Liquid–Liquid Interface Can Promote Micro-Scale Thermal Marangoni Convection in Liquid Binary Mixtures

Abstract

Liquid–liquid phase separation, a physical transition in which a homogeneous solution spontaneously demixes into two coexisting liquid phases, has been a key topic in the thermodynamics of two-component systems and may find applications in separation, drug delivery, and protein crystallization. Here we applied a microscale temperature gradient using optothermal heating of a gold nanoparticle to overcome the experimental difficulties inherent in homogeneous heating: We aimed at highlighting precise structural development by avoiding randomly nucleating/growing microdomains. In response to laser illumination, a single gold nanoparticle immersed in a binary mixture of aqueous 2,6-dimethylpyridine (lutidine) and N-isopropylpropionamide (NiPPA) was clearly sensitive to the phase transition of the surrounding liquid as demonstrated by light-scattering signals: spectral red-shifts and bright-spot images. The local phase separation encapsulating the gold nanoparticle resulted in immediate formation and growth of an organic-rich droplet which was confirmed by Raman spectroscopy. Remarkably, the droplet was stable under a nonequilibrium steady-state heating condition because of strong thermal confinement. Microdroplet growth was ascribed to thermocapillary flow induced by a newly formed liquid–liquid interface around the hot gold nanoparticle. On the basis of a tracer experiment and numerical simulation, it is deduced that the transport of solute to the high-temperature area is driven by this thermocapillary flow. This study enhances our understanding of phase separation in binary mixtures induced by microscale temperature confinement.