Growth of dust

An adequate answer to these questions must be based on the detailed study of the processes of formation and growth of dust particles in these environments. However, dust formation cannot be considered as an isolated problem because due to their huge absorption cross sections even a small contamination of the atmospheres by circumstellar dust may have a significant influence on the radiative transfer and (via energy- and momentum-coupling) on the thermodynamic and hydrodynamic structure of the dust forming shell. 

The previous sections considered the growth of dust grains into planets from a dynamical point of view. Here we discuss aspects of the chemical evolution that occur at each stage of growth. 

Allelopathy found hopeful utilization in plant protection against parasite weeds. The main world parasitic species are the witchweeds (Striga spp.), broomrapes (Orobanche spp.), and eventually dodder (Cuscuta spp.). Witchweeds and broom-rapes attack many economically important crops especially throughout the semi-arid regions. Early detection of parasitic weed infestation and protection is difficult because of the growth habit of a root parasite and huge production of dust-like seeds viable up to 20 years (Kebreab and Murdoch 1999). 

Much of the trace metal demand of plankton in the open ocean may be supplied by the aeolian transport of dust. If trace metals limit plankton growth, the factors that control the transport of dust will also ultimately control the ocean s biological pump and, hence, atmospheric CO2 levels. This suggests the existence of powerful feedbacks among the processes that control global climate. For example, shifts in climate that lead to regional... 

Cosmic ray particles can work as a trigger of the chain of polymerization of formaldehyde adsorbed (e.g., at the surface of interstellar silicate dust). However, the triggering of the polymerization chain is a necessary but still not sufficient condition for obtaining the interstellar polymers. If the addition of any new link of the chain would require a hit of adsorbed monomer layer by another cosmic ray particle (i.e., if there is no spontaneous growth of chains after they are started by some external factor), the formation of polymers in interstellar space would be highly improbable. Therefore the spontaneous growth of polymer chains near absolute... 

Silicosis occurs in industries in which the air is polluted by silica dust, e g., pottery, metal grinding, sandblasting and mining in rock. The inhaled silica gives rise to the production of diffuse fibrosis in the lungs moreover it facilitates the growth of the tubercle bacillus so that tuberculosis is a possible complication. A special form of silicosis, called anthracosis (black lung), occurs in coal miners who are exposed to a mixed dust, mainly of coal, with a small proportion of silica. 

Results of a comprehensive study of the absolute spectral radiance of the infrared emissions from methane—air expins have been reported (Ref 44). The spectral growth of these expanding flames was recorded with a time resolution of one msec in the spectral range 1.7— 5.0 microns. Time resolved spectra were obtained as a function of stoichiometry, nitrogen dilution and Halon dilution. Similar data are also available for coal dust-air explns. Additional applications of rapid scan IR spectroscopy are discussed in Ref 50. In this work, flare spectra (Mk45, LUU-2B and LUU-2B/B) in the 1.7-4.7 and 9—14 micron regions were studied. The Mk-45 and LUU-2B/B showed similar spectral character with Na and C02 emissions superimposed on a gray body continuum, while LUU-2B flares demonstrated variable emittance properties... 

Hertzberg et al, "The Spectral Growth of Expanding Flames the Infrared Radiance of Methane—Air Ignitions and Coal—Dust—Air , BuMines-R 1-7779 (1973) 45) C.F. Smith,... 

The mass flux ms,Gr is the sum of the reaction product deposited on the liquid film and non-converted material flux, of the dust deposited on the particles coming from separator into granulator and of the attrition mass flux coming from separator into granulator, which leads to the growth of particles. 

This dust separated by pneumatic filters or cyclones can be recycled and used as internal seeds (disperse phase) for the production of new particles or for the layered growth (continuous phase) of existing particles. One effect of the production of internal seeds is the formation of a wide particle size distribution, while the deposition of the re-fed dust yields to growth of particles. 

The average lifetimes of dust grains in the ISM of about 0.5 Gyr have to be compared with a turnaround time of about 2.5 Gyr for the matter cycle between stars and the ISM, which would result in a small depletion S 0.8 of the refractory elements in the ISM into dust, if depletion of the refractory elements in the returned mass from stars was strong and if no accretion of refractory elements onto dust occurred in the ISM. This clearly contradicts the high observed depletion in the ISM. Hence, most of the interstellar dust is formed in the ISM and is not stardust (Draine 1995 Zhukovska et al. 2008). The most likely place for dust growth in the ISM is in the dense molecular clouds (Draine 1990), but the processes responsible for growth are presently unknown. 

Observations of protoplanetary disks indicate that these objects remain optically thick for timescales of millions of years, meaning that a population of dust is sustained for that period of time (see Chapter 9 for a detailed discussion of disk lifetimes and dispersal mechanisms). As will be discussed in Chapter 10, the timescale for dust growth and incorporation into planetesimals is less than this time period. Additionally, the timescale for dust settling is much less than the age of these disks. However, the apparent contradictions between these timescales and the observations can be explained within the context of the processes described thus far. 

---