Phase transfer and freezing processes investigated on acoustically levitated aqueous droplets

An acoustic trap was designed and constructed to investigate, on a microsca le, physicochemical processes relevant to the troposphere, mainly focusing on the temperature range below 0 degreesC. Droplets ranging from 0.5 nL to 4 muL (0.1 to 2 min in diameter) were introduced into the cooled reaction c hamber 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 mea sured and calibrated by in-situ chemiluminescence. Freezing of stably posit ioned droplets was observed and documented by means of a microscope and a v ideo camera; this demonstrated the usefulness of the technique for simulati on and investigation of cloud processes. Ex-situ microanalysis of sub-micro liter droplets by use of a fiber optic luminometer was also shown to be a s uitable means of investigating relevant physicochemical processes on a micr o scale.