Kerry J. Knox

Direct measurements of the axial displacement and evolving size of optically trapped aerosol droplets

The axial displacement of optically tweezed liquid aerosol droplets has been studied directly through the application of side imaging at 90° to the trapping laser beam. In conjunction with imaging in the plane of the optical trap and cavity-enhanced Raman spectroscopy (CERS), the optical forces experienced by a trapped aerosol have been interrogated. By varying the power of the trapping laser and observing changes in the axial position of a trapped particle it has been possible to examine the fine balance between the gradient and scattering forces, a key parameter in optical manipulation. Clear differences observed in sizing trapped particles from bright field microscopy and CERS have been reconciled. As a consequence, a novel technique for probing the evolving size of a single aerosol particle is proposed.

Ultrasensitive Absorption Spectroscopy of Optically-Trapped Aerosol Droplets

High-sensitivity optical absorption measurements on individual sub-picoliter aqueous droplets are reported using aerosol optical tweezers to simultaneously manipulate and characterize a sample droplet and a control droplet for comparison. It is demonstrated that the detection sensitivity to trace analytes is set by the weak absorption by the solvent, water, and that absorbances less than 5 x 10(-7) can be measured over pathlengths of less than 10 microm. The potential applications of this approach to analyze aerosol particle composition and to perform trace analysis are discussed.

First observation of capping/uncapping by a ligand of a Zn porphyrin adsorbed on Ag(100)

A significant first step towards creation of catalytically active porphyrin-functionalised metal surfaces has been achieved.

Infrared spectroscopy and phase behavior of n-butane aerosols and thin films at cryogenic temperatures.

Spectroscopic studies of two phase transitions of solid n-butane aerosol droplets performed under conditions representative of those in the lower atmosphere of Titan are presented. Pure n-butane aerosols and mixed ensembles of n-butane/acetylene, n-butane/carbon dioxide and n-butane/water aerosols were generated in a bath gas cooling cell at 78 K and their phase transition dynamics monitored using infrared extinction spectroscopy. For pure n-butane aerosols the volume and surface nucleation constants were found to range from JV = 10(12) -10(14) cm(-3) s(-1) and JS = 10(13) -10(15) cm(-2) s(-1), respectively, for the first observed transition, and JV = 10(9) -10(11) cm(-3) s(-1) and JS = 10(11) -10(13) cm(-2) s(-1) for the second observed transition. The phases of the n-butane aerosols were determined by comparing their spectroscopic signatures with spectra collected from thin films of liquid and solid n-butane. The first observed transition was from an amorphous-annealed phase into the metastable crystalline phase II of n-butane. The second transition was from the metastable crystalline phase II into the crystalline phase III. The effect of the presence of a second aerosol substance (acetylene, carbon dioxide or water) was examined; while this accelerated the first phase transition, it did not directly influence the rate of the second phase transition. The kinetic studies might be important for the understanding of cloud formation on Titan, while the spectral data provided, which include the first reported spectrum of liquid n-butane close to the melting point, are expected to be of use for remote sensing applications.

Timescales of water transport in viscous aerosol

Evaporation studies of single aqueous sucrose aerosol particles as a function of relative humidity (RH) are presented for coarse and fine mode particles down into the submicron size range (600 nm < r < 3.0 μm). These sucrose particles serve as a proxy for biogenic secondary organic aerosols that have been shown to exist, under ambient conditions, in an ultraviscous glassy state, which can affect the kinetics of water mass transport within the bulk phase and hinder particle response to changes in the gas phase water content. A counter-propagating Bessel beams (CPBBs) optical trapping setup is employed to monitor the real-time change in the particle radius with RH decreasing from 75% to 5%. The slow-down of the size change upon each RH step and the deviation from the theoretical equilibrium hygroscopic growth curve indicate the onset of glassy behavior in the RH range of 10-40%. Size-dependent effects were not observed within the uncertainty of the measurements. The influence of the drying time below the glass transition RH on the timescale of subsequent water condensation and re-equilibration for sucrose particles is explored by optical tweezers measurements of micron-sized particles (3 μm < r < 6 μm). The timescale for water condensation and re-equilibration is shown to increase with increasing drying time, i.e. the time over which a viscous particle is dried below 5% RH. These studies demonstrate the importance of the history of the particle conditioning on subsequent water condensation and re-equilibration dynamics of ultraviscous and glassy aerosol particles.

Summary, Conclusions and Future Directions

The work presented in this thesis has sought to address two key deficiencies in the current understanding of aerosol effects on radiative forcing: those related to the aerosol direct effect and the aerosol indirect effect. The work has focussed on improving the understanding of the direct effect via the development of a technique for the quantification of aerosol optical absorption. An improvement of the understanding of the indirect effect was approached through the development of a technique for the quantification of the mass accommodation coefficient of water at aerosol surfaces. For both techniques it was necessary to be able to determine the role played by aerosol composition.

Practical independent research projects in science: a synthesis and evaluation of the evidence of impact on high school students

ABSTRACT Practical independent research projects (IRPs) are a feature of school science in a number of countries. To assess the impact of IRPs on students, a systematic review of the literature was undertaken. Thirty-nine papers met the review inclusion criteria, reporting on work from twelve countries. The review indicates that IRPs are often associated with wider initiatives such as authentic science, problem-based learning, and project-based learning. There is considerable variability in the nature of IRP work in relation to focus, models of provision, assessment, the involvement of external partners such as universities and employers, and funding, and this diversity affects judgements on the quality of the evidence base on impact. The majority of the research reviewed explored areas such as conceptual understanding, motivation to study science once it is no longer compulsory and attitudes to science, and the development of practical skills. Benefits were identified in relation to the learning of science ideas, affective responses to science, views of pursuing careers involving science, and development of a range of skills. Studies focusing on traditionally under-represented groups indicated that such students felt more positive about science as a result of undertaking IRPs. The review findings indicate that further work is needed to enhance the quality of the available evidence, to consider the ways in which IRPs can be validly assessed, to explore more fully the potential benefits for traditionally under-represented groups, and to explore more fully the potential longer-term benefits of participation in IRPs at high school level.

Aerosol Mass Transfer

The size distribution of aerosol particles has a significant impact on their chemical and physical properties, including their optical properties and ability to act as cloud condensation nuclei in the atmosphere [1]. Understanding the factors which determine the particle size underpins all of aerosol science and is essential for the construction of accurate atmospheric models of past, present and future climate. This chapter will outline the thermodynamic and kinetic factors which control the size of aerosol particles, in particular aqueous aerosol droplets containing inorganic and organic solutes. Theory predicting thermodynamic equilibrium droplet size will first be discussed, followed by a description of the kinetic aspects of aerosol mass transfer which can limit the rate at which the equilibrium size is established.

Spectroscopy of Optically-Tweezed Aerosol Droplets Containing Fluorescent Chromophores

The observation and characterisation of fluorescence from aerosol particles has been the subject of considerable research over recent decades. Fluorescence spectroscopy has previously been used in the characterisation of droplet composition, temperature and pH. The potential for using fluorescence spectroscopy to analyse biological aerosol has recently been demonstrated, with unique fingerprints from different fluorophores, including amino acids, recorded in the ultra-violet and visible parts of the spectrum.

Kinetics of Aerosol Mass Transfer

It has been demonstrated that a small change in the power of the trapping laser beam can be used to induce a small change in droplet absorption. This gives rise to a small shift in the droplet temperature and hence in the equilibrium size at a given relative humidity. The change in equilibrium droplet size allows the absorption coefficient along the pathlength of the laser beam to be determined. In the absorption studies it is the magnitude of the droplet size change necessary to re-establish equilibrium that is the important quantity to measure.