Contact Efflorescence on Demand

The phase state of atmospheric aerosols (liquid vs solid) plays an important role in particle growth, cloud formation, climate impact and visibility degradation. In the atmosphere, changes in relative humidity (RH) and temperature cause phase transitions in the atmospheric particulate. Efflorescence, the process of salt crystal nucleation from an aqueous electrolyte solution upon decreasing RH, often occurs at a lower RH than the reverse process of deliquescence. It has been shown that the efflorescence RH can occur at a higher RH in the presence of a heterogeneous surface immersed in a liquid particle. Here we present a new laboratory technique using optically levitated particles to study heterogeneous efflorescence initiated by contact with an external particle. In this work, collisions between aqueous microdroplets and heterogeneous nuclei are monitored in situ using scattered laser light to quantify the number of collisions and to detect phase transitions. We find that when contact initiates the phase transition, efflorescence occurs at a higher RH than when the same heterogeneous nucleus is immersed in the particle. The results of these experiments will be discussed in the context of understanding contact nucleation on a mechanistic level and predicting the relative importance of contact efflorescence in the atmosphere.