# A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows
## 3 Specific Measurement Methods
### Optical Particle Characterization
#### 3.1.1 Particle Image Velocimetry  
PIV is a non-intrusive measurement technique for determining the instantaneous flow field, based on a pair of time-correlated particle images having a short time lag of several to hundreds of microseconds. PIV is able to measure the velocity field, as well as other measurable quantities in real time, such as particle concentration distribution and temperature. Thus, PIV has been widely applied to fields as diverse as aerospace propulsion, biomedicine, aeronautics and micro-electromechanical systems (MEMS). During a typical PIV measurement, the tracer particles are first broadcast into the fluid and excited by a laser pulse, so that the scattered light from these particles is recorded using a CCD. The laser pulse and the CCD are triggered synchronously. Pairs of particle images are shot at short time intervals. The particles' shifts can be calculated using cross-correlation or self-correlation, while the velocity vector of the interrogation window is calculated by averaging the entire particles' shifts within the image area. Finally, a velocity field is reconstructed, after determining all velocity vectors. Figure 4 outlines the steps involved in the PIV technique. In the early 1980s, it was shown by [PERSON] [11] that PIV could be used for the measurement of flow fields. Over the last 20 years, techniques using analog image acquisition and post-processing have been gradually replaced by techniques using digital image capture, such as CCD [3]. Because PIV is popular for fluid mechanical studies, there are abundant reviews on the various advances in PIV, outlined in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Times** & **Authors** & **Reference** \\ \hline \multirow{6}{*}{Before 1990} & & \"Modern optical techniques in fluid mechanics\", \\  & & Ann. Rev. Fluid Mech., vol. 16, no. 16, pp. 223–244, 1984. \\  & & \"Multi-point optical measurements of simultaneous vectors in unsteady flow—a review\", Int. Journal of Heat and Fluid Flow, vol. 7, no. 2, pp. 127–145, 1986. \\  & & \"Particle-imaging techniques for experimental fluid mechanics\", \\  & & Ann. Rev. Fluid Mech., vol. 23, no. 23, pp. 261–304, 1991. \\ \hline \multirow{6}{*}{In the 1990s} & & Bibliography of particle image velocimetry using imaging \\  & & methods: 1917–1995, TAM Report 817, UILU-ENG-96-6004, \\  & & \"University of Illinois (USA), 1996. \\ \cline{1-1}  & & \"Particle image velocimetry: a review\", Proc. Inst. Mech. Eng. C, vol. 211, no. 1, pp. 55–76, 1997. \\ \hline \multirow{2}{*}{After 2000} & & \"Twenty years of particle image velocimetry\", Exp. Fluids, vol. 39, no. 2, pp. 159–169, 2005. \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relevant reviews of advances in particle image velocimetry over time. The references covered in each of these reviews total more than 100, and there are more than 1200 references on PIV published from 1917 to 1995 collated by [PERSON]. The major technical advances over the last 30 years for this technique were presented by [PERSON] et al. [3], as shown in Figure 5. Most recently, high-time resolution PIV and holographic PIV methods have been commercialized, supported by CMOS cameras and high-repetition rate Nd:YAG and Nd:YLF lasers. Figure 4: Schematic of particle imaging velocimetry (PIV) and results: (**a**) showing steps involved in PIV; (**b**) vorticity contours of the wake behind two rotating circular cylinders using PIV [18]. The measurement of instantaneous velocity fields using PIV, effectively pushed forward research on unsteady flow, with PIV results able to directly verify the numerical results of computational fluid dynamics. Since its appearance, PIV has gradually displaced LDA, because it is able to examine the velocity of both the dispersed and carrier phases. At low particle concentrations, a normal laser is sufficient to scatter light from relatively large particles in the continuum regime, and high-contrast particle images can be captured using a basic camera.