pubs.acs.org/NanoLett
Morphology of Air Nanobubbles Trapped at Hydrophobic Nanopatterned Surfaces Antonio Checco,*,† Tommy Hofmann,† Elaine DiMasi,‡ Charles T. Black,§ and Benjamin M. Ocko† †
Condensed Matter Physics and Materials Science Department, ‡ National Synchrotron Light Source, and § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973 ABSTRACT The details of air nanobubble trapping at the interface between water and a nanostructured hydrophobic silicon surface are investigated using X-ray scattering and contact angle measurements. Large-area silicon surfaces containing hexagonally packed, 20 nm wide hydrophobic cavities provide ideal model surfaces for studying the morphology of air nanobubbles trapped inside cavities and its dependence on the cavity depth. Transmission small-angle X-ray scattering measurements show stable trapping of air inside the cavities with a partial water penetration of 5-10 nm into the pores, independent of their large depth variation. This behavior is explained by consideration of capillary effects and the cavity geometry. For parabolic cavities, the liquid can reach a thermodynamically stable configurationsa nearly planar nanobubble meniscussby partially penetrating into the pores. This microscopic information correlates very well with the macroscopic surface wetting behavior. KEYWORDS Hydrophobicity, nanopatterned surface, nanobubble, X-ray scattering
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ecent experiments have demonstrated that trace gases dissolved in water can condense at the interface of planar hydrophobic solid surfaces to form bubbles of submicrometer size.1-3 The existence of such “nanobubbles” defies classical thermodynamic theory, and the details of their formation and stability are still poorly understood.4 As well, the presence of nanobubbles may affect, for example, the biocompatibility of surfaces,5 the adhesion between hydrophobic surfaces in water,6 and the interfacial hydrodynamic friction7,8sall potentially relevant technological issues. Flat surfaces support a highly nonuniform spatial and size distribution of nanobubbles9 which may provide insight into the mechanism of nanobubble stabilization4 but also pose potentially serious limitations for technological applications. One possible route to controlling nanobubble formation is by providing effective sites for the nucleation of the gas phase in the form of topographical patterns on solid surfaces.10 Previous experiments have indirectly probed the occurrence of air trapping inside micrometer- and nanometer-scale topographic features by measuring the macroscopic contact angle of water on such textured surfaces.11-13 These studies have provided indirect evidence for air trapping in the nanotextures without shedding light on the microscopic details of the nanobubble morphology. Such information is essential for a full understanding of the properties of the composite solid/gas surface including how the meniscus protrusion affects hydrodynamic slippage.14
This Letter reports the results of a comprehensive study of the morphology of air bubbles trapped inside nanometerscale cavities as a function of cavity depth. For this purpose, large-area silicon surfaces (cm2) were prepared containing highly uniform, periodic, densely packed 20 nm wide hydrophobic cavities. The ability to experimentally vary the cavity depth allows control of surface topography at sub-100nm length scales, significantly smaller than that achieved in previous studies.11-13 The high degree of nanostructured surface periodicity and uniformity facilitates the use of transmission small-angle X-ray scattering (SAXS) methods to provide an unprecedented detailed view of the composite interface. More specifically, SAXS measurements show stable trapping of air inside the cavities with a water penetration of 5-10 nm into the pores, independent of the pore depth. This behavior is explained by consideration of capillary effects and the cavity geometry. For hydrophobic parabolic cavities, the liquid can reach a thermodynamically stable configuration with a nearly planar nanobubble meniscus by partial penetration into the pores. This microscopic information correlates very well with the macroscopic contact angle measurements showing a monotonic increase in both the surface hydrophobicity and contact angle hysteresis with cavity depth up to depths of ∼30 nm with a plateau in these values for deeper cavities. The advantages of block copolymer self-assembly-based fabrication approaches15 have been leveraged to prepare nanostructured surfaces with properties suitable for studying the details of nanobubble formation at a liquid/solid interface. Block copolymer thin films autonomously form welldefined, periodic patterns of high density (∼1011 cm-2) nanometer-scale features (
