attempts to crystallize the compound resulted in the formation of a slurry with a high yield stress caused by extremely high product supersaturation. By seeding the crystallization and controlling the addition rate of the catalyzing reagent, the process was successfully scaled up into one which was both high-yielding (93% at 1 L scale) and concentrated. These changes altered the crystal morphology, with crystal growth being favored over nucleation, resulting in larger, higher aspect ratio crystals (from 8 to 20). Powder X-ray diffraction (XRD) showed that the solvated Schiff base gradually transformed into a distinct desolvated polymorph, and a quantitative method for assessing solvent content with XRD was developed. The compound is a promising candidate as a model solvate system for drying trials on account of its (a) high-aspect-ratio morphology typical of many organic products; (b) stability at room temperature, facilitating handling and analysis; (c) desolvation temperature exceeding the boiling point of methanol, separating the drying of the free solvent from the period of desolvation. In conclusion, this relatively unexplored Schiff-base was identified as a promising model solvate for studying drying under industrially relevant conditions.

This work investigates the link between the single particle and bulk flow properties of stainless steel (SS316L) powders and their spreading behaviour for additive manufacturing (AM) application. Two different batches of SS316L powders, noted to have significantly different spreading behaviour in AM machines, were investigated. The two batches had almost identical particle size, morphology and bulk flow properties as quantified by angle of repose, Hausner ratio, Carr index and basic flow energy; however one of the batches did not consistently flow during Hall and Carney flow tests and also had very poor spreadability when evaluated using an in-house spreading rig. The flow and spreading behaviours of the poorly spreading batch significantly improved and became almost identical to the good batch when tested following drying, either in a vacuum oven or a freeze dryer. Elemental analysis revealed that batches had some small but significant differences in chemical composition. This study revealed the complexity of defining a correlation between powder properties and spreading behaviour, and the importance of considering the environmental conditions.

Powder bed fusion methods of additive manufacturing (AM) require consistent, reproducible, and uniform layers of powder for the reliable production of high-quality parts, where properties of powder are central to achieving this. Among these properties, powder flowability and spreadability play critical roles in determining the quality of these powder layers.While extensive research has been conducted on powder flow and spreading behaviour, and on their characterisation, there is little critical comparison and review of these terms in the context of AM. Such a review is necessary to further develop and enhance our comprehension of spreading dynamics and its relation to powder properties in AM systems.This review paper aims to build a coherent understanding of the correlation between powder characteristics and spreading in powder based additive manufacturing and its impact on manufactured parts. It highlights the current progress in comprehending spreading dynamics, the influence of powder characteristics, environmental conditions, spreading system, and the development of testing tools to assess powder spreadability. Furthermore, the paper critically discusses the challenge of finding appropriate quantitative metrics and recent advances in the use of standardised methods for evaluating powder spreadability.

In powder-based Additive Manufacturing (AM) the precise control of process parameters plays a significant role in the quality and efficiency of the printing process. Among these, the effect of temperature has received less attention in the literature, although it is a significant factor that influences the inter-particle forces and, consequently the powder flow and spreading behaviour of powders. In selective laser sintering (SLS) or selective laser melting (SLM), pre-heating the chamber and powder bed is a required step prior to sintering, hence, the temperature can significantly influence the layer adhesion and spread quality. In this context, the present study explores the effect of elevated temperature on the flow and spreading behaviours of AlSi10Mg powders. The flow properties of two different grades of aluminium alloy powders are characterised using the Carney and Hall flow tests, angle of repose and shear test techniques at different temperatures and correlated with the spreading behaviour at elevated temperatures, measured using the spreading rig with a heated bed developed at the University of Leeds. This study revealed that at elevated temperatures the spreadability of AlSi10Mg powders worsens because of changes in interparticle forces and particle surface interactions.

A study using both Raman spectroscopy and molecular dynamics (MD) simulations was carried out for alkyl ethoxysulfate (AES) surfactants at various concentrations in solution. Direct comparison between experiment and simulation shows that the conformational changes observed in MD are in good agreement with those obtained via Raman spectroscopy. We show that there is an increase in the relative number of trans conformations with increasing concentration and illustrate the relationship between phase structure and molecular conformation, which is often speculated but difficult to confirm. Our results open up the possibility of applying MD to other surfactants, with the aim of analyzing conformational behavior, which can typically be difficult to study experimentally using spectroscopy methods, due to large numbers of vibrational modes present in large complex molecules.

Identification of the most reliable method to characterise powder flow behaviour in correlation with the conditions of powder spreading in additive manufacturing (AM) is still challenging. In this study, a number of standard and advanced flowability techniques were used to characterise the flowability of two grades of Ti6Al4V powder (gas atomized, GA, and hydride-dehydride, HDH) used for the powder bed fusion (PBF) based AM. In parallel, the powder spreading behaviour was characterised using an in-house spreading rig. It is found that GA powder has better spreading behaviour than HDH, owing to its better flowability due to the regular particle shapes. However, none of the flow test techniques investigated in this paper can offer a correlation between the dynamic powder flow and powder spreadability at varying speeds. The study in this work has revealed the shortcomings in correlating the flowability of powder and their spreadability under real process conditions.

We present a dissipative particle dynamics study of surfactant solutions under shear, which allows us to investigate their rheological properties. We consider a variety of concentrations and phase structures, including micellar solutions and liquid crystal phases. It is shown that the viscosity of micellar solutions increases as a function of concentration, in agreement with what is expected from experimental data. We also show that micelles can exhibit shear-thinning behavior when a shear force is applied, which is a result of micelles breaking down into smaller aggregates. Lamellar and hexagonal phases are found to orientate under the application of shear, in agreement with experimental observations. It is normally suggested that lamellar phases under shear can exhibit a transition between orientations as the shear rate is increased, usually as a result of lower viscosity. We calculate the viscosity for different lamellar phase orientations, showing that, although the viscosity of perpendicular orientations is lower than that of parallel orientations, we do not observe a transition to the perpendicular phase at high shear rates. Finally, we show that the choice of Schmidt number has a significant impact on the results, which is important for determining the correct behavior via simulations.

Particle flocculation in a stirred tank was numerically investigated by a coupled CFD-DEM approach, in which a microscopic test in a typical zone was first proposed instead of solving the full-scale particle field. The flocculation kinetics was described by the Johnson-Kendall-Roberts (JKR) theory, followed by calibration of surface energy parameter. A Volume of Fluid (VOF) model was employed to capture the interface between liquid and air. The two-way coupling of fluid and particle was achieved by resolving pressure gradient force and Gidaspow’s drag force in the momentum equations. Based upon the qualitative and quantitative validation tests in air-water interface pattern and fluid tangential velocity, respectively, the particle coordination number and flocs fractal properties (e.g. fractal dimension, voidage, effective density) were investigated considering the effect of impeller speed. Results show that the number of particles first increases with particle coordination number and then decreases, and is further positively correlated with the stirring intensity in lower coordination numbers (0−3) while negatively correlated in higher coordination numbers (≥5). The influences of turbulence dissipation rate on the average particle coordination number were also investigated in radial, tangential, and axial directions. The mean floc size decreases with impeller speed, however, the fractal dimension increases in a certain range. The voidage increases with the daughter-particle number in a floc while the effective density of flocs is inversely proportional to the floc size. It is recommended to adopt operations that facilitate the one-by-one attachment mode and syneresis for forming relatively large and compact flocs.

The flowability of spray-dried laundry detergent is important for manufacturing process performance and reliability, and product quality; it is strongly dependent on formulation. In this work, a detailed study of four model formulations provides fundamental understanding of the role of particle composition, and micro-structure, on spray-dried detergent powder flow behaviour. All spray-dried powders studied contained the sodium salt of linear alkylbenzene sulphonate (NaLAS) and sodium sulphate. However, their formulation varied depending on either the initial water content (30.0 or 63.0 wt%) of the slurry or the addition of sodium silicate with molar-ratios of 1.6 and 2.35 SiO2:Na2O. The nil-silicate powder, made from low water content slurry, had the poorest flow characteristics. The addition of the silicate binder, significantly improved flowability, 2.35 ratio silicate, providing better flowability than 1.6 ratio. Remarkably, increasing the water content of the slurry also improved flowability significantly. Detail analysis showed that this was due to changes in composition and micro-structure of the composite matrix formed on drying the liquid components of the slurry, and that the improved flowability was due to improvements in mechanical strength of this matrix. These changes in mechanical robustness were significantly more important to the flowability than the powder shape.

Sodium lauryl ether sulfate (SLES) is a common anionic surfactant used in a large number of personal care products. Commercial products typically contain a distribution in the number of ethoxy groups; despite this, there is limited existing work studying the effect of the ethoxy groups on the phase formation and structure. This is particularly important for the effect the structure has on the viscosity, an important consideration for commercial products. Dissipative particle dynamics is used to simulate the full phase diagram of SLES in water, including both micellar and lyotropic liquid crystal phases. Phase transitions occur at locations which are in good agreement with experimental data, and we find that these boundaries can shift as a result of varying the number of ethoxy groups. Varying the ethoxy groups has a significant effect on the micellar shape and crystalline spacing, with a reduction leading to more nonspherical micelles and decreased periodic spacing of the hexagonal and lamellar phases. Finally, while typical commercial products contain a distribution of ethoxy groups, computational work tends to focus on simulations containing a single chain length. We show that it is valid to use monodisperse simulations to infer behavior about solutions with a polydisperse chain length, based on its mean molecular length.

This study investigates the dynamics of a novel, oscillatory, intensified plug-flow reactor - an agitated tubular reactor (ATR) - designed for efficient flow processing of solid-liquid mixtures. The relative movement of the reactor and agitator bar, and associated effects on fluid mixing, were characterised physically with a suite of experimental instruments - utilising laser-based, video-based and acoustic techniques - and numerically via a Lattice Boltzmann method (LBM) computational fluid dynamic simulation. The reactor volume consisted of a cylindrical outer tube containing a free-moving, perforated agitator tube. The position, velocity and angular velocity of the inner agitator relative to the outer tube were measured experimentally and computationally under a range of realistic operating conditions, in terms of applied agitation frequency and displacement distance, along with their effect on the associated fluid velocity and turbulence levels. Additionally, simulations were used to validate a model for the reactor power input. The agreement between experimental and simulation data was very good in all cases, leading to clear recommendations for optimal operating conditions, while an experimentally derived regime map of the types and magnitudes of ATR motion is also presented.

Flocculation occurring in a feedwell plays a critical role in tailings slurry thickening process, which is complicated and significantly influenced by flow characteristics. This work presents a numerical approach to explore the effect of flow characteristics on flocculation performance. It combines an aggregation kernel and a breakage kernel, used to describe the polymer-bridging flocculation kinetics, with a conventional Computational Fluid Dynamics-Population Balance Model (CFD-PBM) coupling to model the complex flocculation-thickening behavior in a lab-scale gravity thickener. The solid-liquid phase interaction is described by an Euler-Euler approach with a modified Schiller-Naumann drag model. The turbulence of liquid phase is resolved by the RNG k-ε turbulence model, while the solid kinematic eddy viscosity is described by a dispersed phase zero equation model. The capability of this proposed model is validated by a good agreement between experimental and predicted results in terms of single-liquid velocity and floc size distribution. The momentum and turbulence dissipation rates are investigated in and around the feedwell over a wide range of feed velocities, showing that the momentum and turbulence dissipation rates have a positive correlation with feed velocity. The momentum and turbulence dissipation rates decrease with the increase in scaled depth in the feedwell. The formation of large vortexes in the feedwell may cause a locally low turbulence dissipation rate. A reasonable increase of feed velocity favours the flocculation, however, an excessive feed velocity can cause a decrease in mean floc size. Modelling tailings flocculation is of great significance for understanding the flocculation behavior and revealing the effect of flow characteristics on flocculation performance.

Spray-dried detergent granules have a complex and multi-scale structure, in which submicron sized crystals of inorganic salts along with multi-lamellar stacks of surfactant molecules are dispersed within a matrix phase. In this study, two model formulations based on sodium linear alkylbenzene sulphonate (NaLAS) and sodium sulphate were used to investigate the molecular structure of stacked lamellae. The parameters of lamellar d-spacing, bilayer thickness (dHH) and water layer thickness (dW) were determined using small-angle X-ray scattering. Four sets of lamellae were detected in nil-silicate detergent powders conditioned at 33 % RH (relative humidity). The exposure of these samples to a higher RH level (75 %) resulted in a reduced number of coexisting lamellar phases by two. This was accompanied by an increase in lamellar d-spacings and a considerable reduction in bilayer thicknesses (dHH). The reduction in number of sets of lamellae was explained by an increase in the fluidity of the lamellar phases and disappearance of water-poor solid polymorphs. This was consistent with the results of FTIR for nil-silicate samples, indicating an increase in the conformational disorder of the alkyl chains at a higher RH value. Interestingly, in the presence of sodium silicate the lamellar phases display a greater degree of swelling at a relatively low RH value (33 % RH) as compared with those in the absence of sodium silicate. Nonetheless, the thickness of water layer was found to slightly decrease at a higher RH (75 %), which was attributed to the moisture-induced glass transition and kosmotropic effect of sodium silicates.

High Speed Sintering (HSS) is a powder bed fusion additive manufacturing technology that relies on inkjet printing of an infrared (IR) radiation absorbing material (RAM) onto a polymer powder bed in order to produce cross-sections which can be selectively sintered using an IR lamp. The RAM used in HSS is carbon black, which is suspended in a petroleum-based carrier fluid to form an ink. The use of carbon black in this way means that any parts currently produced by HSS are inevitably some shade of black or grey depending on the amount and distribution of RAM on the part surface. As well as affecting aesthetics, the effect of printing different densities of RAM has previously been shown to influence part sintering and subsequently mechanical properties, which demonstrates the importance of RAM distribution in determining the final properties of parts made using HSS. Anecdotally, it has also been observed that parts produced with the same type, amount and printed density of ink, but using different polymer powders, can be significantly different colours. This suggests that the wetting interaction between the ink and the polymer powder is also significant factor in determining the distribution of RAM on a part surface. However, to date there has been no investigation into the nature of this relationship. By measuring part colour, in this work we quantify differences in RAM distribution when different polymers are processed via HSS using the same printed density of ink, with parts made from commercially available powders of poly(propylene) (PP), pol(ether-b-amide) (PEBA), poly(styrene) (PS), poly(amide)-11 (PA11), and poly(amide)-12 (PA12) having Relative Luminance (i.e. brightness) of (5.3 +/- 0.2)%, (5.5 +/- 0.5)%, (12 +/- 1)%, (15 +/- 2)%, and (18 +/- 1)%, respectively. The contact angles of the ink on the powder beds were measured in a simple model system, and these were found to be (46.1 +/- 0.8)degrees, (80 +/- 3)degrees, 82 degrees, (90 +/- 2)degrees, and (93 +/- 2)degrees for PP, PEBA, PS, PA11, and PA12, respectively. With the exception of one anomalous data point for PEBA, it is demonstrated that part Relative Luminance can be correlated to contact angle, and that we can describe that relationship to good precision using a semi-empirical model. Knowledge about the effect of ink wettability on RAM distribution should prove useful for continuing the optimisation of the parts obtained by HSS.

Maximizing separation sharpness is critical for the design and operation of hydrocyclones but remains difficult to achieve. This work presents a numerical study on the relationship between separation performance and the characteristics of axial velocity wave zone to understand the reasons for the limited separation sharpness. The results showed that this zone is featured as an inherent transition region, with extensive secondary vortices formed between inner and outer spiral flows. The presence of this zone adversely affects the fluid-solid momentum transfer, causing prolonged residence time and accumulation of intermediate-sized particles. The separation sharpness shows a strong dependence on the characteristics of axial velocity wave zone, which are further controlled by geometric parameters. Increasing the symmetry of flow field and optimizing the spatial distribution of this zone can help increase the separation sharpness while its size shows little effect.