## 3 Plasma and Aerosols: Challenges  
Compared to plasma discharges in and over liquids, biphasic aerosol-plasmas have been far less investigated, and many aspects of their nature are still unknown. The generation of plasma in such finely dispersed biphasic media is very challenging for physical and chemical diagnostics and modeling studies, where very local, transient and fast dynamic behaviors, and long-distance effects should be accounted for. Nowadays, the significant lack of basic understanding concerning the mechanisms governing these plasmas hinders the optimization of their applications. The scientific questions that need to be addressed to improve our knowledge on plasma-aerosol interactions can be ideally sorted into three main categories (see Figure 1):  
* Modification of the liquid droplets due to plasma
* Multiphase species transport between plasma and liquid droplets  
In the first category all the phenomena governing the plasma discharge propagation and intersection with the droplets in a biphasic environment can be found. Based on this, some pioneering studies [31; 32] highlighted how the propagation of streamers can be significantly disturbed by the presence of droplets due to the local modification of the electric field that results from the droplets polarization and charging [33]. This local modulation of the electric field in turn modifies the local rate of ionization. Other possible effects include charge removal from the streamer and shadowing of the photoionization behind the droplets; both mechanisms strongly depend on liquid characteristics (e.g., permittivity) and droplet size. Small droplets (tens of \(\upmu\)m or less) of moderate permittivity are expected to be enveloped by the streamer and do not significantly perturb its propagation, while larger drops, with a big enough capacitance, can intercept and reinitiate the streamer or stall its further propagation. Droplets may thus be considered as controlling and tracing the path where the discharge energy is deposited [31]. Other mechanisms that can possibly play a role and that deserve further investigation include: droplets induction of streamer branching; extraction of electrons from the droplet surface owing to photons emitted from the streamer head; alteration of the streamer propagation velocity. Besides, arising from surface evaporation, the interfacial layer in proximity of the droplet surface is suspected to contain a cocktail of positively and negatively charged clusters with a large size range, along with a very high localized vapor density, possibly up to tens of times that possible in a normal plasma before extinction. We can speculate that this may lead to amplified non-equilibrium gas-phase chemistry, the products of which may be sequestered by the droplet boundary. Even if the analysis of transient phenomena (streamer propagation, streamer droplet interaction) at the microscale requires a great deal of effort, a combined simulative and experimental approach to the subject greatly benefits the community in addressing the role of different phenomena in the plasma-droplet interaction. In particular, a better understanding of the effect of liquid (conductivity, droplet dimension, colloidal state) and gas (electron density, electron temperature, electric field) properties will help researchers to optimize the design of plasma processes and sources. Concerning droplets modifications due to the interaction with plasma, a droplet entering a plasma region is subjected to charging and subsequently experiencing deformation and possibly splitting due to Coulombic fission as a result of exposure to a plasma-related electric field [20]. In addition, the rate of evaporation of such droplets could be enhanced by additional factors other than those  
Figure 1: Schematic representation of the interactions between the plasma and an aerosol droplet. encountered in non-reactive gases, such as irradiation by ultra-low energy electrons [34].