The morphologies of micro-drops of pure water on natural mica and SUS304 steel surfaces were observed with an AC non contact mode of the atomic force microscope. Shape changes of micro-drop pure water over time were also observed in situ. The surface elements before wetting were examined by X-ray photoelectron spectroscopy. The potential for accurate shape measurements and the stability of micro-drop pure water are discussed.
Micro-drop pure water observations carried out with the AC non contact mode of the atomic force microscope demonstrated that the morphologies on natural mica and SUS304 steel surfaces were different. Four morphological types of micro-drop pure water on mica were identified; they comprised semi-sphere, lump, film and assembly. Groups of semi-spheres were the most common. The morphology of micro-drop pure water on the SUS304 steel surface was fairly irregular. The difference might be related to the difference in surface roughness-smooth for natural mica and rough for SUS304 steel. Water droplets on both mica and SUS304 steel remained stable for more than six hours, whereas macro-drops evaporated in a few minutes. The authors speculate that the water vapor pressure is equilibrated near the surface of the micro-drop pure water and the mica or SUS304 steel specimen. To fully evaluate the micro-drop water wetting behavior on materials, a micro-order smooth surface and contact angle lower than 90° will be required.