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The study of respiratory droplets evaporation and formation of crystalline solid residues has become more and more important with the spread of respiratory diseases such as COVID-19. Evaporation time and droplet sizes greatly influence the dispersion of respiratory aerosols and their subsequent airborne transmission to susceptible hosts. In this study, a saline droplet is used as a surrogate for a respiratory droplet and a mathematical model is developed for its evaporation. Predictions agree well with experimental data reported in the literature. The model includes partial differential equations (PDEs) for the diffusion of dissolved solute and heat conduction within a saline droplet and water evaporation from the droplet surface. The internal domain of the droplet is discretized in space using the finite volume method, transforming the (PDEs) into a set of ordinary differential equations in time, which are solved using the Rosenbrock method. The calculation terminates when the solute concentration at the droplet surface reaches a value corresponding to a critical supersaturation level for the on-set of crystallization. The radial concentration profiles at different time intervals highlight the solute concentration enrichment at the droplet surface as it dries, by which the rate of solvent evaporation is affected. The model is also applied to a free-falling saline droplet evaporating under room temperature and relative humidity. The outcome reveals a strong dependency of the initial solute concentration on the ratio of initial to final droplet size. Lastly, the capability of the model predictions is demonstrated at a high relative humidity, where condensation occurs, broadening the model's applicability. [ABSTRACT FROM AUTHOR]

Triboelectrification is a ubiquitous and poorly understood phenomenon in powder processing, particularly for pharmaceutical powders. Charged particles can adhere to vessel walls, causing sheeting; they can also cause agglomeration, threatening the stability of powder formulations, and in extreme cases electrostatic discharges, which present a serious fire and explosion hazard. Triboelectrification is highly sensitive to environmental and material conditions, which makes it very difficult to compare experimental results from different publications. In this work, density functional theory (DFT) is used to investigate the charge transfer characteristics of several functional groups of paracetamol in order to better understand the mechanisms of charging at the nanoscale and the influence of the environmental and material properties on charge transfer. This is achieved by studying the structure and electronic properties at the molecule–substrate interface. Using this molecule–substrate approach, the charging contributions of individual functional groups are explored by examining the Hirschfeld charges, the charge density difference between the molecule and substrate, the density of states, and the location of the frontier orbitals (HOMO and LUMO) of a paracetamol molecule. Charge density difference calculations indicate a significant transfer of charge from the molecule to the surface. Observable regions of electron density enrichment and depletion are evident around the electron-donating and -withdrawing groups, respectively. The density of states for the paracetamol molecule evolves as it approaches the surface, and the band gap disappears upon contact with the substrate. Hirshfeld charge analysis reveals asymmetry in the charge redistribution around the molecule, highlighting the varying charging tendencies of different atoms. [ABSTRACT FROM AUTHOR]

A spiral jet mill was simulated using Discrete Element Method modelling and Computational Fluid Dynamics. The particle behaviour and fluid motion were analysed as a function of hold-up and grinding pressure. Particle collision energy was predicted to be prevalent along the bed surface and in front of the grinding jets, as shown through the collision data recorded. The bed itself affects the fluid flow field, as momentum is transferred to the particles. Increasing the grinding pressure does not result in a proportional increase in the kinetic energy of the particle system, as the high pressure jets begin to penetrate the bed with greater ease. The particle bed moves as ‘plug-flow’, with the layers of the bed closest to chamber wall.

Milling is an important process for tailoring the particle size distribution for enhanced attributes, such as dissolution, content uniformity, tableting, etc., especially for active pharmaceutical ingredients and excipients in pharmaceutical industries. Milling performance of particulate solids depends on the equipment operating conditions (geometry, process conditions and input energy etc.) as well as material properties (particle size, shape, and mechanical properties, such as Young’s modulus, hardness and fracture toughness). In this paper the particle dynamics in a pin mill is analysed using Discrete Element Method (DEM), combined with a novel approach for assessing particle breakability by single particle impact testing. A sensitivity analysis is carried out addressing the effect of the milling conditions (rotational speed and feed particle flow rate), accounting for feed mechanical properties on the breakage behaviour of the particles. Particle collision energy spectra are calculated and shown to have a distribution with the upper tail end being close to the maximum energy associated with the collision with the rings. Breakage is primarily due to collisions with the rings, except for large particles that are comparable in size with the gap between the rings, nipping is also a contributory breakage mechanism.

Soft granular materials are assemblies of highly deformable grains interacting via surface forces. The large grain deformations of these materials differ them from hard granular systems, in which, their behaviors are essentially governed by grain rearrangements. In this paper, we study the uniaxial compression of soft granular materials using a numerical approach based on the Material Point Method allowing for large grain deformations, coupled with the Contact Dynamics method for the treatment of unilateral frictional contacts between grains. Considering the neo-Hookean and elasto-plastic grains, the compaction of 2D soft granular packings is analyzed. We focus essentially on the evolution of the packing vertical stress as a function of the packing fraction and the predictive models are proposed.