## 3 Results and Discussion  
In previous research, Venturi reactors/absorbers have been successfully used in the case of chemical reaction/absorption between a reactant in gaseous form and an absorbent liquid, whether or not it contained another reactant [26]. This is because these types of reactors allow high performance in gas-liquid mixing due to the turbulence of the outgoing mixed flow [27]. They have been recently proposed as a valid solution for CO\({}_{2}\) capture [28] and the abatement of pollutants like SO\({}_{2}\) and NO from a gas stream [29].  
The choice of CaCO\({}_{3}\) NP synthesis for the present study has been made for many different reasons. The first motivation is the total absence of toxicity of this inorganic compound [30] and its excellent tolerability for in vivo [31] and in vitro [32] human cells, making it a very promising compound in theranostic nanomedicine [33]. Very recently, CaCO\({}_{3}\) NPs have been successfully proposed in nanocomposites as a multi-drug carrier for odontoiatric uses, with interesting results in bone repair and bacterial eradication [34]. [PERSON] et al. [35] synthesised CaCO\({}_{3}\) NPs and tested their fire-retardant properties. Furthermore, these inorganic NPs have proven to be efficient nanosorbents for the removal of heavy cations [36], thus representing a viable alternative to biosorbents [37]. Likewise, the process described here can be used for the synthesis of other particles of organic or inorganic composition. The latter include hydroxides, sulfides, or otherwise nanostructured solid phases, provided they are insoluble in the carrier liquid of the Venturi scrubber.  
In the experimental campaign realised here, a first attempt was made using the setup described in the left image of Figure 3, namely, adopting an indirect nebulisation technique. However, this choice was affected by a serious drawback that actually hindered its use. In many cases, the atomisation of a saline solution does not imply a significant change in its surface tension \(\
u\), with respect to its value in the case of a pure solvent. However, the situation is different in the case of a concentrated solution of electrolytes like Na\({}_{2}\)CO\({}_{3}\). [PERSON] et al. [38] reported a value of \(\
u=77.15\) mN/m at 23 \({}^{\circ}\)C for a saturated aqueous solution of Na\({}_{2}\)CO\({}_{3}\), against a value \(\
u=72.72\) mN/m of pure water at the same temperature.  
Figure 5: **(a)** A photograph of the driver module, plugged with its piezoelectric transducer, as marketed by the manufacturer. **(b)** A scheme of the piezoelectric transducer in its original commercial form, with the electric connectors located on the same side of the plate. **(c)** A scheme of the piezoelectric transducer, specially modified to make it suitable for atomising the conductive solutions used in this work, with the electric connectors placed far from each other. The dashed area represents the surface treated with an insulating paint, specially added here as not foreseen by the manufacturer.  
This trend, which is different from that of NaHCO\({}_{3}\), has been ascribed to the water dipole moment distribution at the air/solution interface [39]. These aspects, owing to a non-tunable power of the atomiser originally designed for diluted solutions of therapeutic use, led to a dramatic loss of aerosolisation efficiency, thus making this setup unusable. For this reason, the indirect ultrasonic aerosolisation was discarded in favour of a direct ultrasonic aerosolisation schematised in the right-hand image of Figure 3, from which all the experimental data reported in this paper were obtained.