## 2 Materials, Methods and Experimental Setup
### The Experimental Setup  
In Figure 1, a scheme of the comprehensive experimental setup relying upon ultrasonic aerosolisation is reported. The overall setup components are as follows:  
* A glass vessel, whose upper flange is crossed by a water supply line, a water outlet line and a Venturi nozzle outlet line. From this point on, this vessel will be named the \"Venturi vessel\" to avoid confusion with the aerosoliser container, called the \"atomising vessel\"
* A Venturi nozzle, whose outlet pipe draws directly into the vessel. A five-chamber diaphragm pump (Seaflo, 24 V, 240 W) for circulating the water phase, operating at a maximum outlet pressure of 4 bar. This pump draws water from the vessel and sends it to the Venturi reactor inlet. - An analogic manometer measuring water pressure at the Venturi reactor inlet; - Two ultrasonic aerosolisers, each one comprising a reservoir (the atomising vessel) containing chemical reagents A and B indicated in Equation (2), namely one reagent for each aerosoliser, equipped with an inlet and outlet duct. The former receives the outgoing air from the Venturi vessel; the latter conveys the gaseous stream containing the liquid droplets into a T-junction where the outgoing streams from each aerosoliser merge together. This T-shaped device acts as a chemical reactor, promoting the coalescence between droplets containing two different reagents, thus forming bigger droplets containing NPs of the product C. - A boosting gas compressor (Tungfull Digital Technology, Shenzhen, China, 24 V, 10 W), operating with adjustable flow rate, is located on the gas line between the outlet of the Venturi vessel and the inlet of the two aerosolisers. Its purpose is to improve the circulation of the gas stream by counteracting pressure drops in the gas line. The compressor motor is equipped with a dedicated, custom-made water-cooling coil. - Two adjustable DC power suppliers (Longwei, LW-K3010D; 0-30 V; 300 W) were adopted. One of the two is used to run the water circulation pump in the Venturi nozzle, while the other powers the boosting compressor mentioned in the previous point. - One fixed DC power supplier (5 V; 20 W) to power the electrical circuit of each atomiser and the relevant cooling fan. In essence, the experimental setup described in Figure 1 is based on two closed-circuit streams, namely,  
* A closed-loop water circulation to ensure the operation of the Venturi nozzle, whose sole purpose is to capture the liquid droplets coming from the T-joint and containing NPs of product C;
* A closed-loop gas stream, exiting the vessel and forming two gas currents of equal flow rate entering the aerosolisers. For a better visualisation, the closed-loop liquid, gas and aerosolised streams are reported in solid, dotted and dashed lines, respectively. Figure 1: Schematic diagram of the experimental setup. For the sake of clarity, the electric components have not been reported. A and B are the reagents indicated in Equation (2). In Figure 2, a scheme of the Venturi nozzle is reported. It consists of a supply duct terminating in a converging nozzle. As the carrier fluid passes through the nozzle, it forms a vacuum by which air and droplets coming from the T-joint are sucked in and further mixed with the carrier liquid. The high turbulence produced downstream of the nozzle causes the abatement of NPs, which are thus trapped by the liquid, finally collected in an underlying vessel. Two different experimental configurations have been tested. Both of them rely on the same Venturi nozzle collector, but they differ from one another in the way in which the aerosolised phases are generated. In fact, the experimental setup initially used was based on an indirect ultrasonic aerosolisation process, while the one subsequently used relied on a direct aerosolisation process. In Figure 3, the differences between the two mentioned nebulisation techniques are highlighted.