Dispersion of Molecular Patterns Written in Turbulent Air.

We study the dispersion of tiny molecular clouds in turbulence by writing patterns in turbulent air and following their deformation in time. The writing is done by fusing O_{2} and N_{2} molecules into NO in the focus of a strong ultraviolet laser beam. By crossing several of these laser beams, patterns that have both small and large scales can be painted. The patterns are visualized a while later by inducing fluorescence of the NO molecules with a second UV laser and registering the image. The width of the lines that make the pattern is approximately 50 µm, a few times the Kolmogorov length η, the smallest length scale in turbulence, while the overall size of the patterns (≈4 mm) is inside the inertial range of the used turbulent jet flow. At small scales molecular clouds disperse under the joint action of molecular diffusion and turbulence. The experiments reveal for the first time this subtle, yet very important interaction. At macroscales (≈200 η) we verify the Batchelor dispersion of objects whose size is inside the inertial range; however, the expected influence of molecular diffusion is smaller than the accuracy of the experiments.

"Puffing" in Sarcoscypha austriaca: Back to Ziegenspeck.

Many discomycetes show the phenomenon of "puffing": the quasi-simultaneous discharge of large numbers of ascospores, rendering them visible to the unaided eye as a whiff of smoke above the fruitbody. Obviously, some kind of synchronization mechanism must be present to make all asci involved burst at about the same time. This mechanism has generally been understood to be of a mechanical nature. For a (small, dung-inhabiting) Ascobolus species, puffing was shown to involve a discharge wave propagating radially outward from a random origin at a speed of about 1.5 cm/s. Using high-speed imaging, we have found no evidence for such a wave in puffing by the (much larger, wood-inhabiting) apothecia of Sarcoscypha austriaca. Rather, extended (but limited) areas seem to become active essentially simultaneously, and within these areas a few percent of the asci discharge their spores randomly. In these large fruitbodies, puffing is found to be preceded by a small but distinct deformation of the apothecium, the geometry of which is such that it increases the lateral compressive stress in the hymenium. The observations suggest that this increase in stress is instrumental in causing many asci to burst quasi-simultaneously in the affected area, a hypothesis essentially already proposed by H. Ziegenspeck, about a century ago.

Dispersion of Droplet Clouds in Turbulence.

We measure the absolute dispersion of clouds of monodisperse, phosphorescent droplets in turbulent air by means of high-speed image-intensified video recordings. Laser excitation allows the initial preparation of well-defined, pencil-shaped luminous droplet clouds in a completely nonintrusive way. We find that the dispersion of the clouds is faster than the dispersion of fluid elements. We speculate that preferential concentration of inertial droplet clouds is responsible for the enhanced dispersion.

Measuring droplet size distributions from overlapping interferometric particle images.

Interferometric particle imaging provides a simple way to measure the probability density function (PDF) of droplet sizes from out-focus images. The optical setup is straightforward, but the interpretation of the data is a problem when particle images overlap. We propose a new way to analyze the images. The emphasis is not on a precise identification of droplets, but on obtaining a good estimate of the PDF of droplet sizes in the case of overlapping particle images. The algorithm is tested using synthetic and experimental data. We next use these methods to measure the PDF of droplet sizes produced by spinning disk aerosol generators. The mean primary droplet diameter agrees with predictions from the literature, but we find a broad distribution of satellite droplet sizes.

Spores do travel.

Model calculations are presented on the horizontal dispersal distance of basidiospores from their source (any typical agaric). The results are compared to old and recent experimental data obtained by sampling on sticky microscope slides placed on soil. I argue that such experimental data alone are insufficient to determine the dispersion kernel because of sampling paucity: Only a minor fraction of the released spores is sampled, and the fate of the rest is unknown. Spore dispersal is determined largely by wind, whereas deposition may be due predominantly to wash-out by rainfall.

Writing in turbulent air.

We describe a scheme of molecular tagging velocimetry in air in which nitric oxide (NO) molecules are created out of O2 and N2 molecules in the focus of a strong laser beam. The NO molecules are visualized a while later by laser-induced fluorescence. The precision of the molecular tagging velocimetry of gas flows is affected by the gradual blurring of the written patterns through molecular diffusion. In the case of turbulent flows, molecular diffusion poses a fundamental limit on the resolution of the smallest scales in the flow. We study the diffusion of written patterns in detail for our tagging scheme which, at short (micros) delay times is slightly anomalous due to local heating by absorption of laser radiation. We show that our experiments agree with a simple convection-diffusion model that allows us to estimate the temperature rise upon writing. Molecular tagging can be a highly nonlinear process, which affects the art of writing. We find that our tagging scheme is (only) quadratic in the intensity of the writing laser.

Monitoring neurotransmitter release using surface-enhanced Raman spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is a promising tool to monitor neurotransmitter release at the single-cell level: it is a sensitive technique that provides structural information of the released compounds and spatial information about their release sites. In this study we demonstrate that depolarization-evoked catecholamine secretion by rat phaeochromocytoma (PC12) cells can be spatially resolved by SERS using silver colloids. A suitable SERS substrate was created by adding silver colloids to the cell culture medium. Nomarski-DIC microscopy combined with reflection confocal laser scanning microscopy showed that the colloids were primarily present on top of the cell membrane. The SERS spectra were successfully corrected for the contribution of cell constituents. Dopamine and noradrenaline were localized by examining the correlation coefficient between spectra and reference catecholamine spectra. Potential improvements of the temporal resolution of the technique are discussed.

Quantitative imaging through a spectrograph. 1. Principles and theory.

Laser-based optical diagnostics, such as planar laser-induced fluorescence and, especially, Raman imaging, often require selective spectral filtering. We advocate the use of an imaging spectrograph with a broad entrance slit as a spectral filter for two-dimensional imaging. A spectrograph in this mode of operation produces output that is a convolution of the spatial and spectral information that is present in the incident light. We describe an analytical deconvolution procedure, based on Bayesian statistics, that retrieves the spatial information while it avoids excessive noise blowup. The method permits direct imaging through a spectrograph, even under broadband illumination. We introduce the formalism and discuss the underlying assumptions. The performance of the procedure is demonstrated on an artificial but pathological example. In a companion paper [Appl. Opt. 43, 5682-5690 (2004)] the method is applied to the practical case of fuel equivalence ratio Raman imaging in a combustible methane-air mixture.

Quantitative imaging through a spectrograph. 2. Stoichiometry mapping by Raman scattering.

The Bayesian deconvolution algorithm described in a preceding paper [Appl. Opt. 43, 5669-5681 (2004)] is applied to measurement of the two-dimensional stoichiometry field in a combustible methane-air mixture by Raman imaging through a spectrograph. Stoichiometry (fuel equivalence ratio) is derived from the number density fields of methane and nitrogen, with a signal-to-noise ratio of approximately 10 in a 600-laser-shot average. Prospects for single-shot Raman imaging are discussed.

Quantitative spectrally resolved imaging through a spectrograph.

A grating spectrograph can be used for spectrally selective two-dimensional imaging if it is operated with a broad entrance slit. The resulting intensity distribution in its exit plane is a one-dimensional convolution of the spatial and spectral distributions of incident light. We present a dedicated deconvolution filter to reconstruct the spatial image from the spectrograph output. The algorithm is illustrated on Raman imaging of an underexpanded dry air jet. Recorded Raman images correspond to density maps convolved with the Raman spectrum of air; the latter essentially acts as a blurring function for the density map. The deconvolution filter combines the individual images recorded in the O2 and N2 Raman bands into a single image of relative air density.