Anomalous Persistence of Evaporating Dense Sprays – APIEDS

The "Anomalous Persistence of Evaporating Dense Sprays" (APIEDS) project is intended to address significant gaps in our understanding of dense spray dynamics by leveraging the collective expertise of leading researchers from LPENS/Université Paris Cité, specialists in wetting, nucleation, and particle transport, and IRPHE/Université Aix Marseille, experts in mixing and fragmentation. This collaborative endeavour seeks to elucidate the collective behaviours of dense sprays, which have far-reaching implications across many areas, from industrial processes to public health strategies and environmental conservation efforts.

The proposal underscores the necessity for new research to comprehend the Anomalous Persistence of Evaporating Dense Aerosols, which are prevalent in nature. Droplets typically form within a cloud or swarm, exhibiting widely distributed sizes and close spacing—a reality that profoundly influences spray longevity. In numerous scenarios, such as disease transmission via pathogen inhalation or pollutant dispersion during agricultural activities, these sprays exhibit remarkable persistence, contrary to what is expected from the conventional academic depiction of isolated droplets evaporating in dry environments. APIEDS aims to address this disparity through a comprehensive approach that integrates advanced experimental techniques with state-of-the-art theoretical modelling.

Following an introduction to the practical contexts where dense sprays are important, and a review of current knowledge and outstanding inquiries, the project outlines a research agenda structured around three key objectives. First, to unravel the dynamic mechanisms governing the persistence of dense spray in the atmosphere, focusing on the interplay between evaporation/condensation processes and the transport and mixing of vapor among droplets. Understanding how these factors affect aerosols lifetimes could lead to long awaited advances in minimizing harmful spray drift in agriculture, optimizing combustion processes in engines, and the delivery of aerosolized medications. This research is also critical for improving our understanding of rain initiation as it will clarify the role of droplet collisional aggregation across a broad range of sizes, including those for which collisions are only weakly efficient.

Second, to explore the control of nucleation in a situation limited by mixing — of vapour, condensation nuclei and heat in the case of a spray of water droplets. Two types of nucleation experiments limited by mixing are proposed, one studying nucleation in the bulk and the other nucleation on a surface, revisiting the problem of breath figures. The integration of advanced image analysis techniques further exemplifies the project's commitment to leveraging cutting-edge technology to push the boundaries of aerosol science.

Third, in a more exploratory part, APIEDS delves into the nuanced role of electrostatic effects in dense sprays. In the case of induced droplet charges, electrostatic interactions can significantly extend the lifetime of aerosol droplets, influencing their coalescence and dispersion patterns. In the presence of an external electric field, induced dipoles are predicted to enhance coalescence, favouring droplets settling by gravity, while net negative charges per droplet will delay it. This line of inquiry has not only potential applications in atmospheric science and cloud microphysics, but also for the technology of air purifiers and bioaerosol samplers.

In summary, APIEDS represents a multidisciplinary effort to decipher the complex dynamics of evaporating and condensing dense sprays. Through a combination of experiments, theory and collaborative synergy, the project aspires to uncover fundamental insights that will inform some of the most challenging issues facing society today, particularly concerning climate change and emerging respiratory pathogens.