## 3 Results and Discussion
### Synthesis by Direct Ultrasonic Aerosolisation and Comparison with Non-Aerosolised Synthesis
Separately, a CaCO\({}_{3}\) synthesis was carried out by impulsively mixing, in a stirred beaker, the same volumes of reagents that were employed in the aforementioned aerosol synthesis (5 cm\({}^{3}\) each), injecting them in the same volume V\({}_{0}\) of water circulating in the Venturi nozzle circuit. The reagent concentrations were the same as shown in Table 1. This choice was adopted in order to compare the characteristics of particles obtained by two different synthesis processes, but with different mixing modes and, in particular, with different mass confinement. From this point on, AER-1 and AER-2 will indicate the aerosol-mediated synthesis methods using equal precursors concentrations at C\({}_{0}\) = 0.472 M and C\({}_{0}\)/4, respectively, while STR-1 and STR-2 will indicate the stirred-tank synthesis by impulsive injection, in a water volume V\({}_{0}\), of the same liquid volumes employed in aerosolisation, each containing the relevant precursor at concentrations C\({}_{0}\) and C\({}_{0}\)/4, respectively. These experimental conditions are summarised in Table 2.  
In Figure 6, the lognormal distribution function of the NPs' diameters by number, measured by DLS, is reported for aerosolised and non-aerosolised CaCO\({}_{3}\) synthesis. In the former case, the NPs' average diameter was 34 nm with a standard deviation of 6 nm, while in the latter case, the average diameter was 52 nm with a standard deviation of 9 nm. The CaCO\({}_{3}\) NPs' average diameter obtained by aerosolisation is significantly smaller than that obtained by a standard mixing in a batch mode, whose solid phase produced cannot be considered nanostructured, as shown in Table 3, where the restrictive convention used by the physicist community is cautiously adopted, assuming that a nanoparticle should have a diameter \(\leq\)100 nm. Even more importantly, the polydispersity of particles is considerably limited in the case of aerosolised synthesis with respect to a non-aerosolised one, and this fact is observable by comparison of the standard deviations according to the data reported in the above-mentioned Table 3. \begin{table}
\begin{tabular}{c c c} \hline
**Sample Type** & **Synthesis Method** & **Precursors Concentrations** \\ \hline AER-1 & Direct aerosolisation & 0.472 M \\ AER-2 & Direct aerosolisation & 0.118 M \\ STR-1 & Impulsive mixing in stirred tank & 0.472 M \\ STR-2 & Impulsive mixing in stirred tank & 0.118 M \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of key experimental parameters and techniques adopted in the present study. \begin{table}
\begin{tabular}{c c c} \hline
**Sample Type** & **Average Diameter (nm)** & **Standard Deviation (nm)** \\ \hline AER-1 & 34 & 6 \\ AER-2 & 52 & 9 \\ STR-1 & 342 (microparticles) & 105 \\ STR-2 & 697 (microparticles) & 118 \\ \hline \end{tabular}
\end{table} 
Table 3: Physical characteristics of the CaCO\({}_{3}\) solid phase produced according to different experimental conditions using aerosolised and non-aerosolised synthesis processes. A possible explanation of these results could be given considering the differences in precursor confinement and mixing between the two synthesis techniques. On that theme, [PERSON] et al. [40] focused on the role of the reagent flow rate and addition order in the synthesis of Ag- and Au-NPs by a redox technique in a batch reactor, and they found a significant influence of these variables on average particle diameter and polydispersity. On the other hand, micromixing [41] and mass transfer have even greater importance in the microfluidics of open systems, where the droplet reactors [42] are in the hotspot for organic [43] and inorganic NP synthesis [44], including the promising field of Janus particles [45]. A common point between droplet reactor synthesis, emulsion techniques [46] and aerosol-assisted synthesis [47] lies in discretising and confining reagents in mobile microreactors, acting as NPs micro/nanofactories, actually damping particle aggregation.