Liquid–Liquid\nPhase Separation in Single Suspended\nAerosol Microdroplets

Abstract

Liquid–liquid phase separation (LLPS) is ubiquitous\nin ambient\naerosols. This specific morphology exerts substantial impacts on the\nphysicochemical properties and atmospheric processes of aerosols,\nparticularly on the gas–particle mass transfer, the interfacial\nheterogeneous reaction, and the surface albedo. Although there are\nmany studies on the LLPS of aerosols, a clear picture of LLPS in individual\naerosols is scarce due to the experimental difficulties of trapping\na single particle and mimicking the suspended state of real aerosols.\nHere, we investigate the phase separation in individual contactless\nmicrodroplets by a self-constructed laser tweezer/Raman spectroscopy\nsystem. The dynamic transformation of the morphology of optically\ntrapped droplets over the course of humidity cycles is detected by\nthe time-resolved cavity-enhanced Raman spectra. The impacts of pH\nand inorganic components on LLPS in aerosols are discussed. The results\nshow that the increasing acidity can enhance the miscibility between\nthe hydrophilic and hydrophobic phases and decrease the separation\nrelative humidity of aerosols. Moreover, the inorganic components\nalso have various impacts on the aerosol phase state, whose influence\ndepends on their different salting-out capabilities. It brings possible\nimplications on the morphology of actual atmospheric particles, particularly\nfor those dominated by internal mixtures of inorganic and organic\ncomponents.