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406 13 Nanoaerosol

13.4.2 Adhesion Efficiency and Nanoaerosol Thermal
Rebound

In conventional filtration theory, the adhesion efficiency is assumed unity (η ad ≡ 1).
However, it is not certain for nanoaerosol particles. Conventional filtration theory
(in Sect. 6.5) indicates that nanoaerosol particle filtration efficiency increases
inversely with particle size. Base on this hypothesis, filtration efficiency of nano-
aerosol particles can reach 100 % for a properly designed filter. In reality, however,
there should be a critical size from which filtration efficiency drops with the
decrease of particle diameter (see Fig. 13.4). Otherwise, gas molecules, which are
indeed extremely small particles, should be captured by filters resulting in no
separation of aerosol particles and the carrier gas. Knowledge of this critical size is
important to the design of effective nanoaerosol filters.
When an aerosol particle impacts on a filtration surface, there is an interfacial
adhesion force attempting to hold them together. When the adhesion force is strong
enough to offset the outgoing momentum at the end of impact, the particle is
captured by the filtration surface. It has been well accepted that aerosol particles
always stick on the surface in contact. However, this may not be true for nano-
aerosol particles because the impact between a solid nanoparticle and a solid surface
is most likely elastic because of the small contact area, high speed, and unique
properties of nanoaerosol [10]. As a result, nanoparticles may rebound from the
filtration surface.
Most researchers (e.g., [60]) assume that the thermal speed of nanoaerosol
particles follow the Maxwell–Boltzmann distribution, which is described in
Eq. (13.18).

m mv im
3=2 2
2
f ðv im Þ¼ 4pv exp ð13:18Þ
im
2pkT 2kT

Fig. 13.4 Filtration efficiency
vs aerosol particle diameter
(not in scale)