On droplets and sprays

This thesis, "On Droplets and Sprays," explores the fundamental fluid dynamics governing liquid droplet formation, flight, and impact. While addressing practical challenges in agricultural pesticide application—specifically mitigating airborne drift and droplet rebound—the research provides deep physical insights into complex interfacial phenomena and non-Newtonian fluid behavior.
First, the study examines spray formation under high wind speeds. External airflow induces bimodal droplet size distributions via bag and sheet breakup mechanisms driven by the Rayleigh-Taylor instability. Introducing dilute polymer additives significantly increases average droplet sizes, reducing the volume of drift-prone fines by up to threefold. The polymer's strain-hardening properties fundamentally modify the fragmentation physics, establishing new scaling laws for viscoelastic sprays in crossflows.
Second, the thesis investigates droplet impact on hydrophobic surfaces. For viscous droplets, the dynamic receding contact angle is identified as the critical parameter governing retraction rates, dictated by a balance of capillary forces and viscous dissipation. For complex non-Newtonian fluids, the study resolves a long-standing debate: the suppression of droplet rebound is caused by elastic normal stresses from polymer stretching near the receding contact line, rather than interfacial adsorption. Using novel sliding-drop rheometry, the research reveals that polymer chain tumbling limits extension at high deformation rates, explaining the distinct transition between fast 'inertial' and slow 'elastic' retraction regimes.
Ultimately, this work delivers fundamental physical models crucial for advancing our understanding of complex fluid dynamics and designing highly efficient, environmentally safe spray technologies.