Mark Seaver

Size and Fluorescence Measurements for Field Detection of Biological Aerosols

This paper describes an instrument developed to monitor the biological fraction of an aerosol. The instrument simultaneously sizes individual particles in a flowing air stream and measures their total fluorescence following excitation at 266 nm. Laboratory data show that these two parameters enable discrimination between individuals of certain bacterial species. Field measurements are presented in which bacterial samples were aerosolized and subsequently detected 800 m downwind.

Bragg grating-based fibre optic sensors in structural health monitoring

This work first considers a review of the dominant current methods for fibre Bragg grating wavelength interrogation. These methods include WDM interferometry, tunable filter (both Fabry–Perot and acousto-optic) demultiplexing, CCD/prism technique and a newer hybrid method utilizing Fabry–Perot and interferometric techniques. Two applications using these techniques are described: hull loads monitoring on an all-composite fast patrol boat and bolt pre-load loss monitoring in a composite beam in conjunction with a state-space modelling data analysis technique.

Aerosol Characteristics in a Subway Environment

This paper presents an attempt to characterize biological and nonbiological aerosols in a subway environment. This opportunity to study a subway station atmosphere was approached as a collaboration of different organizations within the Department of Defense (DoD) and a suite of instruments was assembled for real-time monitoring, sample collection, and subsequent sample analysis. Based on ultraviolet (UV) fluorescence, aerosols of a biological nature were found to comprise a small fraction of the total aerosols (typically <1%). The total number concentration of aerosols exhibits a diurnal cycle that depends on the station usage. Several bacterial species were identified using polymerase chain reaction (PCR) analysis. The most common element in the aerosols is iron. Sodium chloride is also prevalent in the aerosol mass.

Excitonic optical nonlinearity in quantum‐confined CuCl‐doped borosilicate glass

The nonlinear refractive index n2 is reported for quantum‐confined CuCl microcrystallites in borosilicate glass near the Z3 excitonic resonance. Induced index changes were estimated using Kramers–Kronig analyses of absorption bleaching data from 370 to 388 nm. The nonlinearity was found to increase with increasing particle radius over the range 22 to 34 A, in agreement with theoretical predictions.

ArF excimer laser multiphoton-ionization mass spectrometry of organic molecules

Abstract Mass spectra are obtained for 27 organic molecules following irradiation with focussed 193-nm (6.42-eV) photons from an ArF excimer laser. The molecules chosen represent several classes of organic compound. Comparisons between the multiphoton-ionization mass spectra and low-energy electron-impact mass spectra indicate that the dominant ionization pathway is non-resonant two-photon absorption. Data for benzaldehyde and CS 2 provide clear exceptions to this mechanism. These two molecules exhibit enhanced fragmentation owing to the presence of one-photon resonances.

Use of data-driven phase space models in assessing the strength of a bolted connection in a composite beam

This work explores the role of empirical dynamical models in deducing the level of preload loss in a bolted connection. Specifically, we examine the functional relationship between data gleaned from locations on either side of the connection using nonlinear predictive models. This relationship, as quantified by a measure of prediction error, changes as a function of bolt loosening, thus allowing both the presence and magnitude of the axial load to be identified. The models are based on a phase space portrayal of the system dynamics and require only that the structures response be low dimensional. The technique is demonstrated experimentally on a composite beam fastened to steel plates with four instrumented bolts. Results are compared to a similar approach using an auto-regressive (AR) modeling technique.

Acoustic levitation in a free-jet wind tunnel

In this article we describe a laboratory device which provides stable acoustic levitation in the free‐jet working section of a horizontal wind tunnel. This apparatus combination provides a covenient means for conducting experiments while maintaining a wall‐less environment and relative motion between particle(s) and gas. This facility has been calibrated against the known evaporation of pure water. Applications of this facility to studies of the evaporation of volatile nonideal liquid mixtures, the condensation of water vapor onto an evaporating drop of 1‐butanol and the remote thermometry of water drops using laser‐induced fluorescence are described. We have successfully levitated drops of liquid ranging in diameter from ∼150 μm to ∼3 mm in laminar air streams whose velocities span the range from 25 to 350 cm/s. The lower limit in size is air speed dependent. Measurements of the evaporation of water agree exactly with literature predictions illustrating the lack of perturbations due to the acoustic field.

Continuous, rapid biological aerosol detection with the use of UV fluorescence: Outdoor test results

This report describes the design and principles of operation of an instrument for separately monitoring ambient aerosol concentrations of biological and nonbiological composition. A method of outdoor testing, and recent results, are also described. The single-particle fluorescence analyzer (SPFA) draws ambient air into an optical chamber through a sampling nozzle. Detection signals from each aerosol particle consist of an elastic scattering response as well as fluorescent emission in two distinct spectral bands. Differentiation of biological aerosols is based on the relative intensity of ultraviolet (UV) fluorescence measured from the particles following excitation with a 266-nm-wavelength laser pulse. During outdoor tests two types of organisms were released as aerosol sprays: Erwinia herbicola vegetative cells and Bacillus subtilis spores. The SPFA participated in 39 of a total set of 40 outdoor tests. Twenty-seven tests released E. herbicola and nine released B. subtilis. Analysis of these results indicates that a reliability of detection of about 90% for E. herbicola, and of about 87% for B. subtilis were achieved under the conditions of the tests, with a sensitivity near 1 particle/l. Some tests involved release of nonbiological aerosols that did not result in a significant change in fluorescent signals in the instrument.

Noncontact fluorescence thermometry of acoustically levitated waterdrops

Noncontact thermometry based on the fluorescence excitation spectrum of aqueous Eu(3+) (EDTA) near 579 nm allows us to measure the temperature of an evaporating drop of water. The results for drop diameters in the 500-microm-3-mm range confirm theoretical temperature predictions for steady state evaporation. Calibration of the excitation spectrum in a constant temperature cell indicates that +/-1.0 degrees C resolution is possible for temperatures below 20 degrees C. The spectrum depends only on the solution temperature when Eu(3+) (EDTA) concentrations are below 1 x 10(-3) M and when the solution pH is between 4.0 and 10.0. Excitation spectra from levitated waterdrops contain additional noise which degrades the temperature resolution to +/-1.2 degrees C. With this technique we are able to follow the temperature change in an evaporating drop of water as a monolayer of 1-octadecanol forms on the surface and retards the evaporation.

Detecting impact damage in experimental composite structures: an information-theoretic approach

This work describes a procedure for detecting the presence of damage-induced nonlinearities in composite structures using only the structures vibrational response. Damage is assumed to change the coupling between different locations on the structure from linear to nonlinear. Utilizing concepts from the field of information theory, we are able to deduce the form of the underlying structural model (linear/nonlinear), and hence detect the presence of the damage. Because information theoretics are model independent they may be used to capture both linear and nonlinear dynamical relationships. We describe two such metrics, the time delayed mutual information and time delayed transfer entropy, and show how they may be computed from time series data. We make use of surrogate data techniques in order to place the question of damage in a hypothesis testing framework. Specifically, we construct surrogate data sets from the original that preserve only the linear relationships among the data. We then compute the mutual information and the transfer entropy on both the original and surrogate data and quantify the discrepancy in the results as a measure of nonlinearity in the structure. Thus, we do not require the explicit measurement of a baseline data set. The approach is demonstrated to be effective in diagnosing the presence of impact damage in a thick composite sandwich plate. We also show how the approach can be used to detect impact damage in a composite UAV wing subject to ambient gust loading.