Phase State of Interfacial Nanobubbles

Interfacial nanobubbles represent an interesting state of matter: tiny bubbles decorating the interface between a solid and liquid. Yet, there is only sparse evidence supporting the idea that they are indeed gaseous. Here we present evidence that nanobubbles are composed of air rather than oil and that the solid surface underneath the nanobubbles is exposed to air rather than water. The differentiation between air and water was achieved by creating a sensor on the solid surface. The solid was coated in a hydrophobic monolayer containing dansyl fluorophores, which acts as a reporter for the environment around the fluorophore: at an excitation wavelength of 340 nm the emission maximum is 515 nm under water and 480 nm in dry or humid air. The difference in emission could thus be used to determine whether individual parts of the monolayer were in air or in water. Interfacial nanobubbles, created using the standard technique of ethanol exchange, were imaged with fluorescence microscopy, interference contrast microscopy, and phase contrast microscopy. Results show that the positions of nanobubbles shown by interference contrast microscopy or phase microscopy coincide with air pockets shown by fluorescence emission, thereby demonstrating that the interfacial nanobubbles are indeed bubbles. Differentiation between air and oils was achieved by absorption of a fluorescent dye, Nile blue. Whereas oils absorb Nile blue, the interfacial nanobubbles do not absorb the dye and therefore are not composed of oil.