Liquid-Liquid Phase Separation in Single Suspended Aerosol Microdroplets

Liquid-liquid phase separation (LLPS) is ubiquitousin ambientaerosols. This specific morphology exerts substantial impacts on thephysicochemical properties and atmospheric processes of aerosols,particularly on the gas-particle mass transfer, the interfacialheterogeneous reaction, and the surface albedo. Although there aremany studies on the LLPS of aerosols, a clear picture of LLPS in individualaerosols is scarce due to the experimental difficulties of trappinga single particle and mimicking the suspended state of real aerosols.Here, we investigate the phase separation in individual contactlessmicrodroplets by a self-constructed laser tweezer/Raman spectroscopysystem. The dynamic transformation of the morphology of opticallytrapped droplets over the course of humidity cycles is detected bythe time-resolved cavity-enhanced Raman spectra. The impacts of pHand inorganic components on LLPS in aerosols are discussed. The resultsshow that the increasing acidity can enhance the miscibility betweenthe hydrophilic and hydrophobic phases and decrease the separationrelative humidity of aerosols. Moreover, the inorganic componentsalso have various impacts on the aerosol phase state, whose influencedepends on their different salting-out capabilities. It brings possibleimplications on the morphology of actual atmospheric particles, particularlyfor those dominated by internal mixtures of inorganic and organiccomponents.