Nobuo Maeda

Stability of Interfacial Nanobubbles

Interfacial nanobubbles (INBs) on a solid surface in contact with water have drawn widespread research interest. Although several theoretical models have been proposed to explain their apparent long lifetimes, the underlying mechanism still remains in dispute. In this work, the morphological evolution of INBs was examined in air-equilibrated and partially degassed water with the use of atomic force microscopy (AFM). Our results show that (1) INBs shrank in the partially degassed water while they grew slightly in the air-equilibrated water, (2) the three-phase boundary of the INBs was pinned during the morphological evolution of the INBs. Our analyses show that (1) the lifetime of INBs was sensitive to the saturation level of dissolved gases in the surrounding water, especially when the concentration of dissolved gases was close to saturation, and (2) the pinning of the three-phase boundary could significantly slow down the kinetics of both the growth and the shrinkage of the INBs. We developed a one-dimensional version of the Epstein-Plesset model of gas diffusion to account for the effect of pinning.

Evaporation and Instabilities of Microscopic Capillary Bridges

The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young–Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2rKcosθ, where rK is the Kelvin radius and θ is the contact angle. During the course of slow evaporation (at H > 2rKcosθ) and subsequent regrowth by capillary condensation (at H < 2rKcosθ), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.

Effects of Surfactants on the Formation and the Stability of Interfacial Nanobubbles

Contamination has previously been invoked to explain the flat shape and the long lifetimes of interfacial nanobubbles (INBs). In this study, the effects of surfactants on the formation and the stability of INBs were investigated when surfactants were added to the system before, during, and after the standard solvent exchange procedure (SSEP) for the formation of INBs. The solutions of sodium dodecyl sulfate (SDS) above critical micelle concentration were found to have little effect on the bubble stability. Likewise, cleaning of the substrate with a surfactant solution had little effect. In contrast, addition of a water-insoluble surfactant during the formation dramatically reduced the INBs. Finally, repeated application of SSEP to surfactant-coated substrates progressively rinsed the surfactant off the system. Thus, we found no evidence to support the hypothesis that (1) INBs are stabilized by a layer of insoluble organic contaminant or that (2) SSEP introduces surface-active materials to the system that could stabilize INBs.

Thermodynamic Stability of Interfacial Gaseous States

We studied the thermodynamic stability of interfacial gaseous states on atomically smooth highly ordered pyrolytic graphite (HOPG) in water using atomic force microscopy. Quasi-two-dimensional gas layers (micropancakes) required a higher supersaturation of gas than spherical-cap-shaped nanobubbles. The two forms of gas coexisted at a sufficiently high supersaturation of gas where one or more of the nanobubbles may sit on top of a micropancake. The micropancakes spontaneously coalesced with each other over time. After the coalescence of two neighboring micropancakes which each had had a nanobubble on top, one nanobubble grew at the expense of the other. We analyzed these results assuming temporal and local quasi-equilibrium conditions.

Micromanipulation of phospholipid bilayers by atomic force microscopy.

The molecular details of adhesion mechanics in phospholipid bilayers have been studied using atomic force microscopy (AFM). Under tension fused bilayers of dipalmitoylphosphatidylcholine (DPPC) yield to give non-distance dependent and discrete force plateaux of 45.4, 81.6 and 113+/-3.5 pN. This behaviour may persist over distances as great as 400 nm and suggests the stable formation of a cylindrical tube which bridges the bilayers on the two surfaces. The stability of this connective structure may have implications for the formation of pili and hence for the initial stage of bacterial conjugation. Dimyristoylphosphatidylcholine (DMPC) bilayers also exhibit force plateaux but with a much less pronounced quantization. Bilayers composed of egg PC, sterylamine and cholesterol stressed in a similar way show complex behaviour which can in part be explained using the models demonstrated in the pure lipids.

Direct observation of surface effects on the freezing and melting of an n-alkane

Abstract We have used optical microscopy and multiple-beam interferometry to study the phase behaviour of n-octadecane (n-C 18 ) confined between mica surfaces in the vicinity of and below its bulk melting point ( T m =28.2°C). n-C 18 adsorbs from vapour (relative vapour pressure p / p 0 ≈0.97) to an isolated mica surface, forming 2.8±0.2 nm thick film. At a separation between two mica surfaces of 11±1 nm capillary condensation occurs, and liquid n-C 18 fills the gap between the surfaces and pulls them into contact, both above and below T m . The condensate does not freeze down to Δ T =14°C below T m while the surfaces are in contact, although slow growth of crystals from the liquid into the vapour phase occurs for Δ T ≥5°C. At an isolated surface, e.g. after separation of the surfaces far enough for the liquid condensate to snap into two droplets, freezing occurs readily at all temperatures below T m . As the temperature is lowered, freezing of bridging necks of liquid between two surfaces starts to occur at progressively smaller surface separations. Finally, for Δ T ≥5°C, freezing may occur during separation, as the surfaces come apart. The observations can be rationalised by comparing variations in the condensate volume to variations in the interfacial areas (n-C 18 mica and n-C 18 vapour). In general, the mica-condensate interface prevents freezing, while the vapour-condensate interface appears to promote nucleation of solid. The results are in accord with the idea that proximity to a crystalline surface like mica leads to a very large melting-point depression for any wetting liquid.

EXPERIMENTAL OBSERVATIONS OF SURFACE FREEZING

Capillary phase transitions and those induced by interfaces, like pre-melting, have been studied for decades. The related phenomenon of surface freezing has not been explored so extensively. We review experiments on surface freezing, those of long-chain n-alkanes in particular, and place the results within the wider thermodynamic framework of surface phase transitions. Surface freezing plays an important role in nucleation and crystallization of bulk long-chain n-alkanes. Implications for capillary melting and freezing of substances at nanoscales are discussed. Theoretical aspects of condensed capillary phase transitions will be reviewed separately.