The separation of oil and water is an important technology for solving a wide range of water‐related problems such as purification of industrial wastewater. Herein, the fabrication of an extremely thin oil–water separation membrane is reported by precisely controlling the wettability of a honeycomb (HC) porous membrane. The developed membrane can easily separate water and oil by gravity and capillary force. The method of wettability control by UV–O3 treatment and the resultant change in the separated liquid are discussed. Amphiphilic perforated HC films with controlled wettability can be used in membranes that can separate different liquids according to their densities when the initial liquid added to the surface of the HC film is changed.

Underwater highly bubble and oil-repellent surfaces were prepared based on honeycomb- and pincushion-structured films prepared by breath figure technique and post modifications including UV-ozone treatment and peeling the top layer. Furthermore, bubble generation from the plastron-like honeycomb gas chamber by attaching oil droplet onto the surface of honeycomb films was first observed. Both controlling gas bubbles and oil droplets underwater are important issues in the field of microfluidics since they are useful and may solve the maleficence to liquid transportation in narrow microchannels.