# On the Micromixing Behavior of a Spinning Disk Reactor for Metallic Cu Nanoparticles Production
## 3 Results and Discussion  
### Experimental Results  
In the first step, two batch runs were carried out at different stirring rates to check the performances in term of minimum size of produced copper nanoparticles using different reducing agents. The results are reported in Figure 2.  
Figure 1: Scheme of the adopted spinning disk rotator (SDR). Figure 2: Results obtained by adopting the stirred tank reactor. It is possible to observe that the size of the produced copper nanoparticles is smallest when sodium borohydride is used at maximum stirring conditions that are equal to 55 nm. As a consequence, for further tests adopting the SDR, sodium borohydride was selected as reducing agent. In the second step, the outcome of the reaction, performed by means of the SDR as a function of the disk velocity, was checked. In this work, two different feeding point configurations were used, that is, feeding by two (nf = 2) or four (nf = 4) injection points, respectively. The configurations are reported in Figure 3.  
For the experiments concerning changes to the disk velocity N, the relevant results of this experimental campaign are reported in Figure 4 and Table 1. In this case, some operating conditions were fixed, that is, a flow rate from a single feeding point Qi, a distance of the feeding point from the disk center ri, and a number of feeding points nf equal to 151 h\({}^{-1}\), 2.0 cm and 2, respectively. It appears that a greater value of the disk velocity leads to the production of smaller nanoparticles, with the smallest one produced at the highest limit of rotational speed of the available equipment. As a consequence, the next experiments were all carried out by adopting a disk velocity N equal to 1500 rpm. The results obtained during the experimental work, and here expressed as the modal size of the produced particles dm, are reported in Figure 4 as a function of Qi, ri, and nf. \begin{table}
\begin{tabular}{c c} \hline
**N (rpm)** & **Dm (nm)** \\ \hline
500 & 45 \\
1000 & 41 \\
1500 & 39 \\ \hline \end{tabular}
\end{table} 
Table 1: Copper nanoparticle size as a function of the disk velocity. Figure 4: Average size of copper nanoparticles as a function of the injection distance from the center of the disk using sodium borohydride: (**a**) for nf = 2; (**b**) nf = 4. Figure 3: Scheme of the different configuration adopted in this work: (**a**) for nf = 2; (**b**) nf = 4. It is possible to observe that for both configurations, the minimum size of produced copper nanoparticles is obtained if a distance of 2.5 cm from the disc center is used, regardless of the feed flow rate. Moreover, it appears that fixing the feed flow rate equal to 9.0 l h\({}^{-1}\) and 4.5 l h\({}^{-1}\) for nf equal to 2 and 4, respectively, leads to the formation of smallest particles. For nf equal to 2, the produced particles are characterized by a dimension ranging from 18 nm to 41 nm, with an average size equal to 29 nm. On contrary, in case of using four injection points, the range was reduced to 16--37 nm, with average size of 26 nm. Therefore, a sensible difference on the produced particle size can be observed if two or four injections were in use. The exact discussion about the resulting dimension of the nanoparticles as a function of the flow rate is complex and involves many aspects, which are out of the scope of this paper. Nevertheless, it should be observed that the film thickness (hundreds of micron) over the disk surface is much smaller than the diameter of the injections points (mm) and the resulting size of the stream entering the available height of the liquid film. In the first step, it appears justified to consider the average Re number only as a function of the disk radius (r), neglecting the vertical distance from the disk surface.