title: Metastable nanobubbles at the solid-liquid interface due to contact angle hysteresis.
abstract: Nanobubbles exist at solid-liquid interfaces between pure water and hydrophobic surfaces with very high stability, lasting in certain cases up to several days. Not only semispherical but also other shapes, such as micropancakes, are known to exist at such interfaces. However, doubt has been raised as to whether or not the nanobubbles are gas-phase entities. In this study, surface nanobubbles at a pure water-highly ordered pyrolytic graphite (HOPG) interface were investigated by peak force quantitative nanomechanics (PF-QNM). Multiple isolated nanobubbles generated by the solvent-exchange method were present on the terraced areas, avoiding the steps of the HOPG surface. Adjacent nanobubbles coalesced and formed metastable nanobubbles. Coalescence was enhanced by the PF-QNM measurement. We determined that nanobubbles can exist for a long time because of nanoscale contact angle hysteresis at the water-HOPG interface. Moreover, the hydrophilic steps of HOPG were avoided during coalescence, providing evidence that the nanobubbles are truly gas phase.
authors.affiliation: Department of Aeronautics and Astronautics, ‡CREST, §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), and ∥Department of Mechanical Engineering, Kyushu University , Fukuoka 819-0395, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Nishiyama
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Yamada
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Ikuta
authors.forename: Koji
authors.initials: K
authors.lastname: Takahashi
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Takata