Fractal-like structures in colloid science

The present work aims at reviewing our current understanding of fractal structures in the frame of colloid aggregation as well as the possibility they offer to produce novel structured materials. In particular, the existing techniques to measure and compute the fractal dimension df are critically discussed based on the cases of organic/inorganic particles and proteins. Then the aggregation conditions affecting df are thoroughly analyzed, pointing out the most recent literature findings and the limitations of our current understanding. Finally, the importance of the fractal dimension in applications is discussed along with possible directions for the production of new structured materials

Combustion soot nanoparticles: Mechanism of restructuring and mechanical properties

Soot, a product of incomplete combustion of fossil fuels, is a global warming agent. The effect of soot particles on climate depends on their morphology. Freshly released soot particles are fractal lacey aggregates, but they often appear collapsed in atmospheric samples collected away from emission sources. A body of work has concluded that the collapse is caused by liquid shells when they form by vapor condensation around soot aggregates. However, some recent studies argue that soot remains fractal even when engulfed by the shells, collapsing only when the shells evaporate. To reconcile this disagreement, the effects of the condensation and evaporation on restructuring are separated in this study, by anchoring coated and coated-denuded soot, after condensation and condensation-evaporation, respectively. The morphology of the particles collected in both ways is characterized by using scanning electron microscopy images. It is shown that wetting and non-wetting liquids act differently in soot restructuring. Liquids capable of wetting the surface of soot aggregates can induce a significant restructuring by condensation. With non-wetting liquids, such as water, it is the evaporation that drives most of restructuring and there is almost no restructuring during condensation. Fractal soot particles released by combustion are typically hydrophobic, but can become hydrophilic after acquiring a coating layer made of hygroscopic atmospheric chemicals. To determine if absorption of water vapor by a thin hygroscopic coating can result in soot particle compaction at moderate relative humidities, the morphological response of soot thinly coated with hygroscopic chemicals is investigated upon humidification. Mass-mobility analysis, scanning electron microscopy, and condensation models confirm that even under subsaturated conditions capillary condensation of hygroscopic chemicals can occur into the junctions between carbon spherules, driven by the saturation pressure depression caused by the concave menisci. Furthermore, the concave menisci promote absorption of a significant amount of water vapor by the condensate at moderate relative humidities, exceeding the amounts achievable for flat and convex surfaces. Results imply that exposure of fractal soot particles to subsaturated vapors of hygroscopic chemicals in the atmosphere may be an important route towards soot compaction even at moderate relative humidities. The mechanistic details of soot aggregate restructuring remain poorly understood. In this study, atomic force microscopy is used for force-displacement measurements on bare, coated, and coated-denuded soot aggregates to determine their mechanical properties. The force curve is determined by measuring the deflection of the cantilever as it approaches and retracts from the sample. Peaks observed in the distributions of forces for bare soot may be related to the processes that occur during aggregate stretching, such as detachments between the monomers, unfolding, and breaking of the connection between monomers. These forces are significantly affected in the presence of a condensate. These results are expected to contribute to the development of physical models for soot restructuring. Overall, the findings of this dissertation advance understanding of the processes governing the transformations and environmental impacts of soot that will benefit the experimental and modeling atmospheric research communities

Droplets, Evaporation and a Superhydrophobic Surface: Simple Tools for Guiding Colloidal Particles into Complex Materials

The formation of complexly structured and shaped supraparticles can be achieved by evaporation-induced self-assembly (EISA) starting from colloidal dispersions deposited on a solid surface; often a superhydrophobic one. This versatile and interesting approach allows for generating rather complex particles with corresponding functionality in a simple and scalable fashion. The versatility is based on the aspect that basically one can employ an endless number of combinations of components in the colloidal starting solution. In addition, the structure and properties of the prepared supraparticles may be modified by appropriately controlling the evaporation process, e.g., by external parameters. In this review, we focus on controlling the shape and internal structure of such supraparticles, as well as imparted functionalities, which for instance could be catalytic, optical or electronic properties. The catalytic properties can also result in self-propelling (supra-)particles. Quite a number of experimental investigations have been performed in this field, which are compared in this review and systematically explained

Soot-PCF: Pore condensation and freezing framework for soot aggregates

Atmospheric ice formation in cirrus clouds is often initiated by aerosol particles that act as ice-nucleating particles. The aerosol–cloud interactions of soot and associated feedbacks remain uncertain, in part because a coherent understanding of the ice nucleation mechanism and activity of soot has not yet emerged. Here, we provide a new framework that predicts ice formation on soot particles via pore condensation and freezing (PCF) that, unlike previous approaches, considers soot particle properties, capturing their vastly different pore properties compared to other aerosol species such as mineral dust. During PCF, water is taken up into pores of the soot aggregates by capillary condensation. At cirrus temperatures, the pore water can freeze homogeneously and subsequently grow into a macroscopic ice crystal. In the sootPCF framework presented here, the relative humidity conditions required for these steps are derived for different pore types as a function of temperature

Light scattering studies of irregularly shaped particles

Doctor of PhilosophyPhysicsChristopher M. SorensenWe present light scattering studies of irregularly shaped particles which significantly affect the climate. We built and calibrated our apparatus which was able to measure all six independent scattering matrix elements. Our apparatus detects light from 0.32° to 157° simultaneously. We studied all six scattering matrix elements of irregularly shaped Arizona Road Dust which behave differently than those of spheres. We strongly focused on the most important scattering matrix element – the phase function, scattered intensity vs. the scattering angle, which we applied Q-space analysis to. Q-space analysis involves plotting the scattering intensity vs. the magnitude of the scattering wave vector q or qR with R the radius of a particle, on a double logarithmic scale. We measured and studied the phase functions of Al₂O₃ abrasives; compared the scattering from the abrasives with the scattering of spheres. To generalize the study, we collected a large amount of experimental and theoretical data from our group and others and applied Q-space analysis. They all displayed a common scattering pattern. The power law exponents showed a quasi-universal functionality with the internal coupling parameter ρ'. In situ studies of the soot fractal aggregates produced from a burner were also conducted. A power law exponent -1.85 is seen to imply the aggregates have fractal dimension of D[subscript f]=1.85. The overall work presented shows Q-space analysis uncovers patterns common to all particles: a q-independent forward scattering regime is followed by a Guinier regime, a power law regime, and sometimes an enhanced back scattering regime. The description of the patterns applies to spheres as well, except the power law regime has more than a single power law. These simple patterns give a unified description for all particle shapes. Moreover, the power law exponents have a quasi-universal functionality with ρ' for non-fractal aggregates. The absolute value of the exponents start from 4 when ρ' is small. As ρ' increases, the exponents decrease until the trend levels off at ρ'≳10 where the exponents reach a constant 1.75±0.25. All the non-fractal particles fall on the same trend regardless of the detail of their structure

The Measurement and Characterisation of Aerosol in the Urban Atmosphere (PM10) and an Evaluation ofthe Sources of these Particles by Number

Merged with duplicate record 10026.1/816 on 08.20.2017 by CS (TIS)The link between human health and the mass of fine particulate matter below 10 µm(PM10) in air is well documented. Current research suggests that the number, size and shape of particles may be of most concern and that in the urban atmosphere combustion sources of PM10, especially diesel engine sources, dominate the line (< 1 µm) and ultra-fine ( <0.1 µm) particles. Despite this, the number, size and shape of particles in urban air has not been reported to any great extent or detail, and the percentage contribution to the numbers of particles from different sources is largely unknown. The objectives of this research were to characterise fine particles with respect to their morphology and thus apportion the sources of particles by number. Urban aerosol above 1 µm was initially examined to study the fluctuations in PM10 number and make retrospective analysis of periods of elevated PM10 for source identification in Plymouth, UK. Aerosol was collected via a Burkard spore trap and examined using light microscopy with image analysis between 16 March 1995 and 31 August 1996, at a background site in Plymouth, UK. Two periods, 19 January-4 February and 10-25 March 1996, identified as UK wide PM10 episodes, were retrospectively studied and compared with PM10 mass measurements. The mean number count for the whole period was 10.5 x 104 ± 7.9 x 104 particles m ˉ³ The two PM10 episodes had elevated average number concentrations of 13.5 x 104 ±7.6 x 104 particles m ˉ³ for 19 January- 4 February 1996, and 13.0 x 104 ± 9.7 x 104 particles m ˉ³ for 10-25 March 1996. During the periods of elevated PM10 the tapered element oscillating microbalance (TEOM) mass of panicles had a low correlation with the particles less than 5 µm and an increased correlation to the particles greater than 5 µm in size. Outside of these peak periods the PM10 TEOM mass was most closely correlated with the number of particles less than 5 µm in size. This work shows the difference in urban aerosol during periods of air quality guideline exceedence. These findings agree with literature that an aged continental aerosol source has a key role in the generation of UK wide PM10 mass exceedances. Further analysis of the line urban aerosol (< 1 µm) was made using direct sampling of urban aerosol on to porous carbon films (PCF) developed in this research. The efficiency of collection was low (ea. 5%) but the samples were representative and enabled transmission electron microscopy (TEM) for sub-micron particle analysis. Measurement was made of the fractal dimensions and diameter of particles. This was used to identify any ageing and ultimately the sources of aerosol. PCF were used in the simultaneous collection of urban roadside and background aerosol, on seven dates between December 1996 and August 1997 in Plymouth, UK. The average perimeter fractal dimension (PFD) of aerosol was consistently significantly greater at the roadside than the background (+ 0.02), indicative of a smoother, aged aerosol at the background site. The sampling of a variety of combustion engines was made for source identification purposes. The particle morphology produced from the diesel engines showed great uniformity of particle morphology with varying speed and load; no consistent significant differences were found. The morphology results were comparable to other density fractal dimensions and perimeter fractal dimension values found in other studies for diesel. A natural log relationship between the median particle size and the median PFD was found for the diesel engine sources but not in petrol samples. This natural log trend was considered as a tentative 'fingerprint' of diesel engine combustion and was in harmony with literature values of PFD for diesel engine particles. Using the fractal measures, size and particle classification the bulk of aerosol was identified as from hydrocarbon combustion sources; ea. 88-92% of the roadside and ea. 77-86% of background. A component of carbon ceno-spheres were identified contributing ea. 6-12% of both the roadside and background aerosol. Non-combustion particles increased from ea. 1-4% of the roadside to ea. 7-9% of the background, as did the proportion of aged combustion particles, from 0-1% of roadside to 2-3% of the background aerosol. A strong correlation for the median size vs. PFD morphology curve between, the roadside and diesel sources (0.93 - 0.95) and the background and petrol sources was found (0.95). The roadside aerosol was significantly different to the petrol source in the l20-220nm size range (p=0.007) and there was a low correlation of the petrol and the roadside size vs. morphology curve (0.66). This suggests the domination o