Rudolf Karch

Bipolar charging of ultrafine particles in the size range below 10 nm

Bipolar diffusion charging of aerosol particles in the size range of 2.3–10 nm was studied experimentally. Charging probability for WOx nanoparticles as a function of particle size was measured using an 241Am aerosol neutralizer for aerosol charging and a tandem DMA (Reischl type) with a VIE-06 Faraday cup electrometer for aerosol measurement. In spite of small deviations in the predictions from the theory of N. Fuchs on bipolar aerosol charging at 2–4 nm range, the theory was seen to be valid. Correction of the deviations may be performed by adjusting the free parameters in the theory (ion mass and mobility). As a rule of thumb, the charging probability of 2 nm particles was seen to be around 0.6% for negative and around 0.4% for positive particles.

Staged Growth of Optimized Arterial Model Trees

AbstractThere is a marked difference in the structure of the arterial tree between epi- and endocardial layers of the human heart. To model these structural variations, we developed an extension to the computational method of constrained constructive optimization (CCO). Within the framework of CCO, a model tree is represented as a dichotomously branching network of straight cylindrical tubes, with flow conditions governed by Poiseuilles law. The tree is grown by successively adding new terminal segments from randomly selected points within the perfusion volume while optimizing the geometric location and topological site of each new connection with respect to minimum intravascular volume. The proposed method of “staged growth” guides the generation of new terminal sites by means of an additional time-dependent boundary condition, thereby inducing a sequence of domains of vascular growth within the given perfusion volume. Model trees generated in this way are very similar to reality in their visual appearance and predict diameter ratios of parent and daughter segments, the distribution of symmetry, the transmural distribution of flow, the volume of large arteries, as well as the ratio of small arterial volume in subendocardial and subepicardial layers in good agreement with experimental data. From this study we conclude that the method of CCO combined with staged growth reproduces many characteristics of the different arterial branching patterns in the subendocardium and the subepicardium, which could not be obtained by applying the principle of minimum volume alone.

Voronoi Polyhedra Analysis of Optimized Arterial Tree Models

AbstractTopological and metric properties of Voronoi polyhedra (VP) generated by the distal end points of terminal segments in arterial tree models grown by the method of constrained constructive optimization (CCO) are analyzed with the aim to characterize the spatial distribution of their supply sites relative to randomly distributed points as a reference model. The distributions of the number Nf of Voronoi cell faces, cell volume V, surface area S, area A of individual cell faces, and asphericity parameter α of the CCO models are all significantly different from the ones of random points, whereas the distributions of V, S, and α are also significantly different among CCO models optimized for minimum intravascular volume and minimum segment length (p < 0.0001). The distributions of Nf, V, and S of the CCO models are reasonably well approximated by two-parameter gamma distributions. We study scaling of intravascular blood volume and arterial cross-sectional area with the volume of supplied tissue, the latter being represented by the VP of the respective terminal segments. We observe scaling exponents from 1.20 ± 0.007 to 1.08 ± 0.005 for intravascular blood volume and 0.77 ± 0.01 for arterial cross-sectional area. Setting terminal flows proportional to the associated VP volumes during tree construction yields a relative dispersion of terminal flows of 37% and a coefficient of skewness of 1.12.

Smog chamber studies of aerosol formation in atmospheric mixtures

During a joint experiment of the Department of Chemical Engineering (California Institute of Technology) and the Institute of Experimental Physics (University of Vienna), measurements have been performed to study the dynamics of particle inception in photochemical smog. The goal of this study was to obtain an understanding of the fundamental mechanisms in the formation of organic aerosols during atmospheric photochemical reactions. This paper presents results with new approaches to mobility analysis for aerosol-measurements that have been performed to trace the dynamics of ultrafine aerosol particles in smog chamber experiments.