Summary
The transition to low global warming potential propellants for pressurised metered dose inhalers (pMDIs) presents reformulation challenges and opportunities due to differences in the physicochemical properties of these new propellants. Controlling the performance of propellant-based formulations and demonstrating equivalence of reformulated products may require a deeper understanding of their evaporation kinetics. Single-droplet studies provide a route to precise measurements of evaporation rates. We present a multi-instrument approach, using an ultrafast stroboscopic droplet analysis method with µs time resolution and an electrodynamic balance, enabling measurements from droplet formation, over timescales equivalent to inhalation. This approach is applied to HFA 134a, HFA 152a, and HFO 1234ze, and resolves different initial evaporation rates of ~ 10,000, ~ 20,000, and ~41,000 µm2s-1, respectively. Importantly, water condensation was observed at both 50 % and 90 % relative humidity (RH), meaning propellant droplets can maintain larger sizes than expected during inhalation, even for sub-saturated humidities. The data from the two methods are combined to provide a theoretical lower limit to the evolving size of droplets.