Graeme Bushell

On techniques for the measurement of the mass fractal dimension of aggregates.

A review is presented of a number of techniques available for the characterisation of the structure of aggregates formed from suspensions of sub-micron particles. Amongst the experimental techniques that have been commonly used are scattering (light, X-ray or neutron), settling and imaging and these are the focus of this work. The theoretical basis for the application of fractal geometry to characterisation of flocs and aggregates is followed by a discussion of the strengths and limitations of the above techniques. Of the scattering techniques available, light scattering provides the greatest potential for use as a tool for structure characterisation even though interpretation of the scattered intensity pattern is complicated by the strong interaction of light and matter. Restructuring further complicates the analysis. Although settling has long been used to characterise particle behaviour, the absence of an accurate permeability model limits the technique as a means of determining the porosity of fractal aggregates. However, it can be argued that the determination of fractal dimension is relatively unaffected. The strength of image analysis lies in its ability to provide a great deal of information about particle morphology and the weaknesses lie in the difficulties with image processing and sample size as this is a particle counting technique. There are very few papers which compare the fractal dimension measured by more than one technique. Light scattering potentially provides a useful tool for checking settling results. However, further work is required to develop proper models for aggregate permeability and flow-through effects.

Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks

The effect of shear history on the evolution of the polystyrene—alum floc size, density, and structure is investigated by small-angle light scattering during cycled-shear and tapered-shear flocculation in a stirred tank using a Rushton impeller. First, various sampling schemes are experimentally evaluated. The floc structure is characterized by the mass fractal dimension, Df, and the relative floc density. During turbulent shear flocculation, small floc structures are shown to be more open (Df = 2.1) than larger floc structures (Df = 2.5) as a result of shear-induced restructuring during steady state attainment. Flocs produced by cycled-shear flocculation are grown at shear rate G-50 s−1 for 30 min, are fragmented at Gb = 100, 300, or 500 s−1 for one minute, and then are regrown at G = 50 s−1. This shear schedule decreases the floc size but compacts the floc structure. When flocs are produced by gradual reduction of the shear rate from G-300 to 50 s−1 (tapered-shear flocculation), smaller though equally dense flocs are produced compared with cycled-shear flocculation. The cycled-shear flocculation method produces the largest flocs with the highest potential for sedimentation when the fragmentation shear rate is Gb = 300 s−1.

Understanding the role of restructuring in flocculation: The application of a population balance model

Abstract The effect of shear on floc properties was observed through population balance to comprehend the mechanisms of flocculation, in particular the role of restructuring. Little fundamental attention has been given before to the shear influence responsible for creating compact aggregates, while the floc characteristics might differ in other conditions. It is crucial to understand how aggregates evolve to steady state, if their properties are to be ‘tailored’ to suit subsequent solid–liquid separation. From a previous experimental study (Langmuir 18(6) (2002) 1974), restructuring was observed to occur extensively in the flocculation of 380 nm latex particles in couette-flow, and was proposed to be responsible for the decrease in floc size on their transition to equilibrium. On the other hand, flocs of larger primary particles (810 nm ) were more susceptible to breakage, with densification occurring as a result of fragmentation and re-aggregation. Denser flocs were found when structural deformation dominated, particularly in the initial stage of the process, while comparatively tenuous ones were observed when formation and breakage kinetics were the governing mechanisms. The distinct manners in which aggregates of different primary particle sizes evolved with time, were replicated with a population balance that incorporated the floc structural variation; verifying that restructuring indeed played a crucial role under certain flocculation conditions.

Moderation of peer assessment in group projects

It is shown here that a grade distribution scheme commonly used to moderate peer assessments where self assessment is excluded is based on a false premise and will give an erroneous ranking in the situation where the best performer in a student group ranks the second best performer much higher than the other group members. An alternative to normalisation is proposed based on the idea that the rank order of peer grades should be preserved and should as far as possible be consistent between assessors. It is shown that the method correctly recovers the rank order of students within the group for all cases examined, while still eliminating biases that can result from differences in marking standards in the group. It is suggested that the approach could also be used to check for bias when self assessment is included.

Characterisation of short-range structure of silica aggregates—implication to sediment compaction

Abstract In this study, both short-range and long-range structures of silica aggregates were studied by small-angle light scattering. The silica particles were aggregated by using different concentrations of KCl and MgCl 2 , with and without continuous shear. It was found from both small-angle light scattering and transmission electron microscopy (TEM) measurements that the aggregates had a compact short-range structure and a looser long-range structure. The floc sediments were studied by allowing the silica aggregates to settle under gravity, as well as having them consolidated by centrifugation. The results show that under gravity and lower centrifugation force (50× g ), the short-range structure of silica aggregates affects the compacted sediment structure, while under higher centrifugation force (453× g ), the sediment structure is independent of the short-range structure of the aggregates.

The effect of a bimodal primary particle size distribution on scattering from hematite aggregates

Abstract The structure of aggregates of bidisperse particles was probed with small angle light scattering. Various mixtures of nearly spherical hematite particles with diameters of 70 and 216 nm were induced to aggregate by the addition of KCl. The fractal dimension of the resulting aggregates was unable to be determined because of effects from the aggregate edges and short range non-fractal behaviour. Significant restructuring of the aggregates was found to occur as a result of pumping.

The Effect of Polydispersity in primary particle Size on Measurement of the fractal Dimension of Aggregates

The use of the mass fractal dimension has become a popular method of characterising the structure of aggregates of fine particles. This parameter is often inferred from scattering experiments that exhibit a power law correlation between scattered intensity and the scattering vector. In this paper we demonstrate deviations from this behaviour that occur when the particles making up the aggregate are not monodisperse even though the aggregate maintains the same fractal structure as observed in the monodisperse case. We have performed light scattering experiments with aggregating colloidal haematite and performed DLCA computer simulations to explain the observed behaviour. The behaviour can influence the determination of the mass fractal dimension, as can other factors such as scattering effects from primary particles.

Characterization of large fractal aggregates by small-angle light scattering

Fractal geometry is a useful tool for describing the complex branched structures seen in certain aggregated systems. This study utilizes small-angle light scattering to examine some of the larger structures produced by these systems. The Fraunhofer scattering approximation is used to describe the scattering from aggregates containing large, opaque primary particles. Scattering from simulated aggregates is compared with experimental results of alumina tri-hydrate aggregated using FeCl3 and NaHCO3. A CMOS pixel array detector having logarithmic response to the incident light is utilized to obtain high-resolution scattering patterns. The applicability of both radial and azimuthal data components for extracting the fractal dimension is examined.

Do ethics courses make engineering students more ethical

To decide whether teaching ethics improves the ethical reasoning of engineering students, an online survey was used to assess their ethical reasoning before and after taking a course in which ethics is taught. Students were asked to consider scenarios requiring ethical judgment and their preferences for pre-conventional, conventional and post-conventional reasoning were measured. Overall, students taking the course with the most emphasis on activities related to ethics showed small but significant movement away from pre- and towards post-conventional reasoning. Local and international and male and female results were compared.