Monitoring Growth of Surfactant-Free Nanodroplets Dispersed in Water by Single-Droplet Detection

Abstract

Nanodroplets of n-decane (50−300 nm in diameter) containing Coumarin 540 were dispersed ultrasonically in water under a surfactant-free condition, and their growth process was observed by a method of a single-droplet detection in which fluorescence-bunching signals from individual nanodroplets passing through the focal volume of an excitation laser were collected and detected in a confocal fluorescence microscope. The volume of the nanodroplets determined from the fluorescence-bunching signals increased stepwise with time; time regimes of gradual and rapid increases were attributed to Ostwald ripening and coalescence, respectively. The alternative manifestation of the two growth processes is consistent with an observed oscillatory behavior of the surface charge density of the nanodroplets.