Judy A Raper

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.

How Much Particle Surface Corrugation Is Sufficient to Improve Aerosol Performance of Powders

No HeadingPurpose.The current study aimed to quantify the different degree of particle surface corrugation and correlate it to the aerosol performance of powders.Methods.Powders of different degree of surface corrugation were prepared by spray drying under varying conditions. The solid-state properties of the powders including particle size, morphology, crystal form, true density, and moisture content were characterized. The degree of surface corrugation was quantified by the surface fractal dimension (DS) obtained by light scattering. The aerosol performance was studied by dispersing the powders using the Rotahaler at 60 L/min into a multi-stage liquid impinger. Fine particle fraction (FPF) was expressed as the wt% of BSA particles of size ≤5 μm in the aerosol.Results.Four powders of increasing degree of particle surface corrugation were prepared, with DS ranging from 2.06 for the least corrugated to 2.41 for the most corrugated. The powders had a similar size distribution (VMD 3 μm, span 1.4–1.5) and solid-state properties. Increasing the surface corrugation, DS, slightly from 2.06 to 2.18 enhanced the FPF significantly from 27% to 41%. This was explained by the reduced area of contacts and increased separation distance between the particles. Further increase of corrugation (DS ≥ 2.18) did not improve FPF.Conclusion.Powders with varying degrees of corrugation were successfully obtained by spray drying with their surface roughness quantified by fractal analysis. It was shown that only a relatively small degree of surface corrugation was sufficient to accomplish a considerable improvement in the aerosol performance of the powder.

Effect of fulvic acid adsorption on the aggregation kinetics and structure of hematite particles

Abstract In this study, the effects of adsorbed fulvic acid, a naturally occurring organic acid, on the kinetics of hematite aggregation and on the resulting structure of hematite aggregates are investigated. A model based on the colloid stability theory, which also accounts for the structure of the aggregates formed, is used to describe the aggregation kinetics of these adsorbed particles. The study of aggregate structure shows that the fractal dimensions of hematite aggregates that are partially coated with fulvic acid molecules are higher than those obtained with no adsorbed fulvic acid. The scattering exponents obtained from static light-scattering experiments of these aggregates range from 2.83 ± 0.08 to 3.42 ± 0.1. The scattering exponents of greater than 3 indicate that the scattering is the result of an object that contains pores that are bounded by surfaces with a fractal structure and can be related only to surface fractal dimension. The high fractal dimensions are due to restructuring within the aggregates, which only occurred at low coverage by the organic acid. At high fulvic acid concentrations, restructuring is not observed and flocculation bridging occurs, resulting in more open structures, with scattering exponents ranging from 2.07 to 2.72.

Fractal structure of hematite aggregates

Abstract The structure of hematite aggregates, considered to behave as fractal objects, is studied using static light scattering methods. The fractal dimension obtained from the scattering exponent is found to be dependent on the aggregation mechanism and it ranges from 2.3 for rapid (diffusion limited) to 2.8 for slow (reaction limited) aggregation. Polydispersity and restructuring of aggregates do not affect the relationship between scattering exponent and fractal dimension in these aggregates. Excellent correspondence over a range of temperatures and ionic strengths is obtained between results of experiments using dynamic light scattering to determine aggregation kinetics and those predicted using a modified Smoluchowski model incorporating fractal dimensions.

What is a Suitable Dissolution Method for Drug Nanoparticles

PurposeMany existing and new drugs fail to be fully utilized because of their limited bioavailability due to poor solubility in aqueous media. Given the emerging importance of using nanoparticles as a promising way to enhance the dissolution rate of these drugs, a method must be developed to adequately reflect the rate-change due to size reduction. At present, there is little published work examining the suitability of different dissolution apparatus for nanoparticles.MethodsFour commonly-used methods (the paddle, rotating basket and flow-through cell from the US Pharmacopia, and a dialysis method) were employed to measure the dissolution rates of cefuroxime axetil as a model for nanodrug particles.ResultsExperimental rate ratios between the nanoparticles and their unprocessed form were 6.95, 1.57 and 1.00 for the flow-through, basket and paddle apparatus respectively. In comparison, the model-predicted value was 7.97. Dissolution via dialysis was rate-limited by the membrane.ConclusionsThe data showed the flow-through cell to be unequivocally the most robust dissolution method for the nanoparticulate system. Furthermore, the dissolution profiles conform closely to the classic Noyes–Whitney model, indicating that the increase in dissolution rate as particles become smaller results from the increase in surface area and solubility of the nanoparticles.

Structure and kinetics of aggregating colloidal haematite

Abstract Photon correlation spectroscopy is used here to measure the hydrodynamic radius of haematite particles induced to aggregate by the addition of various concentrations of KCl. The kinetics of aggregation are followed over long time intervals (as long as the particles remain in suspension) and the results modelled using Smoluchowskis kinetic equation with rate constants modified to account for interaggregate repulsive and attractive forces and account taken of the nature of packing within the aggregate structure. The aggregating haematite particles are well described by a fractal geometry with a fractal dimension of 2.3 at salt concentrations greater than the critical coagulation concentration. A somewhat higher fractal dimension appears to be appropriate for aggregation under salinity conditions where a repulsion barrier exists.

Characterization of ceramic composite membrane filters for hot gas cleaning

Abstract Novel composite ceramic membrane filters suitable for hot gas cleaning operations have been prepared from fly ash and titania particles on stainless steel woven mesh substrates. Both membranes had a structure like a solid packed bed with a thickness of less than 100 μm. In the domain of laminar flow, significant differences in the pressure drop between experimental results and those predicted from the Kozeny-Carman relationship were found, particularly for flow through the titania membrane. Membrane filters with extremely small pores such as the titania membrane are greatly affected by slip up to 80% in permeability at elevated temperatures. Efficiencies of filtration of typical coal fired power station exhaust fly ashes were found to be greater than 909% for both titania and ash membranes, whilst there was greater variation in the pressure drops in the ash membrane than in the titania at typical gas flows.

Modelling the Settling Behaviour of Fractal Aggregates; A Review

Sedimentation is one of the simplest and cheapest methods used for the removal of particulate matter from wastewater. However, the method is only viable for large particles and hence pollutants need to be flocculated to form large aggregates, normally behaving as fractals. Knowledge of the structure of fractal aggregates and their settling behaviour are crucial for optimisation of the sedimentation process. The structure of fractal aggregates can be readily measured using methods such as light scattering. However, the characteristics of the settling behaviour of fractal aggregates are not well understood since these aggregates do not behave as spheres with constant density. The settling behaviour of fractal aggregates depends on various aspects, such as the porosity, size, and buoyant density. The models used in this paper to predict settling velocity of fractal aggregates show that aggregates with particles packed closely together settle more easily than open flocs of particles when the size of aggregates exceeds the transition radius, RT. RT exists due to porosity of the flocs. When the sizes of the flocs are smaller than RT, the passage of fluid through the interior of the aggregates allows the reduction of the drag coefficient, which further results in faster settling of loose aggregates compared to compact aggregates. However, when flocs are bigger than the RT, gravity plays a more important role, which allows compact aggregates to settle faster than loose aggregates. This result suggests that by controlling aggregation processes to ensure aggregates have compact structure and are bigger than RT, sedimentation could be further utilised in pollution control processes. Additionally, the usage of coagulant can be greatly reduced since less coagulant is needed to produce compact aggregates.

Characterisation and aerosolisation of mannitol particles produced via confined liquid impinging jets.

Mannitol particles, produced by spray drying (SD), have been used commercially (Aridol) in bronchial provocation test. In this study, we propose an alternative method to produce inhalable mannitol powders. The elongated mannitol particles (number median length 4.0microm, and axial ratio of 3.5) were prepared using a confined liquid impinging jets (CLIJs) followed by jet milling (JM). Spray dried and jet milled raw mannitol particles were compared in an attempt to assess the performance of the particles produced by the new method. Aerosol performance of the three different powders (CLIJ, SD, and JM) was relatively poor (fine particle fraction or FPF(loaded) below 15%) when dispersed by the Rotahaler. Dispersion through the Aeroliser led to better aerosol performance of the CLIJ mannitol (FPF(loaded) 20.3%), which is worse than the JM (FPF(loaded) 30.3%) and SD mannitol particles (FPF(loaded) 45.7%) at 60 L/min, but comparable (FPF(loaded) 40.0%) with those of the JM (FPF(loaded) 40.7%) and SD (FPF(loaded) 45.5%) powders at 100L/min. Hence, the optimum use of these elongated mannitol particles can be achieved at increased air flow with a more efficient inhaler. In addition to crystallinity, morphology, and particle size distribution, the surface energies of these powders were measured to explain the differences in aerosol performance. A major advantage of using the CLIJ method is that it can be scaled up with a good yield as the precipitate can be largely collected and recovered on a filter, compared with spray drying which has a low collection efficiency for fine particles below 2microm.