In Situ Probing of the Surface Properties of Droplets in the Air

Surface properties of nanodroplets and microdropletsare intertwinedwith their immense applicability in biology, medicine, production,catalysis, the environment, and the atmosphere. However, many meansfor analyzing droplets and their surfaces are destructive, non-interface-specific,not conducted under ambient conditions, require sample substrates,conducted ex situ, or a combination thereof. Forthese reasons, a technique for surface-selective in situ analyses under any condition is necessary. This feature articlepresents recent developments in second-order nonlinear optical scatteringtechniques for the in situ interfacial analysis ofaerosol droplets in the air. First, we describe the abundant utilizationof such droplets across industries and how their unique surface propertieslead to their ubiquitous usage. Then, we describe the fundamentalproperties of droplets and their surfaces followed by common methodsfor their study. We next describe the fundamental principles of sum-frequencygeneration (SFG) spectroscopy, the Langmuir adsorption model, andhow they are used together to describe adsorption processes at planarliquid and droplet surfaces. We also discuss the history of developmentsof second-order scattering from droplets suspended in dispersive mediaand introduce second-harmonic scattering (SHS) and sum-frequency scattering(SFS) spectroscopies. We then go on to outline the developments ofSHS, electronic sum-frequency scattering (ESFS), and vibrational sum-frequencyscattering (VSFS) from droplets in the air and discuss the fundamentalinsights about droplet surfaces that the techniques have provided.Finally, we describe some of the areas of nonlinear scattering fromairborne droplets which need improvement as well as potential futuredirections and utilizations of SHS, ESFS, and VSFS throughout environmentalsystems, interfacial chemistry, and fundamental physics. The goalof this feature article is to spread knowledge about droplets andtheir unique surface properties as well as introduce second-ordernonlinear scattering to a broad audience who may be unaware of recentprogress and advancements in their applicability.