Morphology of Air Nanobubbles Trapped at Hydrophobic Nanopatterned Surfaces

• Published in: Nano letters 2010-04, Vol.10 (4), p.1354-1358
• Main Authors: Checco, Antonio, Hofmann, Tommy, DiMasi, Elaine, Black, Charles T, Ocko, Benjamin M
• Format: Article
• Language: English
• Subjects: Air; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Hydrophobic and Hydrophilic Interactions; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Methods of nanofabrication; Nanoscale pattern formation; Nanostructures - chemistry; Nanotechnology - methods; Particle Size; Physics; Silicon - chemistry; Solid-fluid interfaces; Structure of solids and liquids; crystallography; Surface Properties; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Wetting; X-ray diffraction and scattering; X-ray scattering (including small-angle scattering)
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl9042246

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 configurationa nearly planar nanobubble meniscusby partially penetrating into the pores. This microscopic information correlates very well with the macroscopic surface wetting behavior.