Collision coalescence study through the dynamics of impinging spray jets

• Effective coalescence requires droplets >20 µm;. • Peak D₃₂ and D max occur near the stagnation plane, especially in cross-paired atomizers. • Asymmetric collisions (of large–fast droplets with small–slow) produce the largest droplets. • In-stream collisions (fast droplets overtaking slower ones in same traverse direction) are a key coalescence pathway. • For in-stream droplets, high Weber numbers post collision can be maintained because the size increase from overtaking collisions occurs faster than the loss of velocity. Cloud droplet growth is known to occur through a combination of condensation and collision-coalescence processes. While gravitational collision-coalescence becomes significant for droplets larger than approximately Φ80 μm, and condensation dominates for smaller sizes, a notable gap exists in the intermediate size range of Φ30 – Φ80 μm. In this regime, known as the "condensation-coalescence bottleneck," neither mechanism sufficiently explains the observed rapid droplet growth. To address this gap, the present experimental study investigates droplet dynamics within sprays to simulate the coalescence behavior seen in cloud environments. Focusing on the low Weber number regime, we explore water droplet interactions and growth mechanisms in the Φ5 – Φ80 μm size range, where collision-coalescence may play a crucial but is less understood. Using Vibrating Mesh Piezoelectric Atomizers (VMA) in an impinging, non-reactive spray configuration, we aim to provide new insights into the efficiency and dynamics of droplet growth, contributing to a better understanding of microphysical cloud processes.