# A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows
## 3 Specific Measurement Methods
### Experimental Methods for Sub-micro- and Nano-particle Laden Gas Flow  
Submicron- and nanometer-sized particle-laden gas flows are a common phenomenon in nature and also have been widely studied in the context of many engineering problems, including advanced nanomaterial manufacture, engineering thermophysics, chemical engineering, inhalation toxicology, medical and pharmaceutical manufacturing [44; 45; 46; 47]. Thus, it is very important to study sub-micro- and nano-particle-laden flows to provide theory, models or measurement techniques for these engineering problems. As described above, the scattering ranges of sub-micro- and nano-particles are mainly within the Rayleigh and Mie scattering regime, in which particle scattering intensity is so weak that light signals cannot be reliably acquired by current apparatuses. However, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) can capture the sub-micro- and nano-particle morphology, as they are sampled on substrates (e.g., holey carbon support films). Moreover, the very slight inertia of submicron- and nanometer-sized particles results in the invalidation of particle sieving, using mechanical or aerodynamic methods. The measurement techniques for nano-particles usually require sampling, and to date, no online technique (e.g., for measuring nano-particle velocity) has emerged. It is important to note that the particle diameter acquired using present nano-particle sizers is either an aerodynamic or an electrical mobility diameter; in which case, the electrical mobility diameter of a real spherical or non-spherical test particle is the mobility-equivalent diameter of an ideal spherical particle. Likewise, the aerodynamic diameter of a test particle is the inertia- and settling-velocity-equivalent diameter of an ideal spherical particle in air, with a density of 1000 kg/m\({}^{3}\). Nano-particles are fantastic tracers, even in the supersonic and hypersonic boundary layer, because of their small size and inertia. [PERSON] et al. [48] used nano-particle tracers in optical flow methods to study compressible turbulence structures at high temporal-spatial resolution. However, this comprehensive technology merely estimates the ensemble-average characteristics of nano-particles, but not its single particle ones. An SMPS or scanning electrical mobility spectrometer (SEMS) [49], FMPS and electrical low pressure impactor (ELPI) are three of the most widely-used commercial measuring instruments used to characterize the concentration and distribution of submicron- and nanometer-sized particles, especially for studying aerosols and particle-laden flows [47; 50; 51]. A general particle size distribution measured using FMPS is presented in the Figure 11, while the practical results of SMPS are shown in Figure 8. Both SMPS and FMPS characterize particle size depending on the mobility of a charged particle passing through a cylindrical electric field. In an SMPS, sampled particles are first neutralized using a bipolar charger (e.g., Kr85 or soft X-ray) in a neutralizer to attain a well-defined steady-state distribution of positively- and negatively charged-particles [2; 53; 52]. In contrast, particles in a FMPS are successively charged, using a negative and a positive unipolar corona charger to achieve a predictable charge distribution. A theoretical model to describe the particle charge distribution in an SMPS neutralizer was proposed by [PERSON] [52], which is based on an approximation of [PERSON] diffusion theory for submicron- and nanometer-sized particles [2]. The fractions of particles carrying different amounts of charge, according to this model, are given by:  
\[f_{c}(N)=10^{\left[ \begin{smallmatrix}5&a_{i}(N)(\log\frac{D_{p}}{\mathrm{nm}}) ^{i}\\ \end{smallmatrix}