Introduction to Nanoscience and Nanotechnology. Chris Binns

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Introduction to Nanoscience and Nanotechnology - Chris Binns


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background of nanoparticles in which we live. The effect of naturally occurring nanoparticles on the environment is an enormous multidisciplinary subject and a rigorous discussion is well beyond the scope of this book. It is an important hot topic, however, as it encompasses climate change and nanoparticles are implicated in many of the feedback mechanisms involved in the Gaia hypothesis that treats the Earth as a living organism. The aim of this chapter is to describe, in general terms, where the nanoparticles come from and, as in the previous chapter, emphasize the special nature of particles belonging to the nanoworld (<100 nm – Figure I.1).

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      Source: (a) US Geological Survey. (b) Reproduced with permission from the government of British Columbia. (d) Reproduced with the permission of the Spanish Society for Microbiology from D. Schüler [1]. (e) NASA.

      It is easy to show [2] that for large (micron‐sized or more) particles with a diameter d and a density ρp, their terminal velocity due to gravity in a still gas with a density ρg is:

      (2.1)equation

      where η is the viscosity of the gas (η = 1.81 × 10−5 Pa s for air at Standard Conditions) and g is the acceleration due to gravity. For 1 μm diameter particles with a typical density (1000–5000 kg/m3), this gives ~0.1 mm/s. The equation, however is only valid for relatively large particles. In its derivation, it is assumed that the gas velocity at the particle surface is zero, which is invalid for very small particles whose size is less than the mean‐free path of the gas molecules. To put it crudely, very small particles “slip” through the gaps between the gas molecules and fall faster than predicted by the equation. As the particles get smaller, an increasing slip correction factor needs to be applied and this can get to be a factor of 10 or more. Even so, the fact that the terminal velocity decreases as d2 ensures that small particles do drop more slowly. Applying the slip correction factor to 10 nm diameter particles falling through the air gives a terminal velocity of ~0.1 μm/s (or about a meter every four months). For all practical purposes, nanoparticles can be assumed to be suspended in the atmosphere. This discussion however is only relevant to the settling of particles by gravity. Accumulation, rainout, and washout will remove them more rapidly.

image

      Source: Reproduced with the permission of Wiley from J. J. Seinfeld and S. N. Pandis [3].


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