E. James Davis

Transport Phenomena with Single Aerosol Particles

This is a review of theory and experiments related to single aerosol particle transport processes. The theories of mass, heat, momentum, and charge transfer are outlined, with emphasis on mass transport in the continuum and noncontinuum regimes. Included in the discussion of mass transfer are single and multicomponent droplet evaporation, a comparison of the results of solutions of the Boltzman equations for Knudsen aerosol evaporation and growth and experimental methods for the study of single droplet evaporation or growth. Of particular concern here are the experimental apparati and techniques developed for single particle measurements. These range from the Millikan oil drop experiment through the electrostatic balance (the Millikan condenser with automatic stabilization of the particle) to the electrodynamic balance. The principles and applications of these instruments are reviewed.

The double‐ring electrodynamic balance for microparticle characterization

A simple form of the electrodynamic balance, suitable for a wide range of microparticle measurements, is described and analyzed. The ac electrode of the device consists of a pair of parallel rings, and the dc endcaps are either simple disks or they can be eliminated entirely by applying suitable dc bias voltages to the rings. The stability characteristics of the device are determined by extension of well‐established stability theory, and experiments are compared with that theory. The device is particularly well‐suited for detection of radioactive aerosols, for it has significant advantages over the bihyperboloidal device for radioactivity measurement. The detection of radioactivity levels of less than 20 pCi is feasible. Coupled with a Raman spectrometer the balance serves as a stable ‘‘platform’’ for the study of the chemistry of microparticles, and both qualitative and quantitative analysis of microdroplet chemistry are demonstrated for binary droplets of 1‐octadecene and 1‐bromoctadecane.

Surface-Enhanced Raman Spectroscopy of Bacteria and Pollen

A technique for distinguishing biological material based on surface-enhanced Raman scattering (SERS) is reported in this work. Of particular interest is biological material that can be airborne. Silver colloidal particles with diameters in the range 10 to 20 nm and with a characteristic ultraviolet–visible (UV-VIS) absorption band at 400 nm were used to obtain SERS spectra of Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium bacteria and a number of tree and grass pollens (Cupressus arizonica (cypress), Sequoia sempervirens (redwood), Populus deltoides (cottonwood), Poa pratensis (Kentucky bluegrass), and Anthoxanthum odoratum (sweet vernal grass)). While differences in the SERS spectra among the bacteria were small, we found that the pollen spectra we analyzed could readily be distinguished from the bacteria spectra, and there were significant differences between pollen from different families. In order to obtain reproducible results, we studied the parameters controlling the interaction between the analyte and the nanoscale metallic surface. Our results show that the volume ratio of analyte to colloidal particles must be within a narrow range of values to optimize the signal-to-noise ratio of the SERS spectra and minimize the fluorescence from the analyte. Also, we found that the time-dependent behavior of colloidal/bacterial suspensions (or adsorption rate of the silver colloid particles on the bacteria) is strongly dependent on pH, density of bacteria in solution, and even, to some extent, the type of bacteria.

Measurement of the thermophoretic force by electrodynamic levitation: Microspheres in air

The thermophoretic force on single microspheres has been measured over a wide range of Knudsen numbers and particle thermal conductivities for solid and liquid spheres in air. The microspheres were dioctyl phthalate (DOP) droplets, metallic nickel, polystyrene latex (PSL), and glass. This covers a thermal conductivity ratio (gas to particle) range of 2 × 10−4−0.17. In a companion paper, results are reported for monatomic (helium) and polyatomic gases (carbon dioxide and air) to examine the effects of gas properties. The measurements were accomplished by levitating the particle between heated and cooled plates mounted in a vacuum chamber. Light-scattering phase functions were used to determine the size of all materials except the nickel spheres, and for the highly absorbing metallic spheres the size was obtained by measuring the marginal stability limit of the particle. The apparatus developed for the research and the experimental techniques are discussed, and the experimental results in the Knudsen number range 0.05–20 are compared with previously proposed theories. For particle Knudsen numbers greater than about 10, the effects of the temperature jumps (thermal slip) at the heated and cooled plates become significant. The data in the transition regime overlap Loyalkas (1992, J. Aerosol Sci. 23, 291–300) solution of the linearized Boltzmann equation for hard-sphere molecules, and the data in the slip regime agree with Brocks (1962, J. Colloid Sci. 17, 768–780) theory. The effects of particle thermal conductivity are shown to be nearly negligible in the Knudsen regime, for the data for particles having greatly different thermal conductivities overlap.

Comparison of Psychro-Active Arctic Marine Bacteria and Common Mesophillic Bacteria Using Surface-Enhanced Raman Spectroscopy

Psychro-active bacteria, important constituents of polar ecosystems, have a unique ability to remain active at temperatures below 0 °C, yet it is not known to what extent the composition of their outer cell surfaces aids in their low-temperature viability. In this study, aqueous suspensions of five strains of Arctic psychro-active marine bacteria (PAMB) (mostly sea-ice isolates), were characterized by surface-enhanced Raman spectroscopy (SERS) and compared with SERS spectra from E. coli and P. aerigunosa. We find the SERS spectra of the five psychro-active bacterial strains are similar within experimental reproducibility. However, these spectra are significantly different from the spectra of P. aeruginosa and E. coli. We find that the relative intensities of many of the common peaks show the largest differences reported so far for bacterial samples. An indication of a peak was found in the PAMB spectra that has been identified as characteristic of unsaturated fatty acids and suggests that the outer membranes of the PAMB may contain unsaturated fatty acids. We find that using suspensions of silver colloid particles greatly intensifies the Raman peaks and quenches the fluorescence from bacterial samples. This technique is useful for examination of specific biochemical differences among bacteria.

Electrokinetics of concentrated suspensions and porous media: 2. Moderately thick electrical double layers

Abstract An earlier analysis of electrokinetics of concentrated suspensions of spheres and of unconsolidated porous media (M. W. Kozak and E. J. Davis, J. Colloid Interface Sci.127, 497 (1989)), which was limited to thin electrical double layers, is extended to include thicker double layers, using an alternate solution method. A unit cell model is invoked to take into account the effects of surrounding spheres on the primary particle, and the analysis is constrained to nonoverlapping double layers. The solution reduces to our earlier result when the double layer is thin, and in the limit as the porosity tends to unity the solution of H. Ohshima, T. W. Healy, and L. R. White (J. Chem. Soc. Faraday Trans. 279, 1613 (1983)) for a single sphere in an infinite medium is recovered. Finally, the solution is shown to reduce to the result of S. Levine and G. H. Neale (J. Colloid Interface Sci.47, 520 (1974)) when the zeta potential is low.

Electrokinetic phenomena in fibrous porous media

Abstract This analysis of electroosmosis and electrophoresis associated with an array of circular cylinders extends to fibrous porous systems Henrys theory (D. C. Henry, Proc. R. Soc. London Ser. A 133, 106, 1931) for the electrophoresis of a single isolated sphere and an analysis of electrokinetic phenomena for a swarm of spherical particles (S. Levine and G. H. Neale, J. Colloid Interface Sci. 47, 520, 1974). The fluid flow is considered to be normal to the axes of the cylinders. A correction factor for Smoluchowskis classic theory (M. Smoluchowski, Z. Phys. Chem. 93, 129, 1918) for the electrophoretic velocity is obtained in terms of the porosity of the fiber matrix, the electric double layer thickness, the fiber surface potential and the effects of the surface potential of surrounding fibers. Analytical asymptotic expansions for the correction factor are developed, and the full solution is examined to determine the effects of the system parameters on the electroosmotic (or electrophoretic) velocity. For collapsed double layers the electroosmotic velocity reduces to Smoluchowskis result, and for expanded double layers the velocity decreases significantly as the porosity decreases.

The effects of gas and particle properties on thermophoresis

This paper extends a companion paper on measurements of the thermophoretic force for solid and liquid spheres in air. Using electrodynamic levitation to maintain a microsphere in the space between heated and cooled plates, the effects of the system parameters, gas and particle thermal conductivities, the magnitude of the temperature gradient and other variables are explored. Experimental data obtained using helium, air and carbon dioxide with nickel, polystyrene latex and glass solid spheres and with dioctyl phthalate liquid droplets are compared with available theories. The polarity and charge density of the microparticle are shown to have a large effect on the thermophoretic force, particularly in helium.

The effects of atmospheric organics on aqueous droplet evaporation

Abstract Difunctional organic oxygenates are shown to have substantial effects on the evaporation rates of aqueous solution droplets. These compounds have been detected in both urban and rural environments, and their particulate concentrations assessed by investigators over the past twenty years [Grosjean (1977) Ozone and other Photochemical Oxidants. National Academy of Sciences, Washington, D.C.; Grosjean et al. (1978) Environm. Sci. Technol., 12, 313–317; Rogge et al. (1991) Atmos. Environ. 27A, 1309–1330]. To understand the effect of difunctional organic oxygenates on the transport of water at the air/water interface of single aqueous droplets, the evaporation rates of aqueous systems containing model organics were measured. The influence of organics of this type on the evaporation rate of water was also compared to aqueous solutions containing ammonium sulfate and sodium dodecyl sulfate. Light-scattering techniques were used to measure the droplet size as a function of time for electrodynamically levitated single microdroplets under conditions of controlled humidity and temperature. These techniques consisted of phase functions (angular scattering data) and morphological resonance spectra. The evaporation rates for all the aqueous systems are compared to that of pure water and are found to be reduced by up to an order of magnitude.

Microparticle raman spectroscopy of multicomponent aerosols

Abstract Inelastic light scattering measurements (Raman and fluorescence spectroscopies) offer the possibility of chemically characterizing micrometer-size particles. Quantitative interpretation of such spectra is complicated by morphological or structural resonances associated with elastic and inelastic scattering. This paper explores the feasibility of using Raman spectroscopy to determine the composition of multicomponent microdroplets by examining the effects of such resonances and microdroplet size on the Raman signal. Raman scattering from electrodynamically suspended droplets of 1-octadecene and 1-bromohexadecane was found to be enhanced by morphology-dependent resonances. By applying Mie theory to the elastic scattering many of the increased Raman intensities were found to coincide with the incident beam resonances, but other peaks can be attributed to “output” resonances. It was also demonstrated that intensity fluctuations associated with resonances can be minimized by increasing the signal integration time, which permits changes in composition to be determined. In this way, the distillative evaporation of binary component droplets of 1,8-dibromooctane and hexadecane was followed by recording the Raman spectrum of the CBr bond. The effects of the system parameters on the interpretation of Raman spectra are elucidated.