Chemistry in evaporating sessile droplets

Abstract

Sessile droplet evaporation is a fundamental natural process that plays important role in the inkjet printing-based two-dimensional (2D) thin film fabrication. The deposition kinetics and the nature of deposit for microparticles are well-known; however, study of nanoparticle-laden droplet evaporation has only recently been reported from our laboratory. The unique chemical phenomena associated with the evaporating droplets need special attention in order to harness the potential of their applications. Apart from the physical phenomenon of fluid dynamics and nanoparticle movement, understanding the chemistry on the surface of nanoparticles and chemical interactions between nanoparticles and solvent in the deposits is of great importance. In our investigations, we have used surface-enhanced Raman spectroscopy to monitor the deposition of chemically functionalized nanoparticles, chemical interactions and reactions on the surface of deposited nanoparticles at the three-phase contact line. This feature article highlights the discovery of single-phase transition in the deposition of plasmonic nanoparticles of different size and initial concentration, as opposed to the well-known gradual deposition of microparticles. Importantly, the study led to the distinction of intrananoparticle and internanoparticle plasmon catalyzed reactions. The effect of pH of the medium in transitioning the single to multiple steps of deposition of nanoparticles and their distribution across the annulus of the ring deposit constitute important observations in this regard. A salient feature is the observation of anomalous 2D crystal formation upon evaporation of droplets with molecular solutions. Finally, the article concludes with the potential of the newly discovered chemistry of nanoparticle deposition for future research in liquidbased printing and 2D moiré superlattices.