Phase transfer and freezing processes investigated on acoustically levitated aqueous droplets

An acoustic trap was designed and constructed to investigate, on a microscale, physicochemical processes relevant to the troposphere, mainly focusing on the temperature range below 0 °C. Droplets ranging from 0.5 nL to 4 µL (0.1 to 2 mm in diameter) were introduced into the cooled reaction chamber by means of a piezo-driven micro pump with a reproducibility better than 5%. Uptake of H2O2 from the gas phase by the levitated droplet was measured and calibrated by in-situ chemiluminescence. Freezing of stably positioned droplets was observed and documented by means of a microscope and a video camera; this demonstrated the usefulness of the technique for simulation and investigation of cloud processes. Ex-situ microanalysis of sub-microliter droplets by use of a fiber optic luminometer was also shown to be a suitable means of investigating relevant physicochemical processes on a micro scale.