Influence of Surface-Active Substances and Substrates on the Wettability of Individual Aerosol Particles during Condensation by Environmental Scanning Electron Microscopy

The formation of liquid cloud droplets from aerosol particles in the Earth atmosphere is still under debate particularly because of the difficulties to quantify the importance of bulk and surface effects in these processes. Recently, single-particle techniques have been developed to access experimental key parameters at the scale of individual particles. Environmental scanning electron microscopy (ESEM) has the advantage to provide in situ monitoring of the water uptake of individual microscopic particles deposited on solid substrates. In this work, ESEM was used to compare droplet growth on pure ammonium sulfate (NH4)2SO4 and mixed sodium dodecyl sulfate/ammonium sulfate (SDS/(NH4)2SO4) particles and to explore the role of experimental parameters, such as the hydrophobic-hydrophilic character of the substrate, on this growth. With hydrophilic substrates, the growth on pure salt particles was strongly anisotropic, but this anisotropy was suppressed by the presence of SDS. With hydrophobic substrates, it is the wetting behavior of the liquid droplet that is impacted by the presence of SDS. The wetting behavior of the pure (NH4)2SO4 solution on a hydrophobic surface shows a step-by-step mechanism that can be attributed to successive pinning-depinning phenomena at the triple-phase line frontier. Unlike the pure (NH4)2SO4 solution, the mixed SDS/(NH4)2SO4 solution did not show such a mechanism. Therefore, the hydrophobic-hydrophilic character of the substrate plays an important role in the stability and dynamics of the liquid droplets' nucleation by water vapor condensation. In particular, hydrophilic substrates are not suited for the investigation of the hygroscopic properties (deliquescence relative humidity (DRH) and hygroscopic growth factor (GF)) of particles. Using hydrophobic substrates, data show that the DRH of (NH4)2SO4 particles is measured within 3% accuracy on the RH and their GF could indicate a size-dependent effect in the micrometer range. The presence of SDS does not seem to modify the DRH and GF of (NH4)2SO4 particles. This study shows that the water uptake on deposited particles is a complex process but, once carefully taken into account, ESEM is a suitable technique to study them.