Aggregate size distribution evolution for Brownian coagulation-sensitivity to an improved rate constant.

Brownian motion causes small aggregates to encounter one another and grow in gaseous environments, often under conditions in which the coalescence rate (say, spheroidization by "sintering") cannot compete. The polydisperse nature of the aerosol population formed by this mechanism is typically accounted for by formulating an evolution equation for the joint PDF of the state variables needed for describing individual particles. In the simple case of fractal-like aggregates (prescribed morphology and state, characterized just by the number of aggregated spherules, or total aggregate volume), we use the quadrature method of moments and Monte Carlo simulations to show that recent improvements in the laws governing free molecule regime coagulation frequency (rate "constant") of these aggregates cause systematic changes in the shape of the asymptotic aggregate size distribution, with significant implications for the light-scattering power and inertial impaction behavior of such aggregate populations.

Effective diameters for collisions of fractal-like aggregates: recommendations for improved aerosol coagulation frequency predictions.

Fractal-like aggregate (FA-) drag has been previously calculated/correlated/reported, but "mobility diameter" information is not sufficient to make rational calculations of Brownian coagulation rates (for, say, population-balance modeling). Indeed, until now, only conjectures about gyration-radius scaling behavior have been used to predict FA-FA collision cross sections! But such "scaling relations" are untrustworthy even for FA momentum-, energy-, and mass-transfer purposes, and improved FA-collision rate constants (appearing as "kernels" in the coagulation balance integro-PDE) are overdue. Our premise is that FA collision rates in the free-molecule regime can be predicted using a gas-kinetic type formulation. If (a) carrier gas mean free path and FA persistence length are much larger than any characteristic FA size, (b) FA number density is low, (c) FA velocity and position are uncorrelated, and (d) there is a "hard-sphere" interaction between primary particles of different FAs, such a theory is developed/applied here. We introduce an effective collision diameter, , depending on the geometries of the two participating FAs. Quasi-MC calculations are reported for large ensembles of pairs of FAs, each computer-generated using a tunable cluster-cluster (CC)-algorithm. Our results differ from frequently used theoretical estimates based only on FA gyration (or mobility) radii and D(f). They also confirm that, if the size disparity of the colliding FAs is large, obtained by simply assigning individual diameters to each FA are significantly overestimated. Modified collision rate expressions for FA-coagulation modeling are suggested.

In situ light-scattering measurements of morphologically evolving flame-synthesized oxide nanoaggregates.

Nonspherical Al2O3 aggregates produced in a laminar counterflow nonpremixed methane flame were investigated with an in situ laser light-scattering (LLS) technique in combination with a thermophoretic sampling-transmission electron microscope (TS-TEM) method. These flame-synthesized nanoparticles clearly underwent morphological changes following their formation (from precursor trimethylaluminum hydrolysis), mainly as a result of aggregation and sintering processes in the approximately 3.3 x 10(4) K/s heating environment. To characterize this particulate morphological evolution conveniently we made multiangular absolute LLS measurements and interpreted them based on the Rayleigh-Debye-Gans scattering theory for fractal aggregates. Optically determined fractal dimension D(f), mean radius of gyration, aggregate size distribution, and local particle volume fraction phi(p) were found to be consistent with our independent ex situ TS-TEM experiments. D(f) (optically inferred) increased from 1.60 to 1.84 with axial position, confirming the morphological evolution of alumina aggregates owing to finite-rate, spatially resolved high-temperature sintering. An extension of our TS-TEM method was successfully applied, for the first time to our knowledge, to inorganic particles. Phi(p) inferred by means of this ex situ technique generally agreed with that from the in situ LLS technique, supporting our interpretation of both measurements. Moreover, an optically inferred net sintering rate of alumina aggregates approaching the flame was estimated to be consistent with the available TEM data. The LLS methods and results presented here are expected to permit more comprehensive mechanistic analyses of nanoaggregate sintering and coagulation kinetics in such flame environments, ultimately improving the modeling of more-complex (e.g., turbulent, high-pressure) combustion systems involving nanoparticle formation and evolution.

Plates of the dinosaur stegosaurus: forced convection heat loss fins?

It is suggested that the plates along the arched back and tail of Stegosaurus served an important thermoregulatory function as forced convection "fins." Wind tunnel experiments on finned models, internal heat conduction calculations, and direct observations of the morphology and internal structure of stegosaur plates support this hypothesis, demonstrating the comparative effectiveness of the plates as heat dissipaters, controllable through input blood flow rate, temperature, and body orientation (with respect to wind).