Triboelectric charge saturation on single and multiple insulating particles in air and vacuum.

Triboelectric charge transfer is complex and depends on contact properties such as material composition and contact area, as well as environmental factors including humidity, temperature, and air pressure. Saturation surface charge density on particles is inversely dependent on particle size and the number of nearby particles. Here we show that electrical breakdown of air is the primary cause of triboelectric charge saturation on single and multiple electrically insulating particles, which explains the inverse dependence of surface charge density on particle size and number of particles. We combine computational simulations with experiments under controlled humidity and pressure. The results show that the electric field contribution of multiple particles causes electrical breakdown of air, reducing saturation surface charge density for greater numbers of particles. Furthermore, these results show that particles can be discharged in a low pressure environment, yielding opportunities for improved industrial powder flows and dust mitigation from surfaces.

Numerical Solution of the Electric Field and Dielectrophoresis Force of Electrostatic Traveling Wave System.

Electrostatic traveling wave (ETW) methods have shown promising performance in dust mitigation of solar panels, particle transport and separation in in situ space resource utilization, cell manipulation, and separation in biology. The ETW field distribution is required to analyze the forces applied to particles and to evaluate ETW design parameters. This study presents the numerical results of the ETW field distribution generated by a parallel electrode array using both the charge simulation method (CSM) and the boundary element method (BEM). A low accumulated error of the CSM is achieved by properly arranging the positions and numbers of contour points and fictitious charges. The BEM can avoid the inconvenience of the charge position required in the CSM. The numerical results show extremely close agreement between the CSM and BEM. For simplification, the method of images is introduced in the implementation of the CSM and BEM. Moreover, analytical formulas are obtained for the integral of Green's function along boundary elements. For further validation, the results are cross-checked using the finite element method (FEM). It is found that discrepancies occur at the ends of the electrode array. Finally, analyses are provided of the electric field and dielectrophoretic (DEP) components. Emphasis is given to the regions close to the electrode surfaces. These results provide guidance for the fabrication of ETW systems for various applications.

Toward the utilisation of resources in space: knowledge gaps, open questions, and priorities.

There are many open science questions in space resource utilisation due to the novelty and relative immaturity of the field. While many potential technologies have been proposed to produce usable resources in space, high confidence, large-scale design is limited by gaps in the knowledge of the local environmental conditions, geology, mineralogy, and regolith characteristics, as well as specific science questions intrinsic to each process. Further, the engineering constraints (e.g. energy, throughput, efficiency etc.) must be incorporated into the design. This work aims to summarise briefly recent activities in the field of space resource utilisation, as well as to identify key knowledge gaps, and to present open science questions. Finally, future exploration priorities to enable the use of space resources are highlighted.

Standard method for performing positron emission particle tracking (PEPT) measurements of froth flotation at PEPT Cape Town.

Positron emission particle tracking (PEPT) is a technique for measuring the motion of tracer particles in systems of flow such as mineral froth flotation. An advantage of PEPT is that tracer particles with different physical properties can be tracked in the same experimental system, which allows detailed studies of the relative behaviour of different particle classes in flotation. This work describes the standard operating protocol developed for PEPT experiments in a flotation vessel at PEPT Cape Town in South Africa. A continuously overflowing vessel with constant air recovery enables several hours of data acquisition at steady state flow and consistent flotation conditions. Tracer particles are fabricated with different coatings to mimic mineral surface hydrophobicity and size, and a data treatment derived from a rotating disk study is utilized to produce high frequency (1 kHz) location data relative to the tracer activity. Time averaging methods are used to represent the Eulerian flow field and occupancy of the tracer behaviour based on voxel schemes in different co-ordinate systems. The average velocity of the flow in each voxel is calculated as the peak of the probability density function to represent the peak of asymmetrical or multimodal distributions.•A continuously overflowing flotation vessel was developed for extended data acquisition at steady state flow.•The data treatment enabled the direct comparison of different particle classes in the flotation vessel.•The solids flow fields was described by the probability density function of tracer particle velocity measured in different voxel schemes.

Effect of Particle Size on the Rising Behavior of Particle-Laden Bubbles.

The rising behavior of bubbles, initially half and fully coated with glass beads of various sizes, was investigated. The bubble velocity, aspect ratio, and oscillation periods were determined using high-speed photography and image analysis. In addition, the acting forces, drag modification factor, and modified drag coefficient were calculated and interpreted. Results show that the aspect ratio oscillation of the rising bubbles is similar, irrespective of the attached particle size. As the particle size is increased, the rising bubbles have a lower velocity and aspect ratio amplitude, with the time from release to each aspect ratio peak increasing. Higher particle coverage is shown to decrease the bubble velocity and dampen the oscillations, reducing the number of aspect ratio peaks observed. The highest rise velocities correspond to the lowest aspect ratios and vice versa, whereas a constant aspect ratio yields a constant rise velocity, independent of the particle size. Force analysis shows that the particle drag modification factor increases with the increased particle size and is greatest for fully laden bubbles. The modified drag coefficient of particle-laden bubbles increases with the increased particle size, although it decreases with the increased Reynolds number independent of the particle size. The drag force exerted by the particles plays a more dominant role in decreasing bubble velocities as the particle size increases. The results and interpretation produced a quantitative description of the behavior of rising particle-laden bubbles and the development of correlations will enhance the modeling of industrial applications.

A Multicriteria Decision Framework for the Selection of Biomass Separation Equipment.

For the first time, a two-stage decision support framework for equipment selection, applied to biomass separation, is presented. In the first stage, the framework evaluates from a number of equipment based on the process requirements and outputs only those that offer a technically feasible separation. In the second stage, the analytic hierarchy process is applied for performing a multicriteria decision analysis to select amongst the feasible equipment based on separation performance and energy consumption criteria. This approach systematically considers the relative importance of those different alternatives and selection criteria by pairwise comparisons. The output of the framework is an overall ranking of equipment as well as a sensitivity analysis of the results for different weighting of the criteria. These results can be used to equip practitioners in the field of bioseparations with a tool for making more consistent and better-informed equipment selection decisions.

Influence of Design Parameters on Biomass Separation in Mini-hydrocyclones.

Small hydrocyclones are an attractive technology for biomass separation from fermentation processes. The interactive effect of design parameters on the performance of mini-hydrocyclones is, however, not fully explored and studies are often limited by the challenges in manufacturing such small units. Here, 10-mm mini-hydrocyclones are produced by 3D printing and the impact of spigot diameter, vortex finder diameter and height on separation performance is studied. A central composite rotatable design was adopted to obtain information on the relation between the variables and their influence on concentration ratio and recovery of yeast from a highly diluted system. A Pareto front for separation performance was generated and shown to be suitable to select an optimal design for a set of process constraints.

Geometry and Topology of Two-Dimensional Dry Foams: Computer Simulation and Experimental Characterization.

Pseudo-two-dimensional (2D) foams are commonly used in foam studies as it is experimentally easier to measure the bubble size distribution and other geometric and topological properties of these foams than it is for a 3D foam. Despite the widespread use of 2D foams in both simulation and experimental studies, many important geometric and topological relationships are still not well understood. Film size, for example, is a key parameter in the stability of bubbles and the overall structure of foams. The relationship between the size distribution of the films in a foam and that of the bubbles themselves is thus a key relationship in the modeling and simulation of unstable foams. This work uses structural simulation from Surface Evolver to statistically analyze this relationship and to ultimately formulate a relationship for the film size in 2D foams that is shown to be valid across a wide range of different bubble polydispersities. These results and other topological features are then validated using digital image analysis of experimental pseudo-2D foams produced in a vertical Hele-Shaw cell, which contains a monolayer of bubbles between two plates. From both the experimental and computational results, it is shown that there is a distribution of sizes that a film can adopt and that this distribution is very strongly dependent on the sizes of the two bubbles to which the film is attached, especially the smaller one, but that it is virtually independent of the underlying polydispersity of the foam.

Predicting the failure of a thin liquid film loaded with spherical particles.

A model is presented for predicting the failure of a thin liquid film stabilized by attached inert particles. A statistical analysis of roughly 3500 Surface Evolver1 simulations was used to identify the relationship between the packing density of the particles on the film, their contact angle distribution, and the capillary pressure required to rupture the film. The model presented allows a fast and simple method of calculating the range of pressures a thin film in a three-phase froth will fail at based upon three variables: the film loading, mean particle contact angle, and the standard distribution of contact angles round the mean. The predicted range of failure pressures can be used in simulations of bulk froth properties where bubble coalescence is an important factor governing the froth properties.

A model for the stability of films stabilized by randomly packed spherical particles.

Particle stabilized thin films occur in a range of industrial applications where their properties affect the efficiency of the process concerned. However, due to their dynamic and unstable nature they are difficult to observe experimentally. As such, a tractable way of gaining insight into the fundamental aspects of this complicated system is to use computer simulations of particles at interfaces. This paper presents modeling results of the effect of nonuniform packing of spherical particles on the stability of thin liquid films. Surface Evolver was used to model cells containing up to 20 particles, randomly packed in a thin liquid film. The capillary pressure required to rupture the film for a specific combination of particle arrangement, packing density, and contact angle was identified. The data from the periodic, randomly packed models has been used to find a relationship between particle packing density, contact angle, and critical capillary pressure which is refined to a simple equation that depends on the film loading and contact angle of the particles it contains. The critical capillary pressure for film rupture obeys the same trends observed for particles in regular 2D and 3D packing arrangements. The absolute values of P*(crit), however, are consistently lower than those for regular packing. This is due to the irregular arrangement of the particles, which allows for larger areas of free film to exist, lowering the critical capillary pressure required to rupture the film.

Measurements of interactions between particles and charged microbubbles using a combined micro- and macroscopic strategy.

We report the use of atomic force microscopy (AFM) to study the interactions between silica glass colloidal probes and charged microbubbles created using one of two different surfactants: anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethylammonium bromide (DTAB) in an aqueous environment. On close approach between the glass probe and a SDS microbubble, an appreciable repulsive force was observed prior to contact. This was not observed when using a DTAB microbubble, where only attractive forces were observed prior to contact. zeta-potential analysis showed that silica surfaces are negatively charged across the pH range of 3-10 when surfactant is not present. Addition of SDS did not alter the zeta-potential significantly, indicating that adsorption onto the particle surface did not occur. Conversely, the addition of DTAB decreased the negativity of the zeta-potential, reversing the sign, indicating that adsorption had occurred. This analysis was used in the removal of fine particles from suspension using charged microbubbles. Silica particles were recovered using positively charged microbubbles from DTAB but not when using negatively charged microbubbles generated from SDS. Taken together, the data suggest that repulsive long-range interactions were responsible for the selective attachment of silica particles to microbubbles in a charge-dependent manner.