## 3 Plasma and Aerosols: Challenges
In fact, these electrons can have energies much lower than is achievable by any other technique, such as radiolysis. Irradiation, however, may also lead to other processes, such as electron-ion or anion-ion recombination, Auger and UV emission, which have received very little attention to date in this context. Moreover, droplets introduced in atmospheric plasma are also susceptible of being accelerated by the combined effect of the drag forces, ionic wind and electrostatic forces and this could result in various effects including size segregation. While it is generally believed that the weak magnetic field produced by atmospheric pressure plasma discharge [35] cannot be accounted for, for the direct modification of a droplet's properties (Moses effect [36], evaporation and modification of surface energy), the application of an external field can be employed to modify the plasma discharge characteristics and droplet properties [37; 38; 39; 40], inducing breakdown [41] or colloid segregation [42]. Precise control over these mechanisms by means of plasma technology would open new perspectives from the point of view of droplet manipulation, from millimetric to nanometric scales, from a single droplet to an aerosol. From this perspective, a deeper investigation of droplet evaporation, taking into account both thermal and physical effects, can strongly influence the use of plasma for on-demand and in-situ synthesis of chemicals and reactive species. Further efforts should be devoted to the fundamental study of droplet charging and electron impaction, with an important focus on the roles of electron density, temperature and droplet size. Concerning multiphase species transport, in the last ten years, considerable advancements have improved our knowledge on the subject [1]. It is considered that plasma-aerosol droplets act as efficient microreactors where reaction rates, mixing and surface/volume ratio are considerably enhanced, posing new unique challenges in the understanding of transport of a species in this configuration [43; 27; 44]. Computational studies [45; 46; 43] showed the importance of the synergy between the plasma and the liquid, including evaporation and the solvation of ions, electrons and neutral particles. They also investigated the plasma treated water chemical and transport processes. They highlighted the importance of the water microdroplet size (or thin water film thickness) on the transport processes of plasma reactive species. While extremely challenging, devoting additional efforts to the analysis of convection and reaction pathways inside a droplet microreactor would be valuable, as well as comparing results with scientific works already published for batch processes. The achievement of better knowledge on this issue will result in the possibility to tune the chemical and physical characteristics of an effluent. Future applications will also take advantage of a better understanding of droplet transportation for the in-situ delivery of chemicals. The understanding of the aforementioned mechanisms is still at an early stage and certainly deserves more attention from the scientific community. The operative ranges for biphasic plasma generation based on aerosol density and granulometry are widely uncharted; the influence of various and numerous process parameters (e.g., pulse repetition frequency, aerosol density) and most of the proposed hypotheses still need to be validated for a wider number of conditions and plasma source architectures. Future studies will have to face the challenges of identifying and validating diagnostic methods able to allow in-situ characterization of this multiphase environment, despite the intrinsic inhomogeneity and transient nature. The measurement of a microdroplet thermal, electrical and chemical characteristics presents a significant challenge that has, to date, seriously limited our understanding of this important system.