Brian D. Swanson

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.

Instrument for studies of homogeneous and heterogeneous ice nucleation in free-falling supercooled water droplets

We have developed an instrument to study the homogeneous and heterogeneous freezing of droplets in free fall. The advantages of this technique are high repetition rates, telemicroscopic imaging of frozen and unfrozen droplets, and the elimination of possible contamination and nucleation effects induced by substrates. Droplets are ejected at a rate of about 5 per s from a generator at the top of a temperature controlled freezing tube. They fall in a stream down the center of the tube as their images are recorded using video-telemicroscopy. The fraction of drops frozen is measured as a function of height (and, hence, as a function of temperature) by illuminating slices of the stream with linearly polarized laser light and monitoring the depolarization of the backscattered light; ice particles depolarize the scattered light while the liquid droplets do not. The use of depolarization for phase discrimination is unique in this context. We have demonstrated the usefulness of our instrument with pure water dropl...

Breakup of levitated frost particles

We have studied the sublimation and breakup of single 100-200 μm frost particles levitated electrically at temperatures in the range -2°C to -30°C. Breakup rates were largely independent of temperature and humidity in this range but strongly dependent on particle shape. Irregular particles of high aspect ratio were most likely to break up. Sublimation (evaporation) was accompanied by an increasing aspect ratio. A linearized sublimation model, presented in the appendix, accounts for the observation that thin neck regions were not subject to enhanced sublimation rates. Estimates of the forces involved in breakup suggest that the breaking strength of these frost particles is considerably less than that of bulk ice. We discuss possible implications of our results for ice particle multiplication in clouds.

Non-isothermal droplet evaporation and condensation in the near-continuum regime

The problem of non-isothermal quasi-steady state evaporation and condensation of aerosol spheres is examined to determine the rates of simultaneous heat and mass transport in the Knudsen (transition) regime. New expressions for the mass and heat 5uxes are obtained that show explicitly the dependence of the rate processes on the Knudsen number, the accommodation coe6cients for mass and energy transport and on the molecular weight ratio of the vapor and gas molecules. The analysis, based on the solution of the Boltzmann equation by the method of Grad for Maxwellian molecules, is shown to yield results in the continuum regime (Kn1) in reasonable agreement with classical methods based on continuum theory and with measured water droplet evaporation rates in dry air. Computations of heat and mass transport rates for ice sublimation for upper tropospheric and stratospheric conditions for sizes that correspond to the continuum and transition regimes show that the process is very nearly isothermal. Parametric studies explore the e<ects of temperature, humidity and accommodation coe6cients on the heat and mass transport processes. Although the method of Grad is known to fail in the free-molecule regime, the results agree with more rigorous theoretical solutions for isothermal processes in the near-continuum regime and with an earlier solution for hard sphere molecules in the near-continuum regime. It is shown that 5ux-matching or resistance models used for the transition regime do not show the correct dependence on the Knudsen number and other parameters. c

Laboratory Measurements of Light Scattering by Single Levitated Ice Crystals

Abstract The authors have measured the differential light-scattering cross sections and phase functions of single vapor-grown hexagonal ice particles levitated in an electrodynamic balance. The ice particles, grown at temperatures −5° > T > −10°C, were typically ≈50 μm in diameter and tended to orient with the c axis either nearly vertical (parallel to the scattering plane normal) or horizontal (in the scattering plane). Helium–neon laser light scattered by a levitated crystal was collected in the angular ranges 20°–65° and 115°–160° with a 1024-element linear photodiode array with an angular resolution of about 0.05°. The particle size and orientation were measured a few seconds before and after the scattering measurements with top- and side-view video telemicroscopes. Three basic features are found in the scattering from vertically aligned crystals: (i) a strong “halo” peak between about 21° and 35°, (ii) a secondary peak with ripple structure between about 30° and 70°, and (iii) a weaker peak in the ba...

Trapping two-particle arrays in a double-ring electrodynamic balance

A new technique has been developed to explore the characteristics and dynamics of the electrodynamic balance (EDB). It is demonstrated that by trapping a pair of microparticles, the electric field of and EDB can be characterized and particle stability can be investigated. The electric field in the neighborhood of the null-point was examined by comparing the oscillatory motion of the two-particle system with a theoretical analysis. In addition, the relevant balance constants were evaluated by five methods: (i) determination of the stabilization strength constant, C1, using measurements on two-particle arrays, (ii) determination of the levitation strength constant, C0, using measurements on single particles of known mass and charge, (iii) computation of C1 and C0 by solving the three-dimensional Laplace equation for the non-axisymmetric electrode system, (iv) computation of C0 using a ring charge simulation technique, and (v) determination of the ratio C1C0 by measurements of the marginal stability limit. The results of the different methods are compared and shown to be consistent.

How Well Does Water Activity Determine Homogeneous Ice Nucleation Temperature in Aqueous Sulfuric Acid and Ammonium Sulfate Droplets

Abstract Frozen fraction measurements made using a droplet free-fall freezing tube apparatus are presented and used, along with other recent laboratory measurements, to evaluate how well both the water activity idea and the translated melting-point curve idea of Koop et al. predict homogeneous freezing-point temperatures for aqueous ammonium sulfate and sulfuric acid solution droplets. The new freezing-point temperature datasets agree with the previous lowest-temperature results for both solutes. The lowest measured freezing-point temperatures for aqueous ammonium sulfate solutions agree with a curve shaped like the translated melting-point curve. However, those for aqueous sulfuric acid solutions are significantly lower than predicted by the translated melting-point curve idea, and a single water activity freezing-point temperature curve does not represent the lowest-temperature freezing-point temperature data for both solutes. A linear extrapolation of the new aqueous sulfuric acid solution freezing dat...