Force spectroscopy revealed a high gas density state near the graphite substrate inside surface nanobubbles.

The absorption of gas molecules at hydrophobic surface may have a special state and play an important role in many processes in interfacial physics, which has been rarely considered in previous theory. In this paper, force spectroscopic experiments were performed by a nano-sized AFM probe penetrated into the individual surface nanobubbles and contacted with the HOPG substrate. The results showed that the adhesion force at the gas/solid interface was much smaller than that in air measured with the same AFM probe. The adhesion data was further analyzed by the van der Waals force theory, and the result implied that the gas density near the substrate inside the surface nanobubbles was about three orders of magnitude larger than that under the Standard Pressure and Temperature (STP). Our MD simulation indicated that the gas layers near the substrate exhibited a high density state inside the surface nanobubbles. This high density state may provide new insight to the understanding of their abnormal stability and contact angle and have significant impact on their applications.