Direct observation of oil displacement by water flowing toward an oil nanogap

A fluorescence microscope and a light microscope were employed to observe the phenomenon of water flowing toward an oil nanogap between two solid surfaces. It was found that water was able to displace hexadecane in the nanogap confinement, which contradicted previous viewpoints. An increase in water flow speed contributed to entrainment of water into the contact region, due to inadequate oil supply. Surface energy was found to be another factor that influenced the displacement phenomenon. It was easier for water to enter the contact region on the surface with a greater surface energy, since less energy is required to separate the contact of hexadecane and solid surface and to form water’s own contact.